5 passivity

CH.4. CH.4. PPASSIVITYASSIVITY

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PassivityPassivityA state of low corrosion rate due to the formation of a very thin surface films (1~10 nm) under oxidizing conditions with high anodic polarizationASTM G15 P i th t t f t l f h t i d b l

yy

ASTM G15: Passive – the state of a metal surface characterized by low corrosion rates in a potential region that is strongly oxidizing for the metal• Thermodynamically unstable, but possess a kinetic stabilityThermodynamically unstable, but possess a kinetic stability• Fe, Ni, Cr, Al, Ti, Zr, Nb and their alloys.

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PassivityPassivityDefinition 1• A metal is passive if, on

increasing its potential to more

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increasing its potential to more positive values, the rate of dissolution decreases, exhibiting low rates at high potentials (thin-film passivity)(thin-film passivity)

Definition 2• A metal is passive if it

substantially resists corrosion in an environment where there is a large thermodynamic driving force for its oxidation (thick-film passivity)

• Ex) Pb in acid, Mg or Al in H2O.

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Pourbaix Diagram for FePourbaix Diagram for Fegg

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Image Source: http://www.crct.polymtl.ca/ephweb.php

Pourbaix Diagram for CrPourbaix Diagram for Crgg

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Pourbaix Diagram for AlPourbaix Diagram for Algg

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Pourbaix Diagram for TiPourbaix Diagram for Tigg

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PassivityPassivityyy

NOBLEV) transpassive

O2

MxOy

TEN

TIAL

( transpassiveM

MxOy

PPLI

ED P

OT

passive

+

MxOy

M

Epp

EF

AP active

log CURRENT DENSITYip ic

H2 M +

MACTIVE

Epp

log CURRENT DENSITY

Epp = primary passive potentialic = critical anodic current densityE Fl d i l

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EF = Flade potentialip = passive current density

ActiveActive--Passive Corrosion BehaviorPassive Corrosion Behavior

Fe3+

E

Transpassive

2H2O→O2+4H++4e-

Fe2O3

Fe2+

E

Passive

Eppeipass

2H++2e-→H2

Fe3O4

Active dissolution

M→M++e-

icorri M /M

io,H+/H2(Fe)

eeq,H

Ecorr

eeq,M

Fe

5 pH

io,M+/M

log |i|Epp ≅ EM+Z/MxOy

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PassivityPassivityyy

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Anodic polarization curves for iron and 304L stainless steel in 1N H2SO4

ActiveActive--Passive Corrosion BehaviorPassive Corrosion Behavior

Effect of increasing acid concentration and temperature on passivity.

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Corrosion Behavior of ActiveCorrosion Behavior of Active--Passive MetalsPassive Metals

Case 1 :Point A → Ecorr & icorr

h d b d blHigh corrosion rate, not good but predictable.ex) Ti in HCl or H2SO4

Case 2 :f3 points of intersection at B, C, and D.

point C is unstable.point B → high icorr in the active region.point D → low icorr = ip in the passive region.the least desirable case.ex) Cr in air free H2SO4, Fe in dilute nitric acid.

Case 3 : One stable point E in the passive region.The metal will spontaneously be passivated.Most desirable system.ex) stainless steels and Ti in acid solutions containing oxidizers.

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) g

The nature and kinetics of the cathodic reactions (are) critical in determining the corrosion state and the rate of dissolution of an active-passive materials.

Effect of Oxidizer ConcentrationEffect of Oxidizer Concentration

Hysterisis in Fig 4 6 indicates that the amount of oxidizer necessary toHysterisis in Fig. 4.6 indicates that the amount of oxidizer necessary to cause passivation is greater than that required to maintain passivity. To safely maintain passivity, oxidizer concentration should be greater than the minimum amount necessary to produce spontaneous passivation.

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In the “borderline passivity” region (GFEB), Any surface disturbance will destabilize the passive film and the corrosion rate increases.

Effect of Oxidizer ConcentrationEffect of Oxidizer Concentration

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Effect of Solution VelocityEffect of Solution Velocityyy

Corrosion of stainless steel in a dilute acid solution such as seawater• The smaller the critical anodic current density (icrit), the easier a metal will be

passivated by an increase in solution velocity.

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p y y

Criteria for Passivation: Lower icrit and more active Epp is desirable.

Summary Summary -- PassivationPassivationyy

When a stable passive film has formed, The rate of corrosion will be critically When a stable passive film has formed, the current has a steady, low value -

the passive current density

affected by the cathodic curveRapid rate of cathodic reaction leads

to passivation, and low rate of corrosion

Pote

ntia

l

Active corrosion gives normal activation polarization

Current falls as the passive film starts to form the active passive transition

Lower rate of cathodic reaction leads to activity, and high rate of corrosion

But it may also lead to low rate of corrosion?Very slow cathodic reaction leads to

low rate of corrosion

lect

rode

P polarization to form - the active-passive transition

log |current density|

E

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Polarization Behavior of ActivePolarization Behavior of Active--Passive AlloysPassive Alloys

(1) Galvanostatic (定電流) polarization curveGalvanostatic methods are not adequateGalvanostatic methods are not adequate for determining the active - passive behavior ; Above icrit, iapp no longer follows the anodic curve in the passive region, but jumps into the transpassiveregion, but jumps into the transpassive region and oxygen is evolved from the metal surface.

(2) Potentiostatic (定電位) polarization curveUsing a potentiostat, potential is increased from Ecorr in the active state in t ith t d d t th d fsteps with current recorded at the end of

an appropriate time at each step. At potentials above Epp, the potentiostatic anodic polarization curve exactly follows the passive loop of anodic curve.

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the passive loop of anodic curve.(e.g., a step rate of 50 mV every 5 min)


Potentiodynamic polarization curve (動電位分極曲線)• Potentiostat is programmed to increase potential continuously from Ecorr to p g p y corr

desired potential. (e.g., 0.6 V/h)• Most anodic polarization curve are now obtained potentiodynamically.

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Image Source: ASTM G5-94


E

Anodic Current

Ecorr

Cathodic (Current) Loop

Anodic Current

ioM/M+ ioM/M+

log |i| log |i|

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Activation-controlled Active-passive transitionActive peak for ironTranspassive corrosion of Oxygen Overall anodic curve

E dissolutionActive passive transitionActive peak for ironCrreductionOverall anodic curve

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log |i|


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Application of Potentiostatic (Application of Potentiostatic (PotiodynamicPotiodynamic) ) Anodic Polarization TechniqueAnodic Polarization Technique

To judge the corrosion resistance of alloys and the corrosivity of solutions.

Anodic Polarization TechniqueAnodic Polarization Technique

solutions.• The result can be obtained in a few hours.

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Alloy EvaluationAlloy Evaluationyy

C

D

B

C

3

A

1

2

DCBA

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Solution CorrosivitySolution Corrosivityyy

• Dependence of Epit on [Cl-]

Epit = A log [Cl-] + B

Epit = 0.168 - 0.88 log [Cl-] (304 stainless steel)Epit= 0.05 - 0.124 log [Cl-] (Al)• Minimum conc. of [Cl-] necessary for pitting

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LimitationLimitation

The growth and dissolution of the passive film is dynamic (Fig. 4.13)Polarization curve cannot be used as a quantitative guideline forPolarization curve cannot be used as a quantitative guideline for long-term corrosion resistancePlant conditions are difficult to characterize and duplicate in the laboratorylaboratory.

Provide useful preliminary guidelines for more extensive in-plant exposure testsexposure tests.

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Using Passivity to Control CorrosionUsing Passivity to Control Corrosion1. A current can be applied by means of a device (a potentiostat) which can set and control

the potential at a value greater than the passivating potential Ep. This method of producing passivity is called anodic protection

g yg y

producing passivity is called anodic protection.2. For environments containing the damaging species such as Cl- that cause local corrosion,

the potentiostat or other devices that control the potential can be used as in 1 above to set the potential to a value in the passive region below the critical potential for pitting Epit.

3 All t l th t t l f i fil f l t i l t l3. Alloys or metals that spontaneously form a passive film, for example, stainless steels, nickel, or titanium alloys, can be used in applications that require resistance to corrosion. Usually a pretreatment such as that described in tactic 4 below is desirable.

4. A surface pretreatment can be carried out on an alloy capable of being passivated. The use of such a pretreatment has been as a standard practice for stainless steels for manyuse of such a pretreatment has been as a standard practice for stainless steels for many years. The passivating procedure involves immersion of thoroughly degreased stainless steel parts in a nitric acid solution followed by a thorough rinsing in clean hot water. The most popular solution and conditions of operation for passivating stainless steel is a 30-min immersion in a 20 vol % nitric acid solution operating at 1200F (490C) Howevermin immersion in a 20 vol % nitric acid solution operating at 120 F (49 C). However, other solutions and treatments may be used, depending on the type of stainless being treated.

5. The environment can be modified to produce a passive surface. Oxidizing agents such as chromate and concentrated nitric acid are examples of passivating solutions that maintain

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chromate and concentrated nitric acid are examples of passivating solutions that maintain a passive state on some metals and alloys.

Alloy SelectionAlloy Selectionyy

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Anodic ProtectionAnodic Protection

Application of DC power to maintain the metal showing “active-passive” transition behavior in a passive state.passive transition behavior in a passive state.

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Anodic ProtectionAnodic Protectionte

ntia

lPo

t

Current Density (log i)

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Any situation in which a active-passive behavior is a potential candidate for anodic protection if a corrosion problems arises.candidate for anodic protection if a corrosion problems arises.

Mild steel or SS in sulfuric acid (conc. H2SO4) storage tanks, and to a lesser extent coolers and pipinga lesser extent, coolers and piping.Mild steel in certain fertilizer solutionsMild steel liquor tanks in pulp and paper industries.

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Combination of CP and AP (Acid Cooler)Combination of CP and AP (Acid Cooler)( )( )

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Alloy EvaluationAlloy Evaluationyy

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Effect of Alloying ElementsEffect of Alloying Elementsy gy g

Ni

S, Mn, Al

Cr

Cr, Ni, W, Ti

Mo, VNi, Mo, Cu,

Cr, Ti

Ni, Mo, Cu

Cr

S, Mn

Cr, Ni, Mo, Cu, Nb, Ti

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Properties of Passive FilmsProperties of Passive FilmsppOxide film theory

The passive film is always a diffusion-barrier layer of reaction products, for example, metal oxide or other compound which separates metal from its environment and which slow down the rate of reactionreaction.

According to this model, the following processes are driven by the electric field across the film.Entry of metal atoms into the film as cations at the metal-film interfaceTransport of the metal cations or of oxygen anions through the oxide.Dissolution of metal cations from the film at the film/environment interface.

Properties required for protective passive filmStability over a wide potential rangeMechanical integrityMechanical integrityLow ionic conductivityGood electron conductivity to reduce the potential difference across the filmLow solubility and slow dissolution in the aqueous medium.

d i hAdsorption theory A chemisorbed layer of oxygen displaces the normally adsorbed H2O molecules and slows down the rate of anodic dissolution involving hydration of metal ions. Adsorbed oxygen decreases the exchange current density.

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M + (H2O)ads → M(O2- adsorbed anion) + 2H+

ReferencesReferences

Textbook (D.A. Jones)R.G. Kelly, J.R. Scully, D.W. Schoesmith and R.G. Buchheit, ElectrochemicalR.G. Kelly, J.R. Scully, D.W. Schoesmith and R.G. Buchheit, Electrochemical Techniques in Corrosion Science and Engineering, Marcel Dekker Inc., New York, 2002.M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill, 1987.Homepage for Prof. Kwon’s Laboratory, http://corrosion.kaist.ac.krLecture Notes by Dr. D.H. Lister, and Dr. W. Cook, Department of Chemical Engineering, University of New Brunswick.J.I. Munro and W.W. Shim, Mater. Perf., May (2001) 104.P.R. Roberge, Handbook of Corrosion Engineering, McGraw-Hill, New York, 2000.ステンレス鋼の選び方使び方 JIS使方シリズ日本規格協會 1994ステンレス鋼の選び方·使び方, JIS使い方シリ-ズ,日本規格協會, 1994.

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Homework ProblemsHomework Problems

Problems 1, 2, 3, 5, 9, 11 of Chapter 4 in textbook.

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5 passivity

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