Stress Corrosion Cracking in Pulp and Paper Systems

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1 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

2 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

3 Cost of Corrosion in the Pulp and Paper Industry Between 1.2 % and 6.0 % of sales depending on the product and process 50% or more of corrosion costs can be avoided by applying existing scientific knowledge and focused research

4 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

5 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 Load Cell Autoclave Tensile or CT Specimen

6 Pulp Mill Issues Anticipated changes in process chemistry Increased sulfide, increased Cl - contamination Increase in alkalinity

7 Duplex Stainless Steels in Pulping 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 Liquors 0.5 g/l NaCl 2 g/l NaCl 10 g/l NaCl 100 g/l NaCl S-rich oxide Ferrite 2 µm

8 Stress (MPa) Cyclic Stress Effect on SCC of Duplex in WL Duplex stainless steel immune to SCC under static load in white liquor Highly susceptible under cyclic loading in identical environment (batch digesters) Monotonic loading Cyclic loading 110% YS, R = 0.5, 173 cycles %YS UTS % Strain

9 Biorefinery Issues Elevated temperature organosolv Room temp fuel grade ethanol

10 SCC of Steels in Mixed-Solvent 316L SS tested in acidified ethanol (ph 3.62) at 220 o C. Extraction Solutions Alloy-20 sample tested in acidified ethanol (ph 2.25) at 220 o C

11 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

12 Ethanol Fuel-Related SCC Issues Ethanol Tank American Petroleum Institute survey found many instances of cracks in carbon steel tanks and equipment used in storage and production of ethanol Cracks R.D. Kane et. al., Mater. Perform., 44, (2005), p.50 N. Sridhar et. al., Corrosion, 62 (8), (2006), p.687 Welds- Air Eliminator Vessel X. Lou et al., Corrosion, 65 (12), (2009), p.785

13 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

14 Cracks per unit length Chloride Effects on SCC in Fuel Grade Ethanol Minor Cl - contamination (few mg/l) major SCC issues SFGE, 0 Cl - 1mm SFGE, 51ppm Cl - 1mm SFGE, 150ppm Cl - 1mm Crack density on samples in SFGE, varied Cl Chloride (ppm) 14

15 Reduction in area (%) SCC in aerated tests Mitigation of SCC Prevention of SCC in deaerated tests, tests with alkaline phe SFGE, 150ppm Cl - Deaerated Alkaline 1mm 1mm 1mm 120 Aerated Deaerated Chloride (ppm) 15

16 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 tip 1,2 Applied stress σ ruptures film at tip Crack initiates, solution enters crack, forms passive layer on crack tip σ σ Steel dissolves anodically (crack grows) until film reforms 1 X. Lou, D.Yang, P.M. Singh, JECS 2010, 157 (2) 2 N. Sridhar et al.,corrosion 2010, 66 (12)

17 Peak current density (A/cm 2 ) Current density (A/cm 2 ) Chloride effects on Repassivation Repassivation & Repassivation Kinetics Behavior in FGE 3.0x x x10-3 Aerated SFGE Baseline 10ppm Cl - 32ppm Cl - 150ppm Cl - 0 Cl - 1.5x10-3 Baseline solution: Water (1 vol%), methanol (0.5 vol%), acetic acid (56ppm), Ethanol (bal) 1.0x x ppm Cl - 10um 3.0x x10-3 T-Tpeak (s) Peak current density vs Cl - Deaerated SFGE SFGE 2.0x x x um 5.0x Chloride (ppm) 17

18 Counts/s (a.u.) Counts/s (a.u.) Surface Analysis of Corrosion Products XPS 60k 50k 40k 30k 20k 10k Baseline SFGE Baseline SFGE 3 Backgnd AFM SFGE, high water content (5 vol% H2O) Preferential dissolution Binding Energy (ev) SFGE, low phe (560ppm acetic acid) 25k 20k 15k 10k 5k Backgnd. SFGE, phe = Dissolution of air-formed film SFGE, high phe (56ppm NaOH) Protective salt film Binding Energy (ev)

19 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 (ethanolwater): 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

20 Thank you. For more information, please see our web page: Contact Information IPST Corrosion Research Group