t ale;/ epri - nrc: home page · 3-12 curves for type 317l stainless steel in solution e3 with a...

M t $tAC Ale;/

EPRIElectric PowerResearch Institute

Keywords:PittingCorrosion testingCorrosionElectrochemistryElectrochemical potentials

EPRI EAR-7489Project 8002-15Final ReportOctober 1991

Using Pitting and ProtectionPotentials to Predict PittingBehavior: New Insights

Prepared byCortest Columbus TechnologiesColumbus, Ohio

- ,'�� J10,

R E P O R T S U M M A R Y

Using Pitting and Protection Potentials to PredictPitting Behavior: New InsightsPining and protection potentials (EpMt and Eprot determined in conven-tional cyclic potentiodynamic polarization~tests, often provide inconsis-tent assessments of the pitting resistance of stainless alloys. A newparameter, Eu, falling between Epit and Ex values, provides a morepractical and consistent assessment of pitting resistance.

INTEREST CATEGORIES

MaterialsFossil steam plant systems

and performanceNuclear plant corrosion

controlService water systems

KEYWORDS

PittingCorrosion testingCorrosionElectrochemistryElectrochemical potentials

BACKGROUND In a cyclic potentiodynamic polarization (CPP) test, the potentialof a stainless alloy specimen Immersed in a corrosive solution is changed at aconstant rate to more noble (positive) values until pitting initiates at Epit; then thepotential is changed to more active (negative) values until the pits repassivate(stop growing) at Ept. Conventional wisdom states that the more noble the valueof Epit or Epr, the more resistant the alloy is to pitting.Unfortunately, the ranking of alloys often depends on whether Elk or Ept ischosen as the ranking parameter; the relationship between Epft and Epro, if any,is unknown; and the value of each parameter is quite sensitive to experimentalprocedures. Consequently, design engineers are reluctant to select materials ofconstruction based solely on the results of the CPP test and instead rely moreon the results of long-term field tests. These tests unfortunately have their ownshortcomings.

OBJECTIVES To understand the relationship between Ept and Epo; to define apitting parameter that is not sensitive to experimental procedure and that can beused to provide an accurate ranking of the pitting resistance of alloys.

APPROACH The project team determined the pitting resistance of Type 317Lstainless steel and alloy G3 In several test solutions that simulated flue gas desul-furization system outlet duct environments. CPP tests, performed at two potentialsweep rates, provided baseline Epit and Epot values. The researchers then heldspecimens at constant potentials between Epit and Ept to determine how pit initia-tion time varied with potential. Finally, they performed modified ASTM StandardF-746 tests to determine the sensitivity of the Epo value to previous pit growth. Inthis test, they held the potential of the specimen at a relatively noble value tostimulate pit growth. After a preselected time of growth, they dropped the potentialto a lower value to establish the repassivation potential, Epro,.

RESULTS The CPP and constant potential tests demonstrated that pitting is astochastic process resulting in significant scatter of the measured EpMt values.Nevertheless, the results clearly showed that Epft decreased as the potentialsweep rate decreased in CPP tests and as the exposure period Increased in con-stant potential tests. The researchers Identified a unique pitting potential (Eu),defined as the potential above which the pit nucleation frequency exceeds zero.Eu corresponds to the most active value of Epit recorded, associated with long

EPIER78sEeti PwrRsac ntttEPRI EAR-7489s Electric Power Research Institute

exposure periods. The modified ASTM Standard F-746 tests confirmedthat Ep"o decreased as the time of previous pit growth increased. Afterminimal pit growth, when the environment within the pit is similar to thebulk environment, Epm is approximately equal to E,.

EPRI PERSPECTIVE E, is a unique pitting potential, falling between theEpit and Epro values determined in CPP tests. E, is not influenced by ex-perimental procedure and, as such, accurately characterizes the long-term pitting resistance of an alloy. Ideally, design engineers should selectmaterials of construction after comparing E, values for candidate alloys inthe expected service environment. If only CPP test results are available,the more conservative Ept value should be used. Ept is less conservative(more positive) than E, and is not an acceptable basis for predicting long-term pitting performance.

PROJECTRP8002-15Project Manager: Barry C. SyrettOffice of Exploratory and Applied ResearchContractor: Cortest Columbus Technologies

For further information on EPRI research programs, callEPRI Technical Information Specialists (415) 855-2411.

Using Pitting and ProtectionPotentials to Predict PittingBehavior: New InsightsEAR-7489Research Project 8002-15

Final Report, October 1991

Prepared byCORTEST COLUMBUS TECHNOLOGIES2704 Sawbury BoulevardColumbus, Ohio 43235

Prepared for.Electric Power Research Institute3412 Hiliview AvenuePalo Alto, California 94304

EPRI Project ManagerB. C. Syrett

DISCLAIMER OF WARRAN11ES ANDLIMITATION OF LIABILITIESTHIS REPORT WAS PREPARED BY THE ORGANIZAMON(S) NAMED BELOW AS ANACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWERRESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANYCOSPONSOR, THE ORGANIZAION(S) NAMED BELOW, NOR ANY PERSON ACTINGON BEHALF OF ANY OF THEM:(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS ORIMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS,METHOD, PROCESS. OR SIMILAR ITEM DISCLOSED IN THIS REPORT, INCLUDINGMERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR 01) THAT SUCHUSE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS.INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR 011) THAT THIS REPORTIS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITYWHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANYEPRI REPRESENWIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES)RESULTNG FROM YOUR SELECTION OR USE OF THIS REPORT OR ANY INFORMATION,APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS REPORT.

ORGANIZATION(S) THAT PREPARED THIS REPORT:CONTEST COWMBUS TECHNOLOGIES

Printed on Recycled Paper Office of Exploratory and Applied Research

Electric Power Research Institute and EPRI are registered service marks dt Electric Power Research InstittAe, Inc.

Copyright © 1991 Electric Power Research Institut% Inc All rights reserved.

ORDERING INFORMATION

Requests for copies of this report should be directed to Research Reports Center(ARC), Box 50490, Palo Alto, CA 94303, (415) 965-4081. There is no charge for reportsrequested by EPRI member utilities and affiliates, U.S. utility associations, U.S. governmentagencies (federal, state, and local), media, and foreign organizations with which EPRI hasan information exchange agreement. On request, RRC will send a catalog of EPRI reports.

ABSTRACT

A literature review was performed to identify test methods that have been used to examine pitting susceptibility ofalloys in chloride (CI- containing environments. Several techniques were identified and a critical analysis of thevalidity of the different electrochemical methods was performed with special attention given to the causes of vari-ations observed in the pitting, or breakdown, potential (Epit) and the protection, or repassivation, potential(Eprot). Experiments were performed on Type 317L stainless steel in three solutions containing high levels of CLand Alloy G3 in two solutions containing high levels of Ca. The test solutions were designed to simulate environ-ments present in flue gas desulfurization systems. Cyclic potentiodynamic polarization (CPP) experiments and con-stant potential-time experiments (up to 60 days) were performed to examine pit initiation and repassivation; andmodified ASTM F-746 tests were performed to establish the protection potential as a function of prior pitting history.

Analysis of the data suggests that there exists a unique pitting potential (Eu) defined by the stochastic models forpit initiation that equals the most active value of Et (longincubation times) and that equals the most noble Eprotvalue (measured following only minimal pit growth).

Ill

CONTENTS

Section Page

Executive Summary .......................................... ES-1

1 Introduction ................................... 1-1

2 Review of Methods to Monitor Pit Initiation ................................... 2-1Pit Initiation .......................................... 2-1Electrochemical Methods .......................................... 2-2

Potentiodynamic Polarization ................. ......................... 2-2Stepping Potentiostatic Polarization ........................................... 2-2Constant Potential-Time .......................................... 2-2Scratch Technique ................................................................. 2-3Pit Propagation Rate (PPR) Curves ............................ 2-3Scanning Vibrating Electrode ............................ 2-4Electrochemical Impedance Spectroscopy (EIS) ............. ............... 2-4Electrochemical Noise ............................ 2-4Artificial Pits ............................ 2-5Galvanostaircase Polarization ............................ 2-5Microscopic Examination .......................................................... 2-5Ellipsometry ............................ 2-6

Validity of Electrochemical Methods ............................ 2-6

3 Cyclic Potentiodynamic Polarization Experiments .3-1Experimental Approach. 3-1Results for Type 317L Stainless Steel. 3-3

Solution El .......... 3-3Solution E2 ...... 3-5Solution. E3 .3-9

V

Section Page

Results for Alloy G3 .......................................... 3-11Solution E4 ........................................... 3-11Solution E5 ........................................... 3-13

4 Constant Potential-Time Experiments .................... ............... 4-1Experimental Approach .......................................... 4-1Results for lype 317L Stainless Steel in Solution E3 ...................................... 4-1Results for Alloy G3 in Solution E4 ..................... ..................... 4-2

5 Modified ASTM F-746 Tests ................................... 5-1Experimental Approach .......................................... 5-1Results for Type 317L Stainless Steel in Solution E3 ...................................... 5-1Results for Alloy G3 in Solution E4 ..................... ..................... 5-5

6 DIscussion ................................... -

7 Conclusions ................................... 7-1

References ........................................ R-1

VI

LIST OF FIGURES

Figure Page

1-1 Schematic of Typical Anodic Cyclic Potentiodynamic Polarization Curves ....................... 1-23-1 Schematic Diagram of the Three-Compartment Electrochemical Cell

Used for the CPP Experiments . ............................................................ 3-23-2 CPP Curve for Type 317L Stainless Steel in Solution El With a PTFE Holder/Specimen Interface

Performed at a Scan Rate of 0.6V/h ........................................................ 3-33-3 CPP Curve for Type 317L Stainless Steel in Solution El With a PTFE Holder/Specimen Interface

Performed at a Scan Rate of 0.06V/h ........................................................ 3-43-4 CPP Curves for Type 317L Stainless Steel in Solution El With a Vapor/Liquid Interface

Performed at a Scan Rate of 0.6V/h . ........................................................ 3-43-5 CPP Curves for Type 317L Stainless Steel in Solution El With a Vapor/Liquid Interface

Performed at a Scan Rate of 0.06V/h ..................... ................................... 3-53-6 CPP Curve for Type 317L Stainless Steel in Solution E2 With a PTFE Holder/Specimen Interface

Performed at a Scan Rate of 0.6V/h .3-73-7 CPP Curve for Type 317L Stainless Steel in Solution E2 With a PTFE Holder/Specimen Interface

Performed at a Scan Rate of 0.06V/h .3-73-8 CPP Curves for Type 317L Stainless Steel in Solution E2 With a Vapor/liquid Interface

Performed at a Scan Rate of 0.6V/h .3-83-9 CPP Curves for Type 317L Stainless Steel in Solution E2 With a Vapor/Liquid Interface

Performed at a Scan Rate of 0.06V/h .3-83-10 CPP Curve for Type 317L Stainless Steel in Solution E3 With a PTFE Holder/Specimen Interface

Performed at a Scan Rate of 0.6V/h . 3-9

3-11 CPP Curve for Type 317L Stainless Steel in Solution E3 With a PTFE Holder/Specimen InterfacePerformed at a Scan Rate of 0.06V/h . 3-10

3-12 Curves for Type 317L Stainless Steel in Solution E3 With a Vapor/Liquid Interface Performedat a Scan Rate of 0.6V/h . 3-10

3-13 CPP Curves for Type 317L Stainless Steel in Solution E3 With a Vapor/Liquid Interface Performedat a Scan Rate of 0.06V/h . 3-11

3-14 CPP Curves for Alloy G3 in Solution E4 With a PTFE Holder/Specimen Interface Performedat a Scan Rate of 0.6V/h ............... 3.............. ......... 3-12

vfl

Figure Page

3.15 CPP Curves for Alloy G3 in Solution E4 With a PTFE Holder/Specimen InterfacePerformed at a Scan Rate of 0.06V/h ........................................................ 3-13

3-16 CPP Curves for Alloy G3 In Solution E4 With a Vapor/Liquid InterfacePerformed at a Scan Rate of 6V/h ................... ...................................... 3-14

3-17 CPP Curves for Alloy G3 In Solution E4 With a Vapor/Liquid InterfacePerformed at a Scan Rate of O.06V/h ................. ....................................... 3-14

3-18 CPP Curves for Alloy G3 in Solution E5 With A PTFE Holder/Specimen InterfacePerformed at a Scan Rate of 0.6V/h ................... ...................................... 3-15

3-19 CPP Curves for Alloy G3 in Solution E5 With a PTFE Holder/Specimen InterfacePerformed at a Scan Rateof 0.06V/h ........................................................ 3-15

3-20 CPP Curves for Alloy G3 in Solution E5 With a Vapor/Liquid InterfacePerformed at a Scan Rate of O.6V/h ................... ...................................... 3-16

3-21 CPP Curves for Alloy G3 in Solution E5 With a Vapor/Liquid InterfacePerformed at a Scan Rate of 0.06V/h ................. ....................................... 3-16

5-1 Current-lime Transients for Type 317L in Solution E3 Following Pit Stimulationat 0.6V (SCE) for 2 Seconds (0.03 mmn) and Polarized to a Potential Near E5rot ..................... i-2

5-2 Current-Time Transients for Type 317L in Solution E3 Following Pit Stimulationat O.6V (SCE) for 15 Seconds (0.25 min) and Polarized to a Potential Near Ep.t ................... 5.2

5-3 Current-Time Transients for Type 317L in Solution E3 Following Pit Stimulation at 0.6V (SCE)for 2 Minutes and Polarized to a Potential Near Eprot(Two Curves Shown for Duplicate Tests at 200mV and 150 mV) ................................ 5-3

5-4 Current-TIme Transients for lyrpe 317L in Solution E3 Following Pit Stimulation at 0.6V (SCE)for 7 Minutes and Polarized to a Potential Near Epmt(Two Curves Shown for Duplicate Tests at 100 mV) ................... ................ 53

5-5 Current-Time Transients for Type 317L in Solution E3 Following Pit Stimulation at 0.6V (SCE)for 25 Minutes and Polarized to a Potential Near Eprot ...................................... 54

5-6 Current-Time Transients for Type 317L in Solution E3 Following Pit Stimulation at 0.6V (SCE)for 40 Minutes and Polarized to a Potential Near Eprot ..................................... 5 5-4

5-7 Current-Time Transients for Type 317L in Solution E3 Following Pit Stimulation at 0.6V (SCE)for 100 Minutes and Polarized to a Potential Near Eprt ......................................... 5-5

5-8 Current-Tlime Transients for Alloy G3 in Solution E4 Following Pit Stimulation at 0.6V (SCE)for 15 Seconds (0.25 min) and Polarized to a Potential Near Eprot ............................... 5-6

5-9 Current-Time Transients for Alloy G3 in Solution E4 Following Pit Stimulation at 0.6V (SCE)for 2 Minutes and Polarized to a Potential Near Eprot .......................................... 5-6

5-10 Current-Time Transients for Alloy G3 in Solution E4 Following Pit Stimulation at 0.6V (SCE)for 40 Minutes and Polarized to a Potential Near Eprot............................... 57

6-1 Potential Versus Time Data for Alloy 317L in Solution E3 for EpIt and Eprot(Epit Versus Time to Pit Initiation and Eprot Versus Time Permitted to Pit) ........................ 6-3

6-2 Potential Versus Tlime Data for Alloy G3 in Solution E4 for Epit and Eprot(Epit Versus Time to Pit Initiation and Eprot Versus Time Permitted to Pit) ........................ 6-3

vill

LIST OF TABLES

Table Page

3-1 Compositions for Solutions Used in the CPP Experiments ..................................... 3-23-2 Values for Er, Epi, and Eprot for Type 317L Stainless Steel Measured by CPP Experiments ..... .... 3-63-3 Values for Ec2,, Eit, and Eprot for Alloy G3 Measured by CPP Experiments ....................... 3-124-1 Data for Constant Potential-Time Experiments for Type 317L Stainless Steel in Solution E3 ..... .... 4-24-2 Data for Constant Potential-Time Experiments for Alloy G3 in Solution E4 ....................... 4-35-1 Values of Ep,,t as a Function of Time of Pitting From the Modified ASTM F-746 Tests ...... ........ 5-56-1 CPP Test Data Including (i) Relationship Between Epit and Time to Scan From Eprt to Epit, and

(ii) Relationship Between Eplt and Time From Pit Initiation to Repassivation ...... .............. 6-2

lx

EXECUTIVE SUMMARY

A literature review was performed to identify test meth-ods that have been used to examine pitting susceptibili-ty of alloys in chloride (Cl) containing environments.Several techniques were identified and a critical analysisof the validity of the different electrochemical methodswas performed with special attention given to the caus-es of variations observed in the pitting, or breakdown,potential (Ed) and the protection, or repassivation, po-tential (Ept). Experiments were performed on Type3l7L stainless steel in three solutions containing high lev-els of C- and Alloy G3 in two solutions containing highlevels of a-. The test solutions were designed to simulateenvironments present in flue gas desulfurization systems.Cyclic potentiodynan-c polarization (CPP) experiments,constant potential-time experiments (up to 60 days), andmodified ASTM F-746 Standard tests were performed toexamine pit initiation and repassivation.

Significant scatter was observed in Epit values but muchless scatter was observed in Eprot values. The extent ofthe scatter was controlled, in part, by the presence or ab-sence of preferred pitting sites which, in turn, was influ-enced by the specimen mounting method: in general,when the junction between the specimen and the PTFEholder was fully immersed in the test solution, the pro-pensity to pitting was higher than when this junctionwas above the liquid level. Pits (but never crevice corro-sion) often initiated preferentially at the PTFE holder/specimen junction. In contrast, pits never initiated pref-erentially at the vapor/liquid specimen interface, i e.when the PTFE holder/specimen junction was heldabove the liquid level. The greatest scatter was observed

for Alloy G3 in a pH 6, high-chloride solution. For testsin which there was a vapor/liquid interface at the spec-imen surface, pitting occurred in only one of three tests;whereas for two tests in which the PTFE holder/speci-men junction was immersed, both specimens pitted inthis pH 6 solution. This and other results suggested thatthe interface between the specimen and the PTFE holderspecimen was a preferred pit initiation site.

CPP test results for lype 317L stainless steel in a pH 1 so-lution containing 1% chloride and for Alloy G3 in a pH6 solution containing 10% chloride were compared withdata generated in constant potential-time tests and mod-ified ASTM F-746 tests. For both of these alloy-solutioncombinations, the value of ENit decreased with increas-ing exposure period in the constant potential-time tests,the most active values corresponding well with the mostactive values obtained in the CPP tests. The modifiedASTM F-746 tests confirmed that Eot is a function ofprior pitting history; the longer the period of pit growthprior to repassivation, the more active is the Epft value.

Analysis of the data suggests that there is a unique pit-ting potential (E,) defined by the stochastic models ofpit initiation as the potential at which the pit nucleationfrequency exceeds zero. E, is equal to the most activevalue of E]Pt recorded in CPP tests or constant potential-time tests (associated with slow scan rates or long expo-sure periods) and is equal to the most noble Frot valuesrecorded when pit growth is minimal.

ES-1

1

INTRODUCTION

Over the past 20 years, electrochemical tests have beenused increasingly to examine the pitting tendencies ofalloys. In particular, electrochemical testing has beenused extensively to rank alloys with respect to their pit-ting resistance in a given environment. In addition, elec-trocheniical tests have been used to establish the effectson pitting due to changes in the environment of a givenalloy. One of the more popular electrochemical testmethods is the cyclic potentiodynamic polarization(CPP) test described in ASTM Standard Practice G61.Schematics of anodic CPP curves illustrating the threeprimary types of behavior are shown in Figure 1-1. Thebehavior of interest in this study, passivation and pit-ting, is given by the solid curve. In the CPP test, twopitting parameters are determined: (1) the pitting, orbreakdown, potential (Epit), and (2) the protection, or re-passivation, potential (Eprot). It is generally believedthat pitting will be initiated very rapidly if the potentialof the alloy exceeds Epit, and that even preexisting pitswill repassivate and stop growing if the potential be-comes more negative than Ept.

What happens between E it and Ep,,t is more controver-sial. It is often claimed tt, at potentials between EpRand Eprot existing pits may grow, but new pits cannotinitiate. However, it is difficult to reconcile this sugges-tion with the observation that Epit decreases as the po-tential scan rate decreases in a CPP test pits initiateduring a slow scan rate test at potentials below the highscan rate Epit value. Also, it has been claimed that Eptis the more unique value. However, it has been shownthat the degree of pitting prior to reversal of the CPPscan can affect the value of E t measured. Amidst thiscontroversy, others claim tEat electrochemical testssuch as the CPP test are of little use in determining the

likelihood of pitting of an alloy in a given environment.Although the relationship between Epit and Eprot hasnot been established, it is generally believed that themore positive the values of Epit and F,.t, the less likelyit is that pits will initiate and propagate. Unfortunately,the ranking of alloys often depends on whether Epjt orEprt is selected as the critical parameter. Thus, the valid-ity of the cyclic potentiodynamic polarization test, as ameans of predicting pitting susceptibility, has beenquestioned.

The primary objective of this work was to determinethe time dependency of pit initiation and the prior pit-ting history dependency on E wt in an attempt to es-tablish a relationship between Kpit and Epot. A secondobjective was to establish the validity of the cyclic poten-tiodynamic polarization test as a means of predictingpitting susceptibility.

The work to accomplish the above objectives was di-vided into four tasks: Task 1-literature Review, Task2-Cyclic Potentiodynamic Polarization (CPP) Tests,Task 3-Potentiostatic Tests, and Task 4-ModifiedASTM F-746 Tests. In Task 1, the existing technical liter-ature was reviewed to establish the present consensuson the validity of electrochemical tests for determiningpitting susceptibility, and to identify useful experimen-tal techniques for this study. In Task 2, CPP tests wereperformed to establish preliminary relationships be-tween Epi, Epct, the corrosion potential (Ecr), and time.In Task 3, potentiostatic tests were performed to de-fine more precisely the time dependence of pit initia-tion in the range between Fit and Ert.-Fially, in Task4, modified ASTM F-746 tests were performed to estab-lish the effect of prior pitting severity on Eprot.

1-1

Introduction

I I H

6

a~ ~ ~ ~~~1a

Eaa

ittpas En

cor

La

Eor = corrosion potentialE=A . potential at which pits initiate on forward scanEprx = potential at which pits repassivate on reverse scanEMaX = potential at active peak

g Current Density

1cxr = current density at the free-corrosion potentialIrna = current density at active peak[PM = current density in passive range

Figure 1-1. Schematic of Typical Anodic Cyclic Potentiodynamic Polarization Curves

1-2

2REVIEW OF METHODS TO MONITOR PIT INITIATION

Task 1-Literature Review was performed to review theelectrochemical methods that have been utilized to ex-amine pitting susceptibility, to establish the validity ofthese electrochemical test methods, and to identify ex-perimental techniques for accomplishing the objectivesof this program. A literature search was performed uti-lizing the DIALOG computerized search to identifytechnical articles concerned with pit initiation. The re-view included Metals and Ceramics Information Center(MCIC), Metadex (Metals Abstracts), 1966-1988, andChemical Abstracts, 1967-1988. Several papers were re-viewed in which a variety. of techniques was used tostudy pit initiation.

Pit InitiationThe primary goal of this research program was to ex-amine the process of pit initiation and relate it to theparameters of Epit and Eprot as measured by CPP meth-ods. Therefore, a review of the pit initiation process ispresented in this section.

Pitting of stainless and nickel-base alloys is a localizedform of corrosion that occurs on an otherwise passive,noncorroding surface. Anyone who has studied pittingin the laboratory or examined failures under operatingconditions will generally agree that the sites at whichpits initiate are randomly oriented on the surface andhighly unpredictable. It is true that pitting quite often isassociated with specific locations on a given structure,e.g., welds, weld spatter, second phase particles, crevicesites, etc; however, within these preferred locations, pitinitiation is random and there are usually areas withinthe preferred locations without pits. Because of this ran-dom nature of pit initiation, stochastic models of pitting

have been developed that attempt to account for the sta-tistical nature of pit initiation.

Williams, Westcott, and Fleischmann reviewed othermodeling efforts and presented their own stochasticmodels of pitting corrosion of stainless steels in two pa-pers.1, 2 Williams et al.1 points out that various authorshave emphasized various phenomena for pit nucleationincluding: inhomogeneity in the metal, failure and slowhealing of the passive film, development of critical acid-ity levels in microscopic flaws, defects and their trans-port in passive films, and chloride absorption orincorporation into localized areas of passive films. Animportant feature of most of the various models and ex-planations of pitting is the induction time before pit ini-tiation occurs for a given set of parameters.

The statistical approach presented by Williams et al.views pit nucleation as a rare event that results in cur-rent and potential fluctuations randomly distributed intime and space over the metal surface. The model hasthe following features: (1) nucleation events have a givenfrequency, (2) these events have a probability of dying,(3) nucleation events that survive beyond a critical agedo not die, and (4) each event has an induction time dur-ing which the local current does not increase but duringwhich the event may die. Thereby, pits are unstablewhen they are first nucleated and become stable only af-ter the pit has survived past the critical age.

The particular model proposed by Williams et al.2 is thatthe pit nucleation phenomenon is a local acidificationcaused by local potential and current fluctuations relat-ed, in part, to the surface roughness of the specimen andboundary layer in the liquid at the metal surface. Of

2-1

Review of Methods to Monitor Pit Initiation

most interest to the present study are predictions of thepotential dependency of pit initiation. The followingconclusions were drawn by Williams et al. First, the nu-cleation frequency of unstable pits is a function of elec-trode potential and varies from zero to some limitingvalue. Thereby, the potential at which the nucleation fre-quency goes to zero defines a unique critical pitting po-tential. Second, the death probability of unstable pits isnot dependent on electrode potential. Third, the criticalage for transition of an unstable to a stable pit and slopeof the current-time transients are electrokinetically relat-ed properties and are strongly dependent on electrodepotential.

Electrochemical MethodsIn any model for pit initiation, the electrode potential isa critical parameter. Furthermore, measurements of cur-rent fluctuations due to unstable pit nucleation eventsand measurements of the significant current increasesthat follow the initiation of stable pitting are quite easilymade utilizing electrochemical methods. Therefore,most of the methods for evaluating pitting corrosion in-volve electrochemical measurements of electrode poten-tial and current, either while controlling the electrodepotential (potentiostatically, potentiodynamically, or acombination of the two), or electrode current (galvano-statically or galvanodynamically), or a combination ofthe two. The following paragraphs review the varioustechniques that have been used to evaluate pittingsusceptibility.

Potentlodynamlc PolarizationThe CPP test is a method for determining Epit and ErDt0by scanning, at a constant rate, the electrode potential ofa specimen while monitoring the corresponding appliedcurrent. Figure 1-1 indicates the type of data expected.This is a widely used technique for examining pittingsusceptibility. The value of Epit denotes the potential atwhich pits initiate on the forward scan and the currentincreases abruptly. The value of Eprot denotes the poten-tial at which pits are repassivated during the reversescan, at which point the current decreases back to pas-sive values. The references cited here include studiesthat examine factors affecting the Edit and lPot valuesor compare data from CPP tests with other electro-chemical methods of determining pitting susceptibility.It should be noted that in several studies, only forwardscans were performed during the potentiodynamicpolarization test. In these tests, only EPit is available. In

other studies, forward and reverse scans (CPP tests)were performed and both Epit and Eprt were deter-mined. 11-8 As will be evident in the final discussion sec-tion, it is believed that measuring both Epit and Eprot iscritical, especially when making an engineering decisionon the susceptibility of a material to pitting corrosion.

One of the arguments against the use of CPP tests formeasuring Epit and Epmt is that neither is a unique valueand both are dependent on experimental parameters. Ithas been shown that the pitting potential becomes morenegative with decreasing scan rate.1 2, 18 This is easily ex-plained by an incubation process for pitting, in which aslower scan rate permits pit initiation to occur at a morenegative potential during the forward CPP scan. Thevalue for E t has been shown to be dependent on thepresence or'revice attack'2 and on the amount of pit-ting permitted prior to reversing the potential scan di-rection.17 18 Nader-Roux et al. 2 showed that, whencrevice corrosion was present, Epot was as much as200mV more negative than when the test specimen wasfree of crevices. Wilde and Williams18 showed that themore pitting permitted prior to reversal of the potentialscan, the more negative Eprot

Stepping Potentiostatic PolarizationStepping potentiostatic polarization is a method of pro-ducing a polarization curve using discrete potentialsteps, usually 25-50mV each, as opposed to a constantpotential scan as used in a CPP test. Because a fixed timeperiod is maintained between each step, an average rateof potential increase in mV/sec can be calculated. Forcomparable average scan rates, Zucchi et al.3 found thatsimilar Epit values were obtained by CPP and steppingpotentiostatic polarization tests.

Williams and WestcottO9 combined a potentiodynamicscanning method with potentiostatic holding of the po-tential for a specified time while monitoring the currentto determine Epit. This method was further used to helpdefine nucleation frequency data required for modelingefforts.3 19

Constant Potentlal-Time

Several studies have used constant potential, potentio.-static polarization, and monitored current as a functionof time to examine the incubation time for pit initia-tion. 2' 4' 9,'3' 16 '19-21 Important variables of this methodtypically include the electrode potential, the length of

2-2


time of the test, passivation treatment prior to testing,and method of monitoring current. In some of thesestudies, current fluctuations with time were examined,i.e., pit nucleation events.I 2AtI9#2O In other studies, thepotential for pit initiation was determined when a netcurrent increase was observed. 13' 16 ,21

Broli and Holtan21 found that, for aluminum in 3%NaCI solution, the time to pit initiation increased as theelectrode potential decreased, but the lowest value ofEpit measured by a constant potential-time method in 17hour tests was approximately equal to Eprot measuredwith the CPP method. This conclusion was differentfrom work performed by these authors for stainless steelin which Epit measured in 83 hour constant potential-time tests remained more positive than the value of Eprotfrom CPP tests. A question remains as to whether longertime periods for the potentiostatic tests for the stainlesssteel would result in Epit equal to Eprt for stainlesssteels.

Scratch TechniqueIn the scratch techniques35 9 a specimen is potentiostati-cally controlled while monitoring the current. The spec-imen is scratched with some hard inert material (siliconcarbide or glass) to break the passive film, and the cur-rent transient is recorded. If the current decreases andthe scratched area repassivates, the control potential ismore negative than a "pitting potential." If the currentcontinues to increase following the initial currentspike, the control potential is more positive than a"pitting potential." By performing the scratch test at aseries of potentials, 10 to 5OmV apart, the value of aapitting potential" can be estimated. In a study byZucchi et al.3 for Type 304 stainless steel in a O.1MNaCI solution, the values of the pitting potential deter-mined by stepping potentiostatic polarization (50mVsteps every hour) and for the scratch technique weresimilar. However, with inhibitor added, the Epit from thescratch technique showed no change while Epit from thestepping potentiostatic polarization method becamemuch more positive. It is the opinion of the author of thisreport that the scratch technique mechanically removesthe inhibiting film and, therefore, the data from thescratch technique do not represent the effects of the in-hibitor. These data appear to indicate a severe limitationof the scratch technique in evaluating inhibitors unless awear process is a part of the intended application.

In a study by Manning et al.,5 CPP tests (scan rate of1OOmV/hr), constant potential-time exposures (170 hoursat each potential), and the scratch technique gave similarvalues for Epit for a duplex stainless steel in a pH 4 1NNaC solution. For a single-phase alloy, the constant po-tential-time method indicated an Epit value that was ap-proximately 160 mV more negative than for the CPP testor the scratch technique. The difference in the findingsfor the two alloy systems could likely be due to the nu-cleation event frequency being much greater on the du-plex stainless steel as a result of the inhomogeneitiescaused by the microstructure. Therefore, the single-phase alloy had a longer incubation time than the du-plex alloy at any given potential and this showed up inthe potentiostatic tests as a more negative value of Epitthan in the CPP or scratch test. It is clear from these datathat of the three techniques evaluated, the long-termconstant potential-time experiments proved the best val-ue of the pitting potential.

Although the above discussion indicates reasonablygood agreement between the scratch technique andother methods, care should be taken when applyingthe technique to systems where the mechanical damageaffects the pitting process, as was shown for inhibitortesting. The problem with the scratch technique is thatthe surface is mechanically deformed to expose baremetal as opposed to a natural film breakdown by an ag-gressive environment. Therefore, relating the passiva-tion/repassivation characteristics of a scratch to anactive pit does not appear to be, in general, an analo-gous situation.

In defense of the scratch technique, the scratching pro-cess produces a relatively high current spike. If the pitnucleation process is one of local acidification producedby current fluctuations? the pit nucleation frequencycould be greatly increased during the scratching process,thereby promoting pit initiation if the electrode potentialis otherwise favorable. However, there remains an incu-bation time factor that is greatly dependent on the exper-imental procedure.

Pit Propagation Rate (PPR) Curves

In the PPR test,17 '22 the following procedure is used: (1)the specimen is scanned (36V/h) to a potential betweenEprot'and Epit and held at this potential for 10 minutesto establish the baseline value of ip. before pitting hasinitiated, (2) the potential is scanned (36V/h) beyond

2-3


Epit until the nominal current density is equal to 100A/m2, (3) the potential is decreased in a single step toa selected potential below Epit and the current (pit) ismonitored for 10 minutes. By subtracting ipas from ipitrand dividing by the surface area of the pits, average pitgrowth rates can be calculated as a function of potential.Epxot is defined as the potential at which the pit propaga-tion rate is zero. Syreft showed that E t from CPP testswere more negative than Eprt from PPR tests for stain-less steels in chloride environments.17 It is speculatedthat the more extensive pitting that occurred prior to re-passivation in the CPP tests led to the development of alower-pH, more aggressive environment within the pits.The value of Eprot measured by the PPR method may beless conservative than Eprt measured by the CPP test,but may be a better indication of long-term pit initiationbehavior under normal service conditions. The PPR testbecame the basis of an ASTM standard procedure for de-termining pitting and crevice corrosion resistance ofstainless steels (ASTM Standard F-746). The advantageof the PPR method for determining Epto is that it pro-vides for a minimum, reproducible amount of pittingprior to evaluating the repassivation (or pit propagat-ing) characteristic of a material.

A very similar technique, referred to as the stationarypotentiokinetic method 1 and also as the quasistationaryanodic polarization method,16 utilizes a stepping proce-dure similar to the PPR test. However, procedures are in-cluded to determine both Ep,,t (similar to PPR test) andEit- Once Epro is determined, Epit is determined by thefollowing procedure: (1) the specimen is passivated for10 minutes, (2) the potential is alternated every 15 min-utes between (i) a potential 0.025V positive to Eprot and(ii) a potential that is increased in 0.025mV steps to morepositive potentials than Eprot. Epit is the potential atwhich the current begins to increase and continues to behigh when returned to the potential in (i) 0.025V positiveto Ept. Though Epit measured this way is always morenoble than Ept, this does not preclude the possibility ofpit initiation at or close to Eprt, if exposure periods wereincreased far beyond the standard 15 minutes.

Scanning Vibrating Electrode

The scanning vibrating electrode method, as describedby Ishikawa and Isaacs,2 utilizes a vibrating electrodeto develop a three-dimensional plot of the current den-sity profile of a pitting surface. The electrode vibratorassembly vibrates a platinum electrode perpendicularto the specimen surface. A mechanical stage scans in the

X-Y direction. The potential gradients generated in thesolution during localized corrosion are measured as thedifferences in the potential measured at the two ex-tremes of vibration of the electrode. The scanning vibrat-ing electrode method provides a surface map for thecurrent distribution on a specimen surface. By doing so,pit initiation and growth can be monitored. This is an ex-cellent technique for mapping the spacial distribution ofpits an."d for examining the growth and repassivationof pits.

The primary drawbacks are the time and cost associatedwith the method. Also, continuous information is notavailable since a finite measurement time is required formapping the surface. However, the size of the specimenmay be decreased to overcome measurement time as asignificant drawback as long as the smaller specimenstill contains a representative number of pit initiationsites. To establish incubation time at a constant potentialwould require continuous scanning, one scan after an-other, until pitting occurred.

Electrochemical Impedance Spectroscopy (EIS)

EIS is a technique that applies a small amplitude(5-20mV peak-to-peak) AC potential signal to a speci-men and simultaneously measures the current re-sponse to establish the impedance as a function offrequency in a range typically between 10,000Hz and0.01 Hz. From the impedance as a function of frequency,electrical analog models of the electrochemical surfacecan be developed that describe the data. Dawson andFerreira15' 24 have shown that EIS can be used to differ-entiate between pitting and passive conditions. Al-though EIS indicated differences between the initiationand propagation stages, EIS is not seen as a viable meansto study the onset of pit initiation and incubation time.The same objections apply to EIS as applied to the scan-ning vibrating electrode method: (1) noncontinuousmeasurement, and (2) intensive analysis and labor re-quirements. The EIS technique is viewed more as a toolto examine the mechanism of pitting rather than ameans to establish Epit and Eprot

Electrochemical Noise

The electrochemical noise technique has been shown tobe quite sensitive in detecting the onset of pit initiation.The concept is simple; prior to the initiation of stable pit-ting, there is a time in which nucleation events are re-corded as current or potential fluctuations. Thereby,

2-4


pitting is usually preceded by an increase, or change, inelectrochemical noise. These data can be recorded in arelatively simple means by a strip chart recorder or dataacquisition system and interpreted by measuring thefrequency of large current fluctuations. 1,2,4 Alternating-ly, more sophisticated equipment, e.g., ultralow noisepotentiostat, a high level of shielding, etc., can be usedin conjunction with spectral analysis for data interpreta-tion.24r25 These techniques are invaluable for moremechanistic studies and model development where esti-mation of nucleation event frequencies are critical. Also,these techniques are important for the development ofmonitoring techniques in which the onset of pitting(high rate of nucleation events) could foretell pit initia-tion and provide an early warning prior to the develop-ment of a significant problem.

Artificial Pits

Artificial pits have been used to study both pit initiationand pit growth. Suzuki and Kitamura used a simulatedpit to examine characteristic pitting potentials3 6 In thisstudy, a simulated pit consisting of an occluded anode(as the pit) coupled to a cathodic surface was used. Thepitting potential discussed was the critical potential forpit growth or propagation (E.). It should be noted thatthe potentials measured are potentials using a Luggincapillary probe inserted into the artificial pit, such thatIR-drop down the pit is eliminated. One of the conclu-sions is that "there exists the critical potential for growthof localized corrosion, below which the already initiatedlocalized corrosion can repassivate. This potential can bemeasured only in an artificial specimen having a localanode insulated from a cathode." Although artificial pitsmay be reasonable for examining the pitting and repas-sivation process from a fundamental point of view, thereare obvious drawbacks when predicting pitting suscep-tibility for an industrial application. Primarily, it is diffi-cult to reproduce the localized environment within a pitand/or to verify whether it has been reproduced whileusing a macro specimen and o~cluded cell. Thereby, thedata would always be suspect.

Galvanostaircase Polarization

Cyclic galvanostaircase polarization (GSCP) is a testmethod designed to determine the relative susceptibilityof an alloy (especially aluminum alloys) to localized cor-rosion (pitting and crevice corrosion). In the gal-vanostaircase method, a current staircase steppingsequence is applied galvanostatically to the working

electrode and the resulting potential transients are re-corded. The current steps are increased at some definedaverage scan rate. Following breakdown, the scan is re-versed. The steady-state potentials at the end of the"up-steps" are extrapolated to zero time, or current, toobtain Epit; and the steady-state potentials at the end ofthe "down-steps" are extrapolated to zero current toobtain Eprot. This current-controlled polarization tech-nique has been shown to predict reproducible protec-tion potentials of aluminum alloys where otherpotential-controlled electrochemical techniques havefailed. Other researchers have used the technique fortype 304L stainless steels with good correlation to con-stant-potential corrosion tests. One key advantage ofthe GSCP technique over potentiostatic techniques(scratch potentiostatic and constant potential tech-niques) is that it requires much shorter test times and iseasier to perform. Both the pitting and protection po-tentials may be obtained by the technique. Another ad-vantage of the technique is that the total charge passedduring the test is precisely controlled and kept to aminimum, thereby allowing corrosion to initiate only atthe most susceptible sites. 3

Microscopic ExaminationMicroscopic examination of the specimen surface maybe a good way to observe pit initiation and the spacialdistribution of pits. However, it is difficult to distinguishpropagating pits from passivated pits. Also, the moni-toring scheme for viewing the surface to establish incu-bation time may be complicated. A video camera may bethe best way to monitor the surface. Beck 32 used a hori-zontally mounted specimen in which a glass windowwas present in the test cell directly across from the 304stainless steel specimen surface to permit microscopicexamination of the pit. Various methods of producing asingle pit were attempted. The most successful was theuse of lacquer applied to the surface and using a sewingneedle tip to create a hole in the lacquer.

As discussed above, the primary problem with this tech-nique is viewing the area in which pitting initiates. Beckeliminated that problem by providing only an extremelysmall area available for pitting. This method may be ap-propriate for pit propagation studies; but, for pit initia-tion, a larger surface is desired to accommodate thestatistical nature of pit initiation. Also, microscopicviewing of a specimen is not likely to be as sensitive aselectrochemical techniques to the early stages of pitinitiation.

2-5


Elilpsom etry

Ellipsometry is a technique used to measure the opticalproperties of a film, from which thickness data can bederived. Sugimoto et al2P present data for an 18-8 stain-less steel in a Nai solution using a microscopic ellip-someter that provides a magnification of 600 times. Thisequipment provided the determination of the thicknessand optical constant of the passive film on an area litimin diameter. By scanning the specimen surface, the spa-tial distribution of film thickness within each grain wasobserved prior to pit initiation. The specimen was po-tentiostatically controlled and the film thickness was re-measured following pit initiation. It was shown that pitsinitiated at thin spots in the passive film.

Ellipsometry belongs in the same category as EIS andscanning vibrating electrode methods, i.e., the tech-nique is labor intensive and does not permit continu-ous measurements.

Validity of Electrochemical MethodsArguments against the use of CPP tests for determiningpitting susceptibility have centered around Epit andEDpot not being unique values. These arguments do notdisqualify the use of CPP tests for determining Epit andEPOt, but merely establish limitations in the interpreta-tion of the CPP data. It is clear that E.,t measured utiliz-ing a slow scan rate is a nearer steady-state value thanEpit measured at a fast scan rate since the slow scan ratepermits a longer incubation time for pit initiation. In re-gard to the stochastic model for pit initiation, a value ofEPit measured by CPP tests is expected to be more posi-tive than a unique value of the critical pitting potentialdefined as the potential at which the frequency of pit nu-cleation events goes to zero. Therefore, E tyCPP tech-niues is a "nonconservative estima

ittin potntial defined by stochastic models.Furthermore, the elatoshi p-between Ep by CPP andthe unique pitting potential have not been established.

The relationship between Eprot and a unique value ofcritical pitting potential is even more unclear than forEpt Suzuki and Kitanura 2 6 define Ept as equal to thesteady-state potential within the pit with no anodic po-larization applied. Thereby, on the reverse potentiody-namic scan, as the potential becomes more negative, thepotential difference between the polarized outside sur-face and the potential within the pit decreases. Ep,,t is

the potential at which the potential on the outside sur-face is the same as the potential within the pit. As the po-tential of the outside surface is decreased further, thepotential within the pit is more positive and a cathodiccurrent will begin to flow into the pit, tending to increasepH and stop pitting.

It is expected that Epot provides a conservative value forestablishing pitting susceptibility since it is expectedthat a unique critical pitting potential will be more posi-tive than Eprote The more severe the pitting prior to mea-suring Ept, the more negative the value of Epxt wouldbe.'8 It is not unreasonable to speculate that the abovedefinition of Eplt would apply to the unique critical pit-ting potential for which the nucleation frequency goesto zero.

One possible means of removing the variation in EPt isto permit significant pitting to occur prior to reversingthe potential scan in a CPP test. A more severe pitting en-vironment within the pit will have been permitted toform and the pH will likely correspond to a limiting val-ue for the hydrolysis reactions according to the chemicalcomposition of the alloy being tested. In this manner, amore consistent Ept will be measured and one that alsois likely to be the most negative value possible. Thereby,a conservative limit for pit repassivation can be obtained,which is desirable from an engineering viewpoint.

From an opposite viewpoint, the PPR techniques mini-mizes the amount of pitting prior to potentiostatic ex-posure, and it is claimed that a more consistent measure-ment is made. It is quite possible that, by approachingeither limit (gross pitting prior to measuring Eprot [CPPtest) or minimum pitting prior to measuring Eprot IPPRtest)), consistent results can be obtained.

The above considerations do not limit the applicabilityof electrochemical methods nor do they indicate thatelectrochemical methods are not valid for establishingED1t and Eprot. Once the causes of variations in Epit andEpr1t produced by experimental procedures are realized,proper interpretation of these pitting parameters can bemade.

Another criticism of CPP tests is that a great deal of sig-nificance is placed in a single test where the processbeing examined is admittedly of a statistical nature.This is a valid argument for Epit and is probably why a

2-6


significant variation in the reproducibility of Epit data isobserved. Fratesilo uses a statistical treatment of Epitwhen examining type 316L stainless steel in 3.5% NaC3solution. Eprot, on the other hand, is not viewed as a sta-tistical-related parameter. Once pits have been initiated,repassivation of the pit is not necessarily related to a sta-tistical model. However, because a range of pit sizes anddensities can occur due to the statistical nature of the pitinitiation process, Epot would be expected to have some

degree of scatter that always lends itself to statistical ap-proaches for presenting data. Therefore, a single CPP ex-periment is viewed as a valid test for establishing Et,However, it is the author's opinion that any critical ex-periment in making engineering decisions should be, ata minimum, duplicated or some other statistical ap-proach used (e.g., statistical experimental design andanalysis).O4

2-7

3CYCLIC POTENTIODYNAMIC POLARIZATIONEXPERIMENTS

The purpose of these experiments was to establish EXtand E t for type 317L stainless steel and Alloy G3 inhigh-cioride (Cli environments as a function of scanrate. CPP experiments were performed in three solu-tions for Type 317L and two solutions for Alloy G3. Thesolutions were selected to simulate environmentspresent in flue gas desulfurization (FGD) systems. Sincethe accuracy of simulation was not a prime objective ofthis study, some modifications were made to the FGDenvironments to simplify their preparation. For in-stance, dissolved oxygen was purged from the solutionsrather than added in controlled amounts. Table 3-1 pre-sents the compositions of the solutions used in thisstudy.

Experimental ApproachThe test procedures followed those outlined in ASTMG61-78 "Conducting Cyclic Potentiodynamic Polariza-tion Measurements for Localized Corrosion" with thefollowing exceptions: (1) the test specimen remained inthe solution at the test temperature overnight (-20hours) prior to performing the CPP test; (2) the scan wasbegun at a potential 50-lOOmV more negative than thefree-corrosion potential; and (3) two scan rates wereused: 0.6V/h (the ASTM standard) and 0.06V/h. Thepurpose of the slow scan was to permit examination ofthe effect of scan rate on Epit. The CPP tests were per-formed using a Santron computer-controlled poten-tiostat. A platinum wire counter electrode was used and

the potential was controlled with respect to a saturatedcalomel reference electrode (SCE). Tests were performedin a three-compartment electrochemical cell with theworking electrode compartment heated by circulatingwater through an outside water jacket (see Figure 3-1).

The working electrode was a cylindrical specimendrilled and tapped to permit mounting on to a specimenholder. A significant effort was made to remove thePTFE holder/specimen interface as an initiation site forpitting. The most successful procedure consisted of thefollowing: (1) the specimen was mounted as tightly aspossible; (2) the specimen and holder assembly weremounted in a lathe and machined to make a verysmooth transition between the PTFE holder and thespecimen; and (3) the specimen surface, including thePTFE holder/specimen interface, was ground to a 600-grit finish. Smearing the relatively soft PTFE over thespecimen surface was avoided by polishing only fromthe metal to the PTFE with fresh polishing paper. Sometests were performed using specimens with this PTFEholder/specimen interface; in other tests, this interfacewas eliminated by using a long specimen that extendedout of the solution, which produced a vapor/liquidinterface on the specimen surface instead. For the alloy-solution combinations tested, enhanced pitting did notoccur at the vapor/liquid interface, while the PTFEholder/specimen interface provided preferred pit initia-tion sites in several cases.

3-1

Cyclic Potentiodynamic Polarization Experiments

Table 3-1Compositions for Solutions Used In the CPP Experiments

Solution Test SolutionVariablesElement* El E2 E3 E4 ES

Al (A 2(S04)3). mg/L 1Mg (MgSO4 ), mg/L 5,000Cr (CrC13), mg/L 1Cu (CuCI2), mg/L 1Fe (FeCI3), mg/L IMo (MoO3), mg/L 200N (NaNO3), mg/L 200Cl (NaCI), mg/L 10,000Br (NaBr), mg/L 0

02,** 0Temperature, C 50pH (adjusted with H2 S04) 1.0

Species added as compound given in parenfteses." Volume percent in cover gas.

2705,000

50.2

1200

1

100,00000

506.0

10,000

50000000

10,00000

501.0

45,000

00001

100,0001

850

6.0

20,0005,000

00001

100,000500

0901.0

L*" W Polah Ied

Om.G_

.out

woMMCOIARTWT

Figure 3-1. Schematic Diagram of the Three-Compartment Electrochemical CellUsed for the CPP Experiments

3-2


Results for Type 317L Stainless SteelType 317L stainless steel was tested in three solutions:El, E2, and E3. For each solution, CPP curves were per-formed for four conditions: specimens with a PTFEholder/specimen interface at scan rates of O.6V/h and0.06V/h, and specimens with a vapor/liquid interface atscan rates of O.6V/h and 0.06V/h.

Solution ElFigures 3-2 to 3-5 show the CPP curves for 317L in Solu-tion El for the four conditions described above. Figure3-2 shows the CPP curve for 317L in Solution El with aPTFE holder/specimen interface and performed at ascan rate of O.6V/h. Pitting occurred on the bulk speci-men surface and no enhanced pitting occurred at thePTFE holder/specimen interface. Significant spikeswere observed starting at O.7, SCE on the forward scan,but stable pitting did not initiate until the reverse scanreached about O.7V (SCE) as evidenced by the sharp in-crease in current The hysteresis observed was quitelarge with Eprot approximately equal to Ecor Table 3-2

presents Eco, EXit, Eprotp and a location of attack for eachType 317L specimen tested in Solutions El, E2, and E3.

Figure 3-3 shows the CPP curve for 317L in Solution Elwith a PTFE holder/specimen interface and performedat a scan rate of 0.06V/h. Pitting was significantly en-hanced at the PTFE holder/specimen interface whencompared to the remainder of the surface. Comparingthe CPP curve in Figure 3-3 to that in Figure 3-2 indicatesthat Epit for the slower scan rate (0.06V/h) test was sig-nificantly more negative than for the faster scan rate(O.6V/h) test Because of the presence of enhanced inter-facial pitting in the slow scan rate test, it is impossible tosay whether the more negative Epit is due to the lowerscan rate or to the preferred pitting sites created by thePTFE holder/specimen interface, or both. Both are ex-pected to make Epit more negative.

Figure 3-4 shows the CPP curves for duplicate 317Lspecimens in Solution El with a vapor/liquid interfaceand performed at a scan rate of O.6V/h. The data for theduplicate specimens indicate approximately 70mV

lb "I-j

I:

I _

x . . . . ...... ........ . . .. .... ... ... . . . . . ................ . . .

B1t l .. tu,_ B ,, , , ,,, *, . _ ..104- 1b'- IFutbiluU---- Iuhb -1I U 'ijb 6_ . .. rag ..a

CURREH E. AA aMZ

Figure 3-2. CPP Curve for Type 317L Stainless Steel In Solution ElWith a PTFE Holder/Specimen Interface Performed at a Scan Rate of 0.6V/h

3-3

Cyclic Potentiodynamic Polariation Experiments

a

Figure 3-3. CPP Curve for Type 317L Stainless Steel In Solution ElWith a PTFE Holder/Specimen Interface Performed at a Scan Rate of 0.06V/h

912ziD0

-10u4 .CURrENT E A/ .22

.lb' "l32

Figure 3-4. CPP Curves for Type 317L Stainless Steel In Solution ElWith a Vapor/Liquid Interface Performed at a Scan Rate of 0.6V/h

3-4


I:Ra,

ILD-4- ' 4 - - -'-'"I4 ' ' lb ' ' " IlbI ' 'CURRENT C A/ .23

Figure 3-5. CPP Curves for Type 317L Stainless Steel In Solution ElWith a VaporlLlquid Interface Performed at a Scan Rate of 0.06Vth

'lb'

difference in Epit. A comparison of Eprct is not availablebecause one test was inadvertently interrupted beforethe return scan was complete. No enhanced pitting wasever observed at the vapor/liquid interface. Compari-son of the CPP curves in Figure 34 (vapor/liquid inter-face: 0.6V/h) to Figure 3-2 (PTFE holder/specimeninterface: 0.6V/h) indicates that E it issiificantly morenegative when the vapor/liqui interface was presentth a n W hen the P T F E h old er/ m en in terface w asFRes-enT. ibs7is somewhait unepctdand is not typi-

cal 0-results for other alloy-solution combinations.Values of Eprot for the two conditions discussed above(vapor/liquid versus PTFE holder/specimen interfaceat O.6V/h) are within approximately 50mV,

Figure 3-5 shows the CPP curve for 317L in Solution Elwith a vapor/liquid interface and performed at a scanrate of 0.06V/h. Comparison of the CPP curve shown in

Figure 3-5 with the curve shown in Figure 34, per-formed at a faster scan rate, indicates both Epit and Eptare somewhat more negative for the slower scan rate.Comparison of the CPP curve in Figure 3-5 (vapor/liq-uid interface: 0.06V/h) to that shown in Figure 3-3(PTFE holder/specimen interface: 0.06V/h) shows thatthe two curves are nearly identical. Values of EFit arewithin 50mV and values of Fat are within 20mV

Solution E2Figure 3-6 shows the CPP curve for 317L in Solution E2with a PTFE holder/specimen interface and performedat a scan rate of O.6V/h. Very little, if any, preferentialpitting was associated with the PTFE holder/specimeninterface. Pitting was initiated at a relatively negativepotential and Eprot is only approximately lOOmV morepositive than Ecor (see Table 3-2).

3-5


Table 3-2Values for Emr, Ep, and Eprot for Type 317L Stainless Steel Measured by CPP Experiments

Solution

ElElElElEl

E2E2E2E2

Interface

H/SH/SV/LV/LV/L

H/SH/SV/LV/L

Scan Rate,V\h

0.60.060.60.60.06

0.60.060.60.06

PittingSites

I SI (S)

SSS

(I) S(I) S

SS

I SI (S)I (S)I (S)

SS

V (SCE)

+0.15+0.20+0.25+0.21+0.22

-0.22-0.15+0.02-0.20

+0.01-0.12+0.06+0.02-0.14-0.16

EvptV (SCE)

+0.70(R)+0.40+0.51+0.58+0.44

+0.06-0.02+0.12-0.01

+0.68+0.78+0.82+0.34+0.54+0.84

EprobV (SCE)

+0.120.00

NA+0.08

0.00

-0.11-0.08+0.01-0.08

-0.05-0.05-0.17-0.08+0.02-0.02

E3 H/S 0.6E3 H/S 0.6E3 H/S 0.06E3 H/S 0.06E3 V/L 0.6E3 V/L 0.06

R: Pitting initiated during reverse scan.1: Pits initiated at PTFE holder/specimen interface.S: Pits initiated on generai surface away from any interface.(): Less favored pit initiation site.NA: Data not available.H/S: PTFE holder/specimen interface used in testV/L: Vapor/liquid interface used in test

Figure 3-7 shows the CPP curve for 317L in Solution E2with a PTFE holder/specimen interface and performedat a scan rate of 0.06V/h. Very little, if any, preferentialpitting was associated with the PTFE holder/specimeninterface. Comparison of the fast scan (0.6V/h) shown inFigure 3-6 to the slow scan (0Q06V/h) shown in Figure3-7 indicates a more negative Epit was observed for theslow scan test. Also, a significant amount of current fluc-tuation was observed for the slow-scan test prior to ini-tiation of stable pitting. The protection potential for theslow scan test was only 3OmV more positive than for thefast scan test.

Figure 3-8 shows the CPP curve for 317L in Solution E2with a vapor/liquid interface and performed at a scanrate of 0.6V/h. No preferential pitting was observed atthe vapor/liquid interface. Comparing the CPP curve in

Figure 3-8 to that shown in Figure 3-6 indicates that Eitand Epret values for the specimen with a vapor/liquidinterface are more positive than for the specimen with aPTFE holder/specimen interface.

Figure 3-9 shows the CPP curve for 317L in Solution E2with a vapor/liquid interface and performed at a scanrate of 0.06V/h. No preferential pitting was observed atthe vapor/liquid interface. Comparing the CPP curve inFigure 3-9 (vapor/liquid interface: 0.06V/h) with that inFigure 3-8 (vapor/liquid interface: 0.6V/h) indicatesEpit and Epot are more negative for the slow scan rate

and 90mV, respectively). Comparing the CPPcurve in Figure 3-9 (vapor/liquid interface: 0.06V/h) tothat shown in Figure 3-7 (PTFE holder/specimen inter-face: 0.06V/h) indicates almost identical behavior withEpit and Epot values within 20mV for the two curves.

3.6


/t~t2 Ct

J..I'C-0.

' 1'0< .* o

CUMENT C A/ u22* b02

Figure 3-6. CPP Curve for Type 317L Stainless Steel In Solution E2With a PTFE Holder/Specimen Interface Performed at a Scan Rate of 0.6V/h

5f,.5 -:0 -

N.'

T L,__... ._.......... .... .... -. ...

CUNRRMT C A/ m23.' '''lb


3-7


I.-

oQ.9

. .....

-I 03-4. . ... 2 rCURRENT E A/ .22

...... . ........ . ........

.*om1 *lbw * 2-"lb2

Figure 3-8. CPP Curves for Type 317L Stainless Steel In Solution E2With a Vapor/Liquid Interface Performed at a Scan Rate of O.6V/h

it

CURRENT UA/ A21

Figure 3-9. CPP Curves for Type 317L Stainless Steel In Solution E2With a Vapor/Liquid Interface Performed at a Scan Rate of 0.06V/h

3-8


Solution E3Figure 3-10 shows CPP curves for duplicate specimensof 317L in Solution E3 with a PTFE holder/specimen in-terface and performed at a scan rate of O.6V/h. Pittingwas observed on specimens in both tests at the PTFEholder/specimen interface. Epit values for the two curvesare lOOmV apart, while values are identical. As canbe seen from the curves, a very large hysteresis is ob-served with Eprot similar to Ecr

Figure 3-11 shows the CPP curves for duplicate speci-mens of 317L in Solution E3 with a PTFE holder/speci-men interface and performed at a scan rate of 0.06V/h.Preferential pitting was observed at the PTFE holder/specimen interface for both specimens shown in Figure3-11. A significant difference is observed in(480mV) for the two curves and a difference of 9Omtisobserved in Eprot Comparison of the CPP curvesshown in Figure 3-11 to those shown in Figure 3-10 in-dicate that the protection potentials are similar for thedifferent scan rates. Because of the large variation inEpit for the duplicate tests shown in Figure 3-11, it is dif-ficult to draw conclusions as to the effect of scan rate onEpit

Figure 3-12 shows the CPP curve for 317L in Solution E3with a vapor/liquid interface and performed at a scanrate of 0.6V/h. No preferential pitting was observed atthe vapor/liquid interface. Comparing the CPP curve inFigure 3-12 (vapor/liquid interface: 0.6V/h) with that inFigure 3-10 (PTFE holder/specimen interface: 0.6V/h)indicates that Epit for the vapor/liquid interface isslightly more negative than for the specimens with aPTFE holder/specimen interface, and 5prot for the va-por/liquid interface is 7OmV more positive than for thespecimens with a PTFE holder/specimen interface.

Figure 3-13 shows the CPP curve for 317L in Solution E3with a vapor/liquid interface and performed at a scanrate of 0.06V/h. No preferential pitting was observed atthe vapor/liquid interface. Comparison of the CPPcurve in Figure 3-13 (vapor/liquid interface: 0.06V/h)with that in Figure 3-12 (vapor/liquid interface: 0.06V/h)indicates that Epit is more negative for the faster scanrate. This finding is contradictory to the hypothesisthat a slower scan rate should provide for longer incu-bation times and, thereby, generally have a more nega-tive Epit. The protection potential for the curves at thetwo scan rates are very similar (within 50mV).

I.a.

a'* lb .UR H ~ .A/ . 3 ... To DI


3-9

Cyclic Potentiodynamic Polarization Experments

I

a

i.1 -. . .

* -. ,clRENT I A/ am2

' * -'r * ' *---lUt


a

Figure 3.12. Curves for Type 317L Stainless Steel In Solution E3With a VaporlLiquld Interface Performed at a Scan Rate of O.BV/h

3.10


I

a

.. ... s

0'*4CURMNT C A/ am2

Figure 3-13. CPP Curves for Type 317L Stainless Steel In Solution E3With a Vapor/Llquld Interlace Performed at a Scan Rate of 0.06V/h

Results for Alloy G3Alloy G3 was tested in two solutions: E4 and E5. Foreach solution, CPP curves are presented for four condi-tions: specimen with PTFE holder/specimen interface atscan rates of 0.6V/h and 0.06V/h, and specimen withvapor/liquid interface at scan rates of 0.6V/h and0.06V/h.

Solution E4

Figure 3-14 shows a CPP curve for Alloy G3 in SolutionE4 with a PTFE holder/specimen interface and per-formed at a scan rate of 0.6V/h. Preferential pitting wasobserved at the PTFE holder/specimen interface for oneof the specimens. Table 3-3 presents Eco, Epit Eprot andlocation of pitting for Alloy G3 specimens in SolutionsE4 and E5.

Figure 3-15 shows the CPP curve for Alloy G3 in Solu-tion E4 with a PTFE holder/specimen interface and per-formed at a scan rate of 0.06V/h. No preferential pittingwas observed at the PTFE holder/specimen interface.Comparing the CPP curve in Figure 3-15 with thatshown in Figure 3-14 indicates Ero is somewhat morenegative for the slow scan than the fast scan. The value

of Fit is much more negative for the slow scan test,which is the expected result.

Figure 3-16 shows the CPP curve for Alloy G3 in Solu-tion E4 with a vapor/liquid interface and performed ata scan rate of 0.6V/h. The very small hysteresis loop ob-served is not associated with pitting and no pits wereobserved on the specimen surface. This is in stark con-trast to the 0.6V/h scan rate tests performed with a PTFEholder/specimen interface in which pitting was ob-served. However, the pitting in the PTFE holder/speci-men tests, was associated with* the PTFE holder/specimen interface. Therefore, for the Alloy G3 in Solu-tion E4, it appears that the PTFE holder/specimen inter-face greatly enhances the initiation of pitting.

Figure 3-17 shows the CPP curves for duplicate spec-mens of G3 in Solution E4 with a vapor/liquid interfaceand performed at a scan rate of 0.06V/h. In one of thecurves (broken line), no pitting was observed with onlya small hysteresis loop at the very positive potentials,while for the other curve (solid line), pitting initiated onthe reverse scan at about 0.6V (SCE). Severe pitting overthe entire surface area was observed with no preferentialattack at the vapor/liquid interface. This was only the

3-11


I:dI

QI IrI I D-.. .... w o~-- l~~b~ ID' ... l

CUDRENT E " NZI

. , . r

Figure 3-14. CPP Curves for Alloy G3 in Solution E4 With a PTFEHolder/Specimen Interface Performed at a Scan Rate of 0.6Vlh

Table 3-3Values for Eor, Epit, and Eprot for Alloy G3 Measured by CPP Experiments

Scan Rate,V\hSolution Interface

PittingSites V (SCE)

E ,V (~CE)

Eprol,V (SCE)

E4E4E4E4E4

H/SH/SV/LV/LV/L

H/SH/SH/SV/LV/L

0.60.060.60.060.06

0.60.60.060.60.06

S

NoneNone

S

NoneNone

+0.02

-0.18

-0.16

-0.02-0.19

+0.16-0.15+0.25+0.14+0.10

+0.70(R)+0.20

NPNP

+0.59(R)

+0.35+0.28+0.80

NPNP

0.00

-0.14NP

NP

+0.13

-0.05

-0.05

+0.01NP

NP

ESE5E5ES

ESR: Pitting initiated during reverse scan.1: Pits initiated at PTFE holder/specimen interface.S: Pits initiated on general surface away from any interface.(): Less favored pit initiation site.NP: No pitting, hence no Ep or Ep,.H/S: PTFE holderlspecimen interface used in test.V/L: Vapor/liquid interface used in test.

3-12


Figure 3-15. CPP Curves for Alloy G3 In Solution E4 With a PTFE Holder/Specimen InterfacePerformed at a Scan Rate of 0.06V/h

second specimen out of the six CPP experiments withAlloy G3 that exhibited pitting on the free surface. Thespecimens for all of these tests were cut from the samepiece of material and the pH of the solutions waschecked to establish that an error in the solution makeuphad not occurred. It is not known whether surface con-tamination or some other experimental procedurecaused the pitting on the free surface for these two spec-imens. The specimens were all prepared in the samemanner, which included the use of new polishing/grinding paper. Therefore, it was highly unlikely thatsurface contamination occurred during the grinding ofthe specimen down to a 600-grit surface. It is obviousthat the presence or lack of a preferred pit-initiation site(PTFE holder/specimen interface) is very important tothe results of the CPP experiments for the nickel alloy inSolution E4. Furthermore, the incubation time for pittingmay be exceedingly long in the absence of a preferredpit-initiation site.

Solution E5

Figure 3-18 shows the CPP curves for duplicate speci-mens of Alloy G3 in Solution E5 with a PTFE holder/spec-imen interface and performed at a scan rate of U6V/h.Preferential pitting was observed at the PTFE holder/

specimen interface. The initial breakdown for the twotests indicate a value of Epit which is less than 100mVapart. The values for Fat for the two tests are identical.

Figure 3-19 shows the CPP curve for Alloy G3 in Solu-tion E5 with a PTFE holder/specimen interface and per-formed at a scan rate of 0.06V/h. Preferential pitting wasobserved at the PTFE holder/specimen interface. Com-paring the CPP curve in Figure 3-19 with that in Figure3-18 indicates that pit initiation occurred at a more posi-tive value for the slow scan test than for the fast scan test.This is not typical and is likely due to preferential pittingsites being different for the specimens. The values forEprot for the two scan rates are within 60mV.

Figure 3-20 shows the CPP curve for Alloy G3 in Solu-tion E5 with a vapor/liquid interface and performed ata scan rate of 0.6V/h. As was observed for Alloy G3 inSolution E4, once the PTFE holder/specimen interfacewas removed, pits did not initiate on the Alloy G3 surface.

Figure 3-21 shows the CPP curve for Alloy G3 in Solu-tion E5 with a vapor/liquid interface and performed ata scan rate of 0.06V/h. Once again, it was observed thatno pitting occurred for Alloy G3 in Solution E5 when thePTFE holder/specimen interface was not present.

3-13


-g Dl '"'bu '"b4' lb' ' ' ' 'lb ' ''CURET IA mM

Figure 3-1B. CPP Curves for Alloy G3 In Solution E4 With a Vapor/Liquid InterfacePerformed at a Scan Rate of, 0.6 Vh

. CURR T At- ..

Figure 3-17. CPP Curves for Alloy G3 In Solution E4 With a Vapor/Llquid InterfacePerformed at a Scan Rate of 0.06V/h

3-14


a4.w

3-4 ' ' ' * E 1 6lbCURREHT t Al.u2M

' . 9. ba

Figure 3-18.

Ia

CPP Curves for Alloy G3 In Solution E5 With A PTFE Holder/Specimen InterfacePerformed at a Scan Rate of M.6V/h

, D - 1 O - - -toOUMREUT C IV am2

Ib'

Figure 3-19. CPP Curves for Alloy G3 In Solution E5 With a PTFE Holder/Specimen InterfacePerformed at a Scan Rate of 0.06VWh

3-15


19:0I.-

0

9! �.1I

a .

if

-OsCURRENT t A/ n21

' '"lbs8 -TrO 2

Figure 3-20. CPP Curves for Alloy G3 In Solution E5 With a Vapor/Liquld InterfacePerformed at a Scan Rate of 0.6V/h

I-

CURRENT C A/ .22

Figure 3-21. CPP Curves for Alloy G3 In Solution E5 With a Vapor/Liquid InterfacePerformed at a Scan Rate of 0.06V/h

3-16

4CONSTANT POTENTIAL-TIME EXPERIMENTS

The purpose of these experiments was to establish thepotential at which stable pit initiation occurs whilemaintaining the specimen at a constant potential in therange between Epit and Eprot. Two alloy-solution combi-nations were selected for examination in this task: Type317L stainless steel in Solution E3 and Alloy G3 in Solu-tion E4.

Experimental ApproachThe experimental procedures were relatively simple. Thespecimen configuration was similar to that described inSection 3, i.e., cylindrical specimen mounted on a PTFEholder with either a PTFE holder specimen interface im-mersed in the solution or the presence of a vapor/liquidinterface on the specimen surface. Specimens were im-mersed in the solution and permitted to stabilize for ap-proximately 20 hours before polarizing to the desiredpotential. Polarization was accomplished with eitherThompson Electrochem Microstats or Cortest Instru-ment Systems Model 2125 potentiostats. Two compart-ment PTFE electrochemical cells having a combinedworking-counter compartment and a separate referenceelectrode compartment were used.

Current measurements were made as the voltage dropacross a resistor placed in series in the counter electrodelead. The resistor was varied from 102 to 106 ohms, de-pending on the magnitude of the current. The voltage

drop across the resistor was recorded on a strip chart re-corder for the initial 24 to 48 hours and switched to acomputer data acquisition for sampling every 15 min-utes. The primary data desired were the times to stablepit initiation. Typical currents, prior to pit initiation, forpassive conditions were 1075 to le A/m2 . Upon pit ini-tiation, the current increased to as high as 2AA/m2 .Therefore, it was typically quite easy to establish thetime-to-pit initiation by monitoring current.

Results for Type 317L Stainless Steel InSolution E3Table 4-1 presents the time-to-pit initiation data for theconstant potential-time tests. The tests for the vapor/liq-uid interface will be considered first. No pitting oc-curred preferentially at the interface in any of these tests.At a potential of 0.4V (SCE) pits initiated between 23 and619 hours following application of the potential. Withthe exception of Test No. 3106, at 03V (SCE), no pit ini-tiation had occurred after up to 1658 hours. Test No.3106 is considered as a separate case because it wasmuch more active (negative free-corrosion potential)when the test was initiated than any of the other tests.The more active behavior prior to application of the con-stant potential was attributed to a much longer (16hours) purging of N2 than normal (1 hour) prior to im-mersing the test specimen. This emphasizes the sensitiv-ity of the pit initiation process to changes in testconditions.

4-1

Constant Potential-rime Experiments

Table 4-1Data for Constant Potential-Time Experiments for Type 317L Stainless Steel In Solution E3

Polarized Potential, Time-to-Pit Initiation,Test No. Interface Present(a) V, (SCE) Minutes

3097 V/L +0.20 >4.0 x 104(b)0103A H/S +0.20 >9.0 x 104(b)3123 H/S +0.30 2.0 x 103

0103B H/S +0.30 9.4 x 1023126 H/S +0.30 >1.1 x 105(b)3102 V/L +0.30 >7.3 x 103(b)3104 V/L +0.30 >1.0 x 105(b)3105 V/L +0.30 >9.1 x 104(b)3106 V/L +0.30 4.1 x 103(c)NA V/L +0.40 1.2 x 104

3099 V/L +0.40 3.7 x 1 043100 V/L +0.40 1.4 x 103

(a) V/L: Vaporfliquid interface.H/S: PTFE holder/specimen interface.

(b) Test terminated after number of minutes shown, no pitting.(c) Specimen had an unusually active free-corrosion potential prior to application of constant potential (see text).NA: Data collected prior to setting up computerized data acquisition system.

Three tests were performed with PTFE holder/speci-men interfaces immersed in the test solution at 0.3V(SCE). Two of these tests initiated pitting following only34 hours and 16 hours, while the other specimen exhib-ited no pitting after 1876 hours. These data indicate thewide scatter to be expected in pit initiation tests. Basedon the above results, pits initiate at potentials more no-ble than 0.2 to 03V (SCE). Furthermore, the presence ofthe PTFE holder/specimen interface provides a pre-ferred pit initiation site that increases the possibility ofpitting.

Recalling data from the CPP tests (Table 3-2), E it andEprot were in the range of 0.75 to 0.34V (SCE) anS -0.05to -0.17V (SCE) respectively, when a PTFE holder/spec-imen interface was present. The most negative value forEpit (0.34V) was recorded on one of the slow scan(0.06V/h) curves. When a vapor/liquid interface waspresent, Epit and Eprot were in the range of 0.84 to 0.54V(SCE) and 0.02 to-0.02V (SCE), respectively. Therefore,the value of Epit from the constant potential-time tests isslightly more negative than the most negative value de-termined by the CPP tests. Also, Epit determined by the

constant potential-time tests was still significantly morepositive than the value of Eprot determined by the CPPtests.

Results for Alloy G3 In Solution E4Table 4-2 presents the time-to-pit initiation data for theconstant-potential tests for Alloy G3 in Solution E4. Thespecimens with the vapor/liquid interface will be con-sidered first. With only a vapor/liquid interface present,pits were not initiated for specimens at 0.6 or 0.4V (SCE)after 1500 hours. At a potential of 0.8V (SCE), pits initi-ated after 138 hours. However, if a crevice collar (similarto the one used in ASTM standard test method F-746)was placed on the specimens, crevice corrosion was ob-served at 0.6V after exposure periods of 73 and 1081hours. Therefore, the presence of the crevice providedpreferred initiation sites for localized corrosion. Itshould be noted that the form of attack with the crevicecollar was significantly different from that observed atthe PTFE holder/specimen interface. Crevice corrosionoccurred beneath the crevice collar, while pits were initi-ated at the PTFE holder/specimen interface.

4-2

Constant Potential-rime Experiments

Table 4-2Data for Constant Potentlal-Time Experiments for Alloy G3 In Solution E4

Test No. Interface Present(") Polarized Potential, Time-to-Pit Initiation,V, (SCE) Minutes(b)

3132 H/S +3134 H/S +3131 H/S 4

3125 H/S 4

3112 VIL 4

3128 H/S 4

3124 H/S 4

3127 H/S 4

3113 V/L 4

3133 V/L 4

3115 Crevice, V/L 4

3116 Crevice, V/L 4

3114 V-L 4

(a) V/L: vaporAiquid interfaceHIS: PTFE holder/specimen interfaceCrevice: ASTM F746 crevice collar

(b) Test terminated after number of minutes shown, no pitting.

Several tests were performed with the PTFE holder/specimen interface present, which quite often provideda preferred site for pit initiation. At a potential of 0.6V(SCE), pits initiated after 4 and 0.7 hours. At potentialsof 0.5, 0.4 and O.3V (SCE) pits initiated in 0.8 hours orless. Two tests performed at 02V (SCE) indicated no pit-ting after 1684 and 1490 hours. With the PTFE holder/specimen interface present, Epit from the constant poten-tial tests was between 0.2V and 0.3V (SCE).

Recalling data from the CPP tests (Table 3-3), Epit andEprot were 0.70 to 0.20V (SCE) and 0.00 to -0.14V (SCE),respectively, when a PTFE holder/specimen interface

.0.20

.0.20+0.30.0.40+0.40+0.50+0.60+0.60+0.60+0.60.0.60+0.60+0.80

>1.0 X 1 0 6(b)

>8.9 x 104(b)3.6 x 1004.8 x 101

>9.5 x 104(b)

4.8x 1012.4 x 1024.2 x 101

>8.6 x 104(b)

>9.1 x 104(b)4.4 x 10 3

6.5 x 1048.3 x 103

was present. When a vapor/liquid interface was present,no pitting at all was observed during two of the CPPtests, while one test exhibited values of Ep1t and Epr't of0.59 and 0.13V (SCE), respectively. The constant potentialtests correspond reasonably well with the CPP tests, i.e.,the presence of a preferred pit initiation site (PTFE hold-er/specimen interface) greatly increased the likelihoodfor pit initiation. Values of Epit determined by constantpotential-time tests were approximately equal to themost negative value of Epit recorded by CPP tests. Also,Epit determined by the constant potential-time tests was0.3V more positive than the values of Eprot recorded forthe CPP tests.

4-3

5MODIFIED ASTM F-746 TESTS

The purpose of these experiments was to examine thedependency of prior pitting history on the measuredvalue of Eplot. The same two alloy-solution combina-tions were selected for examination in this task as wereused in the constant potential-time experiments.

Experimental ApproachModified ASTM Standard F-746 tests were performed todetermine Eprot while providing a well-controlled pit-ting history prior to determining Eprj. In this test, a crit-ical variable is how much the largest active pit growsbefore it iW'repassivated. This largest pit is likely to con-trol the potential, Ep t, of which repassivation occursfor that specimen. After considering several options andperforming preliminary tests, it was decided that the to-tal time at the pit stimulation potential (see below) wasas good a measure of pit growth as total charge passedor time from rapid current increase.

Tests were performed utilizing a working electrode withthe previously described PTFE holder/specimen inter-face. The test procedure was as follows: (1) insert testspecimen; (2) permit to stabilize under freely corrodingconditions (16-20 hours); (3) polarize to a potential of0.60V to produce rapid onset of pitting; and (4) after apredetermined time for pitting, polarize the specimen toa more negative potential and measure the current-timetransient for up to 16 hours for the purpose of establish-ing Ep~t. The above procedure was repeated until a po-tential was established above which pitting continuedand below which complete repassivation occurred. Ascan be seen from the above procedures, modificationsto the ASTM Standard F-746 are in three areas: (1) a crev-ice washer was not utilized, (2) the current criterion for

establishing the pitting history was replaced by a speci-fied time, and (3) the length of time for monitoring thecurrent transient at EFxt was extended from 15 minutesto several hours.

Results for Type 317L Stainless Steel InSolution E3The time of pitting was the primary variable in the testmatrix. Following a specified time of pitting, the poten-tial was stepped to a potential close to the value of Eprotdetermined in CPP tests. By performing several suchtests at different potentials, an independent value ofEprot was established as the most noble (positive) poten-tial at which pits, initiated at +0.60V, repassivated. Uponpolarization to +0.60V, a large current transient oc-curred, followed by a decrease in current to a minimum,followed by a current increase indicating significant pit-ting. Preliminary experiments showed that pits initiateeven during the initial current transient and subsequentcurrent decrease. Therefore, pitting time was calculatedfrom the initial polarization to +0.60V (SCE).

Figures 5-1 through 5-7 show current-time data follow-ing pit stimulation at +0.60V for selected periods andsubsequent polarization at selected more negative po-tentials near the expected Epot for alloy 317L. Eprot is de-fined as the potential below which repassivation occursand above which pits grow. For instance, in Figure 5-1,

was determined to be in the range of 150 to 200mV(SCE). Table 5-1 summarizes the data as a function of pit-ting time. It is seen that Eprot became more positive aspitting time decreased and leveled off at pitting times ofless than 1 minute.

5-1

Modified ASTM F-746 Tests

50

40

&J

CL!

L.

.30

20

10

0

-100 200 400 600 800 1000

Time, minutes

Figure 5-I1. Current-lime Transients for Type 317L In Solution E3 Following Pit Stimulation at O.6V (SCE) for2 Seconds (0.03 min) and Polarized to a Potential Near Eprot

50

40

C

I-3

O.

,30

20

10

0

-100 200 400 800 800 1000

Time, minutes

Figure 5-2. Current-Time Transients for Type 317L In Solution E3 Following Pit Stimulation at 0.6V (SCE) for15 Seconds (0.25 min) and Polarized to a Potential Near Ep,,t

5-2


50

40

4-WC

1-,!

30

20

10

-100 200 400 600 800 1000

Time, minutes

Figure 5-3. Current-TIme Transients for Type 317L In Solution E3 Following Pit Stimulation at 0.6V (SCE) for2 Minutes and Polarized to a Potential Near Ep(ot Two Curves Shown for Duplicate Tests at 20OmV and 150 mV)

50

40

4.1

C)

30

20

10

0

-100 200 400 600 800 1000

Time, minutes

Figure 5-4. Current-lime Transients for Type 317L In Solution E3 Following Pit Stimulation at 0.6V (SCE) for7 Minutes and Polarized to a Potentlal Near Epr, (Two Curves Shown for Duplicate Tests at 100 mV)

5-3


50

40

C.)

30

20

10

-100 200 400 600 800 1000

Time, minutes

Figure 5-5. Current-Time Transients for Type 317L In Solution E3 Following Pit Stimulation at 0.6V (SCE) for25 Minutes and Polarized to a Potential Near Eprot

50

40

*1

4iG)

a

30

20

10

0I

°-° 75 mV03-0 50 mV4-A 25 mV

o' \ ,o

0,o

;'0, _0

-tO0 200 400 600 800 1000

Time, minutes

Figure 5-. Current-Time Transients for Typo 317L In Solutlon E3 Followin Pit Stimulation at 0.6V (SCE) for40 Minutes and Polarized to a Potential Near prot

5-4


50

40

41

C

C,

30

20

10

-100 200 400 600 800 1000

Time, minutes

Figure -7. Current-TIme Transients for Type 317L In Solution E3 Following Pit Stimulation at 0.6V (SCE) for100 Minutes and Polarized to a Potential Near Eprot

Table 5-1Values of Ep,02 as a Function of Time of Pitting From

the Modified ASTM F-746 Tests

Pitting Time,min. ~E rtAlloy

317L317L317L317L317L317L317LG3G3G3

0.030.252.07.025401000.252.040

+150+150+100+50+50+25

0+50+50

to +200to +200to +200to +100

to +100

to +75to +50to +100to +1 00

Results for Alloy G3 In Solution E4During polarization at +0.60V, the current-time behav-ior was very similar to that observed for Alloy 317L.Figures 5-8 through 5-10 show current-time data fol-lowing pit stimulation at +0.60V (SCE) for selected pe-riods and subsequent polarization at selected morenegative potentials near the expected Eat for AlloyG3. Ep,.t is determined for each "time of pitting' as thepotential below which repassivation occurred andabove which pits grow. Table 5-1 gives Ent values as afunction of pitting times at 0.06V (SCE). As for Alloy317L, the value of Epot became more positive with a de-crease in pitting time from 40 to 2 minutes, but showedno further change for times less than 2 minutes.

-50 to O

5-5


Off-Scale Readings

50

40

.4-ICa,I-L

C-)

30

20

10

0

-100 200 400 500 800 1000

Time, minutes

Figure 5-8. Current-Time Transients for Alloy G3 In Solution E4 Following Pit Stimulation at 0.8V (SCE)for 15 Seconds (0.25 mmn) and Polarized to a Potential Near Epro

50

40

30

204.4Ca,

C-)

-100 200 400 600 800 1000

Time, minutes

Figure 5-9. Current-Tlme Transients for Alloy G3 In Solution E4 Following Pit Stimulation at 0.6V (SCE)for 2 Minutes and Polarized to a Potential Near Eprot

5-6


U

50

so

40

20

10

0

-In

0-0 0 mVA-A -50 mV

-I

A- 4,.A-A-A.&_-A--A-A-A-A-A-A-A-A-A-A

. ._

0 200 400 600 800 1000

Time, minutes

Figure 5-10. Current-Time Transients for Alloy G3 In Solution E4 Following Pit Stimulation at O.6V (SCE)for 40 Minutes and Polarized to a Potential Near Ep,ot

5-7

6DISCUSSION

There remains a great deal of controversy concerning thesignificance of Epit and Eprot, the relationship betweenthem, and the most appropriate method to measurethem. In terms of the stochastic models that have beendeveloped, 1,2 Eit is expressed as the most active poten-tial at which the frequency of pit nucleation exceedszero. However, having a pit nucleation frequency great-er than zero is not the only prerequisite for pit initiation.Another is an incubation period to allow the transitionfrom an unstable to a stable pit.

The presence of preferred pitting sites, e.g., a specimen/holder interface or a second-phase inclusion on the freesurface, is an additional factor that can affect the incuba-tion time requirement. Therefore, preferred pitting sitescan affect the value of E.It measured by any method in-volving potential scanning, stepping, or holding a po-tential for a given time period. In the present study, thiswas most obvious in the CPP tests for Alloy G3 in Solu-tion E4 in which pitting was observed in only one ofthree tests with a vapor/liquid interface present on thespecimen; while pitting was observed in all three testswith a PTFE holder/specimen interface (pits preferen-tially initiated at the interface in two of the three tests).From the above discussion, it is clear that pit initiationhas a significant dependence on potential, time, andspecimen geometry and preparation.

Ep~ot has typically been defined by other workers as thepotential at which ongoing pits repassivate. Suzuki andKitamura26 described Eprot as the potential equal to thesteady-state potential within the pit with no anodic

polarization applied. That is, repassivation occurs whenthe potential at the specimen surface is decreased to avalue below that of the inside of the pit, thereby produc-ing a net cathodic current within the pit. Therefore, Eprotwould be dependent upon the environment within thepit which is, in part, controlled by the amount of pittingprior to its measurement (Wilde and Williamsl ), andwhich is directly related to the time of pit propagationprior to measurement of Eprot,

To examine the potential and time dependence for pitinitiation and repassivation, the Epit and Eat data fromthe CPP tests, constant potential-time tests, and modi-fied ASTM Standard F-746 tests were combined in po-tential versus time plots. First of all, the CPP tests wereanalyzed to extract time parameters that correspond tothose in the constant potential-time tests and the modi-fied ASTM F-746 tests. Time prior to pit initiation was se-lected as the time to scan from Eprot (as measured on thereverse scan) to Epit during the forward scan. It is real-ized that the potential is varying during this period, sothis value of time is not exactly equivalent to the time inconstant potential-time tests, but a reasonable compari-son can be made. Also from the CPP data, a time for pitgrowth is calculated as the time following pit break-down (at Epit) to the time, during the reverse scan, atwhich repassivation occurs (Ep,,t). This time can then becompared to the time of pit stimulation in the modifiedASTM F-746 tests, with similar reservations as discussedabove for pit initiation times. Table 6-1 presents thesetime data for the CPP tests for Alloys 317L and G3 in So-lutions E3 and E4, respectively.

6-1

Discussion

Table 6-1CPP Test Data Including (l) Relatlonship Between Eplt and Time to Scan From Eprot to Epity

and (II) Relationship Between Eprot and Time From Pit Initiation to Repassivation

Scan Rate PittingV/h Sites

Epit

Potential Time(")V (SCE) (min)

Eprot

Potential Time(b)V (SCE) (min)Specimen Type

Type 317L In Solution E3

PTFE Holder/SpecimenPTFE Holder/SpecimenPTFE Holder/SpecimenPTFE Holder/SpecimenVapor/LiquidVapor/Liquid

Alloy G3 In Solution E4

PTFE Holder/SpecimenPTFE Holder/SpecimenVapor/Liquid

0.60.60.060.060.60.06

1, S1, (S)1, (S)1, (S)

SS

+0.68+0.78+0.82+0.34+0.54+0.84

7090

760320

45860

-0.05-0.05-0.17-0.08+0.02-0.02

135121

1,100

930116

1,110

0.60.060.6

SNoneNone

S

+0.70(R)+0.20

112350

0.00-0.14

78350

470Vapor/Liquid 0.06Vapor/Liquid 0.06

(a) Time prior to pit initiation.(b) Time following pit initiation to pit repassivation.

R: Pitting initiated during reverse scan.1: Pits initiated at PTFE holder/specimen interface.S: Pits initiated on general surface away from any interface.(): Less favored pit initiation site.

+0.59(R) 1,170 +0.13

Figures 6-1 and 6-2 show the potential versus time plots constant potential-time tests, each Eb.it data point corre-for Type 317L stainless steel in Solution E3 and for Alloy sponds to the time prior to pit initiation at the test poten-G3 in Solution E4, respectively. Data are presented for tial; a data point with an arrow indicates that no piteach of the three measurement techniques: CPP, con- initiation occurred in that particular test for the time pe-stant potential-time, and modified ASTM F-746. For the riod indicated.

6-2

Discussion

1.0

0.8

(/)

0

C0)0~

0.6

0.4

0.2

0.0k-

-0.4

10-2 10-l 100 10 102 10 3 104 105

Time (min)

Figure 6-1. Potential Versus Time Data for Alloy 317L In Solution E3 for Epft and Eprot(Epit Versus Tlime to Pit Initiation and Epo. Versus Time Permitted to Pit)

1.0

0.8

0.6

w0

4.1

0C-

0.4

0.2

0.0

-0.2

-0.4 ' .-10-2 10 100 101 102

Time (min)1i o4 1o5

Figure 6-2. Potential Versus Time Data for Alloy G3 In Solution E4 for Epit and Eprot(Epl, Versus Time to Pit Initiation and Eprot Versus Tlime Permitted to Pit)

6-3

Discussion

There is significant scatter in the data for both alloy/solution combinations. The more important concept be-ing considered is whether the most active value of Epitequals the most noble value of E...t, the latter being as-sociated with repassivation of pits that have been per-mitted to grow only a minimal amount. The data inFigure 6-1 suggest that, for Type 317L stainless steel in alow-pH, high-chloride environment (10 g/L al, Eprotand Epit approach the same potential. For lype 317L, thedifference between the most negative value of Epit andthe most positive value of Eprot is less than 1OOmV, pos-sibly dose to zero. For Alloy G3 (Figure 6-2), this differ-ence is less than 7OmV.

The suggestion that Eprot and Epit approach a singlevalue can be explained in terms of a conventional sto-chastic model of pitting. In this model, a unique Epitvalue is defined as the most active potential at whichthe pit nucleation frequency exceeds zero. A nucleationevent is characterized by a local increase in (anodic)current density which, in the case of a nonpropagatingpit, is transitory; but which, in the case of a stable pitnucleus, remains higher than the initial passive currentdensity. At potentials more active than this unique pit-ting potential (Eu) any tendency to initiate a pit in thebulk environment is overwhelmed by the tendency torepassivate it, so no new pits are initiated. This also sat-isfactorily describes Eprot if pit growth has been negligi-ble. Therefore, in the special case of a pit that has beenpropagating for a very short period, the environmentwithin the pit will be quite similar to the bulk environ-ment and Eprot should have a similar value to E,. Tostate this concept in another way (Equation 6-1), themost active Epit value possible is equal to the potential atwhich the pit nucleation frequency approaches zero,which is also the most noble potential that growing pitscan be repassivated.

Epit =Eu =Eprot (6-1)

Pits that have propagated some significant amount aremore difficult to repassivate because the environmentwithin the confines of the pit is more aggressive (higherin a- and lower pH) than the bulk. Thus, in general,EpOt values measured after significant pitting will belower than Eu.

The above analysis implies that Epit values will drawever closer to Ew the slower the scan rate in CPP tests,the longer the exposure time allowed in constant poten-tial tests, and when incubation times are minimized bythe presence of preferred pitting sites (e.g., PTFE holder/specimen interface). In addition, these 'long-term" val-ues of it should be similar to the values of Eprota de-termined by such means as the modified ASTMStandard F-746 test, where only a minimal time for pitgrowth is allowed prior to repassivation.

In this study, significant scatter was observed in the Epitvalues in the CPP data and in the incubation time for pitinitiation at a constant potential. In a previous study forEPRI for the same alloys in similar solutions, the scatterobserved in the CPP tests was not nearly as great.4 It isspeculated that this increased scatter in Epit is due toachieving a "better" PTFE holder/specimen interface.By removing preferred pit initiation sites, pit initiationoccurred at less preferred sites, which introduced agreater amount of variation. In the previous EPRIstudy,34 no attempt was made to remove the PTFE hold-er/specimen interface as a preferred site for pit initiation,which resulted in more reproducible and consistentlymore negative Epit values.

6-4

7CONCLUSIONS

Based on the literature review and the testing program.the following conclusions are drawn concerning electro-chemical measurements for pitting susceptibility.

1. Pit initiation can be described by a unique pittingpotential, E., which is approximately equal to (a)the most negative Epit value recorded in CPP testsor constant potential tests (associated with slowscan rates or long incubation periods), and (b) themost positive Ep.t value when prior pit growth isminimized.

2. CPP tests tend to separate Fit and Fat by minimiz-ing incubation time and maximizing pit growth fol-lowing pit initiation.

3. Ept from the CPP test is more conservative (morenegative) than Eu and is an acceptable basis for mostengineering decisions.

4. Fit is less conservative (more positive) than Euand is not an acceptable basis for engineering de-cisions involving long-term pitting performancepredictions.

5. Deliberate incorporation of preferential pit initiationsites in a CPP test specimen is desirable from an en-gineering viewpoint to minimize the effect of incu-bation time and maximize the chances of pitinitiation during this relatively short-term test.

7-1

REFERENCES

1. D.E. Williams, C. Westcott, and M. Fleischmann."Stochastic Models of Pitting Corrosion of StainlessSteels, I." Journal Electrochemical Society, 132, 8, p.1796 (1985).

2. D.E. Williams, C. Westcott, and M. Fleischmann."Stochastic Models of Pitting Corrosion of StainlessSteels, II." Journal Electrochemical Society, 132,8, p.1804 (1985).

3. F. Zucchi, I.H. Omar, and G. Trabanelli. "Inhibitionof Pitting Corrosion of AISI 304 by Organic Com-pounds." Werkstoffe Und Korrosion, 38, pp. 742-745(1987).

4. R.K. Dayal, N. Parvathavarthini, and J.B. Gnana-moorthy. "A Study of Various Critical Pitting Poten-tials for Type 316 Stainless Steel in Sulfuric AcidContaining Chloride Ions." Corrosion, 36,8, pp.433-436 (1980).

5. P.E. Manning, D.J. Duquette, and W.F. Savage. "TheEffect of Test Method and Surface Condition on Pit-ting Potential of Single- and Duplex-Phase 304LStainless Steel." Corrosion, 35, 4, 151-157 (1979).

6. P.E. Manning and D.J. Duquette. "The Effect of Tem-perature on Pit Initiation in Single-Phase and Du-plex 304L Stainless Steels in 100ppm Ca Solution."Corrosion Science, 20, pp. 597-610 (1980).

7. "Relative Critical Potentials for Pitting Corrosion of304 Stainless Steel, Incoloy 800, and Inconol 600 inAlkaline High-Temperature Aqueous Solutions."Journal of Nuclear Materials, 115, pp. 339-342 (1983).

8. M. Urquidi and D.D. Macdonald. "Solute-VacancyInteraction Model and the Effect of Minor Alloy El-ements on the Initiation of Pitting Corrosion.' Jour-nal of Electrochemical Society, 132, 3, pp. 555-558(1985).

9. W.R. Clesiak and D.J. Duquette. "An Electrochemi-cal Study of Pit Initiation Resistance of Ferritic Stain-less Steels." Journal of Electrochemical Society, 132, 3,pp. 533-537 (1985).

10. R. Fratesi. "Statistical Estimate of the Pitting Poten-tial of AISI 316L Stainless Steel in 3.5% NaC Mea-sured by Means of Two Electrochemical Methods."Corrosion, 41, 2, pp. 114-117 (1985).

11. D.M. Aylor and P.J. Moran. "A Comparison of Elec-trochemical Techniques for Assessing the Pitting Be-havior of Aluminum Alloys in Seawater." Paper No.216, Corrosion 85, National Association of CorrosionEngineers (1985).

12. J. Nader-Roux, A.M. de Becdelievre, G. BouyssouxR. Roux, and J. de Becdelievre. "Study of Pitting Po-tential and Repassivation of Austinetic Steel inChloride Medium." Metaux-Corros-Ind., 60,716, pp.111-116 (1985).

13. N. Nilsen and E. Bardal. "Short Duration Tests anda New Criterion for Characterization of PittingResistance of Aluminum Alloys." Corrosion Science,17, pp. 635646 (1977).

R-1

References

14. N. Azzerri, F. Mancia, and A. Tamba. "Electrochem-ical Prediction of Corrosion Behavior of StainlessSteels in Chloride Containing Water." Corrosion Sci-ence, 22, 7, pp. 675-687 (1982).

15. J.L. Dawson and M.G.S.Ferreira. "CreviceCorrosionon 316 Stainless Steel in 3% NaCl Solution."Corrosion Science, 26,12, pp. 1027-1040 (1986).

16. D.M. Aylor and P.J. Moran. "The Influence of Incu-bation Time on the Passive Film Breakdown ofAluminum Alloys in Seawater." Journal of Electro-chemical Society, 133,5, pp. 868-872 (1986).

17. B.C. Syrett. 'TPR Curves-A New Method of As-sessing Pitting Corrosion Resistance." Cormsion, 33,6, pp. 221-224 (1977).

18. B.E. Wilde and E. Williams Electrochemical Acta. 16,p. 1971 (1971).

19. D.E. Williams and C. Westcott. "Proceedings of the9th International Conference on Metallic Corrosion,Toronto, 1984." Vol. 4, p. 390, National ResearchCouncil of Canada, Ottawa, Ontario, Canada (1984).

20. G. Daufin, J. Pagetti, J.P. Labbe, and F. Michael. "Pit-ting Initiation on Stainless Steels: Electrochemicaland Micrographic Aspect." Corrosion, 41, 9, pp. 533-539 (1985).

21. A. Broli and H. Holtan. "Determination of Charac-teristic Pitting Potentials for Aluminum by Use ofthe Potentiostatic Methods." Corrosion Science, 17,pp. 59-69 (1977).

22. B.C. Syrett, R. Viswanathan, S.S. Wing, and J.E. Wit-tig. "Effect of Microstructure on Pitting and Corro-sion Fatigue on 17-4 pH Turbine Blade Steel inChloride Environments." Corrosion, 38, 5, pp. 273-282 (1982).

23. Y. Ishikawa and HS. Isaacs. "Study of Pitting Corro-sion on Aluminum by Means of The Scanning Vi-brating-Electrode Technique." MechanicalEngineering Research Laboratory Technical Report,B & L-33059, DE83-013065 (1983).

24. J.L. Dawson and M.GS. Ferreira. "ElectrochemicalStudies of the Pitting of Austenitic Stainless Steel."Corrosion Science, 26,12, pp. 1009-1026 (1986).

25. G. Okamoto, K. Tachibana, S. Nishiyama, andT. Sugita. "The Analysis of Passive Current Noise onStainless Steels Under Potentiostatic ConditionsWith and Without Chloride Ions." Passwity and ItsBreakdown of Iron Base Alloys, National Association ofCorrosion Engineers, Houston, TX, pp. 106-109(1973).

26. T. Suzuki and Y. Kitamura. "Testing Method for Lo-calized Corrosion of Stainless Steel Considering theCorrosion Potential in its Environment." In Proceed-ings of the Fifth International Congress on Metallic Cor-rosion, Tokyo, Japan, NACE, pp. 1070-1073 (1972).

27. S.T. Hirozawa. "Determ ination of the Protection(Pitting) Potential by the Zap-GalvanostaircaseMethod." Paper No. 270, Corrosion/86.

28. A. Viebeck and D.W. DeBerry Journal of Electrochem-ical Society, Volume 131, p. 1844 (1984).

29. D.W. DeBerry and A. Viebeck,, Journal of Electro-chemical Society, Volume 133, p. 32,1986).

30. D.W. DeBerry and A. Viebeck. "Inhibition of PittingCorrosion of Type 304L Stainless Steel by SurfaceActive Compounds." Paper No. 196, Corrosion/86.

31. S.T. Hirozawa. "Galvanostaircase Polarization: APowerful Technique for the Investigation of Local-ized Corrosion." PaperNo. 48 at the ElectrochemicalSociety Meeting, Detroit, MI, October, 1982.

32. T.R. Beck. "Experimental Observations and Analy-sis of Hydrodynamnic Effects on Growth of SmallPits." Interim Technical Report, Martch/78-Febru-ary/79, N00014-76-C-0495 (1979).

33. K. Sugimoto, S. Matsuda, Y. Ogiwara, and K.Kitamura"Microscopic Ellipsometric Observationof Change in Passive Film on 18Cr-8Ni StainlessSteel with the Initiation and Growth of Pit." Journalof Electrochemical Society, 132, 8, pp. 1791-1995 (1985).

34. G.H. Koch, N.G. Thompson, and J.M. Spangler."The Effects of SO 2 Scrubber Environments on AlloyCorrosion." EPRI report CS-4697, Electric Power Re-search Institute, Palo Alto, CA (1986).

R-2

_0

0)CL

-0

0

CD

I-4

zCD

:3CC).

4.TCD,

t ale;/ epri - nrc: home page · 3-12 curves for type 317l stainless steel in solution e3 with a...

Documents