Stress Corrosion Cracking in Pulp and Paper Systems

Lindsey Goodman, Preet M. Singh School of Materials Science and Engineering & Institute of Paper Science and Technology

Georgia Institute Technology, Atlanta, GA

Overview

• Motivations – Cost of corrosion in P&P

– Changing processes changes in equipment life

• P&P-related Stress corrosion cracking (SCC) projects in our lab – Pulping: SCC of Duplex SS in pulping liquors

– Lignin extraction: SCC of SS in high temp organosolv environments

– Biorefinery: SCC in bioethanol

• Summary

Cost of Corrosion in the Pulp and Paper Industry

50% or more of corrosion costs can be avoided by applying existing scientific knowledge and focused research

Between 1.2 % and 6.0 % of sales

• depending on the product and process

Changes in the P&P Industry Related to Equipment Reliability

• Reduced Water Usage - Closed-Loop System Processes

– Increased corrosion problems

• Increased temperatures and concentrations

• Use of new chemicals like biocides in paper machine area

• Reduced Emissions and Increased Efficiency of Recovery Boilers

– Corrosion in mid and upper furnace

– Superheater corrosion (molten salt corrosion)

• New Pulping Processes

– Increased alkalinity and sulfidity

– Other changes in chemical composition (contaminant concentration)

• Older Mills Converted to Biorefineries

– New corrosion issues with extraction, delignification processes, storage, fuel transportation

Testing for Stress Corrosion Cracking • Simulate corrosive industrial

conditions in lab

– Apply stress in environment

– Room-temp tests

– Tests at elevated-temp & pressure

• Evaluate failed test materials visually for crack density, morphology

Autoclave

Tensile or CT Specimen

Load Cell

Pulp Mill Issues

• Anticipated changes in process chemistry

– Increased sulfide, increased Cl- contamination

– Increase in alkalinity

Duplex Stainless Steels in Pulping Liquors

• Duplex SS has superior corrosion and SCC resistance compared with other austenitic grades

• However: SCC susceptible when Cl- present

• Increased corrosion rate as sulfide increases

S-rich oxide

2 µm

Ferrite

0.5 g/L NaCl

10 g/L NaCl 100 g/L NaCl

2 g/L NaCl

Cyclic Stress Effect on SCC of Duplex in WL

Cyclic loading

110% YS, R = 0.5, 173 cycles

Monotonic loading

0

200

400

600

800

0 10 20 30

Stre

ss (

MP

a)

% Strain

UTS

110%YS

Duplex stainless steel immune to SCC under static load in white liquor • Highly susceptible under cyclic loading in

identical environment (batch digesters)

Biorefinery Issues

• Elevated temperature organosolv

• Room temp fuel grade ethanol

316L SS tested in acidified ethanol (pH 3.62) at 220oC.

SCC of Steels in Mixed-Solvent Extraction Solutions

Alloy-20 sample tested in acidified ethanol (pH 2.25) at 220oC

SCC in Mixed-Solvent Extraction Solutions (2)

• SCC is – Temp dependent

– pH dependent

– Water dependent

SCC is: – Alloy dependent

– Temperature dependent

– pH dependent

– Water dependent

Ethanol Fuel-Related SCC Issues

Cracks

Ethanol Tank

Welds- Air Eliminator Vessel X. Lou et al., Corrosion, 65 (12), (2009), p.785

R.D. Kane et. al., Mater. Perform., 44, (2005), p.50

N. Sridhar et. al., Corrosion, 62 (8), (2006), p.687

American Petroleum Institute survey found many instances of cracks in carbon steel tanks and equipment used in storage and production of ethanol

Background: SCC in Ethanol Fuel

• No SCC in pure ethanol • Common contaminants or

non-ethanol constituents lead to changes in SCC susceptibility – Water – Oxygen – Chloride – Organic acids – Inhibitors

• Stress plays large role in SCC susceptibility in FGE – Crack propagation occurs

post-yield

Objectives of our research: • To better understand

mechanism of SCC of pipeline steel FGE – understand causes and

mitigators of SCC in FGE

Chloride Effects on SCC in Fuel Grade Ethanol

Minor Cl- contamination (few mg/L) major SCC issues

14 0 10 20 30 40 50140 150

0

5

10

15

20

25

30Crack density on samples in SFGE, varied Cl

-

Cra

cks p

er

un

it le

ng

th

Chloride (ppm)

1mm SFGE, 51ppm Cl- SFGE, 150ppm Cl- 1mm 1mm SFGE, 0 Cl-

Mitigation of SCC

SCC in aerated tests

15

1mm

1mm

SFGE, 150ppm Cl-

0 20 40 60 80 100 120 140

40

60

80

100

120

Re

du

ctio

n in

are

a (

%)

Chloride (ppm)

Aerated

Deaerated

Prevention of SCC in deaerated tests, tests with alkaline pHe

Deaerated

1mm

Alkaline

Proposed Mechanism • SFGE composition affects crack length, density

• SFGE studies indicate that SCC in FGE is likely due to anodic dissolution of unpassivated steel at the crack tip1,2

1X. Lou, D.Yang, P.M. Singh, JECS 2010, 157 (2) 2 N. Sridhar et al.,Corrosion 2010, 66 (12)

σ σ

Applied stress σ ruptures film at tip

Crack initiates, solution enters crack, forms passive layer on crack tip

Steel dissolves anodically (crack grows) until film re-

forms

Repassivation &

Repassivation Kinetics

17

10um

0 Cl-

10um

32ppm Cl-

0 100 200 300 400

0.0

5.0x10-4

1.0x10-3

1.5x10-3

2.0x10-3

2.5x10-3

3.0x10-3

Cu

rre

nt

den

sity (

A/c

m2)

T-Tpeak (s)

Baseline

10ppm Cl-

32ppm Cl-

150ppm Cl-

Baseline solution:

Water (1 vol%), methanol (0.5 vol%),

acetic acid (56ppm), Ethanol (bal)

Aerated SFGE

0 20 40 60 80 100 120 140 160

5.0x10-4

1.0x10-3

1.5x10-3

2.0x10-3

2.5x10-3

3.0x10-3

Pe

ak c

urr

ent

den

sity (

A/c

m2)

Chloride (ppm)

Deaerated SFGE

SFGE

Peak current density vs Cl-

Chloride effects on Repassivation Behavior in FGE

Surface Analysis of Corrosion Products

XPS AFM

740 730 720 710 7000

5k

10k

15k

20k

25k

3 3 2

1

Co

un

ts/s

(a

.u.)

Binding Energy (eV)

Backgnd.

SFGE, pHe = 4.31

740 730 720 710 700

10k

20k

30k

40k

50k

60k3

1

Co

un

ts/s

(a

.u.)

Binding Energy (eV)

Backgnd.

Baseline SFGE 3

1

2

SFGE, high pHe (56ppm NaOH)

SFGE, high water content (5 vol% H2O)

SFGE, low pHe (560ppm acetic acid)

Baseline SFGE

Dissolution of air-formed film

Preferential dissolution

Protective salt film

Summary • New process conditions can lead to changes in

SCC severity: – DSS in caustic pulping liquors:

• Sulfide and chloride concentration lead to changes in SCC morphology and severity

– SS and carbon steel in high temp organosolv (ethanol-water): • Temperature and alloy composition affect SCC behavior

– Carbon steel in bioethanol: • Cl- and dissolved oxygen exacerbate SCC • Plastic stress necessary for SCC propagation

• Understanding corrosion processes leads to development of strategies for mitigation and prevention

Thank you.

For more information, please see our web page: http://www.ipst.gatech.edu/research/projects/corrosion.html

Contact Information Preet.Singh@ipst.gatech.edu 404-894 6641 Lindsey.Goodman@gatech.edu

IPST Corrosion Research Group