Stress Corrosion Cracking in Pulp and Paper Systems
Transcript of Stress Corrosion Cracking in Pulp and Paper Systems
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)
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 [email protected] 404-894 6641 [email protected]
IPST Corrosion Research Group