Materials Performance

DEC 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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18 DECEMBER 2018 W W W.MATERIALSPERFORMANCE.COM MATERIAL MATTERS Fabrication Practices to Mitigate Risks of Corrosion Cracking in Renery Units A high percentage of mainte- nance costs in a petroleum ref inery are due to failures caused by corrosion. The indi- rect costs of corrosion—unit shut downs, loss of productivity, and environmental pollution—can be even one order higher than the direct corrosion cost. Addition- ally, corrosion failures can be associated with accidents where the cost cannot be estimated. According to Jigneshkumar J. Desai and Ashik S. Murthy with Fluor Daniel India Private, Ltd. (Gurgaon, Haryana, India), and Cathleen Shargay with Fluor Enterprises (A liso Viejo, California, USA), research has shown that about 10 to 40% of corrosion costs can be avoided with appropriate engineering during the design phase and corrosion management during operation. Many process streams in petroleum ref ineries with an aqueous phase contain enough hydrogen sulf ide (H 2 S) to cause corrosion or cracking in susceptible materials. The most common ty pes of cracking due to wet H 2 S are sulf ide stress cracking (SSC), hydrogen-induced crack- ing (HIC), and stress-oriented hydrogen- induced cracking (SOHIC). Additionally, amine, caustic, and wet carbonate streams can cause stress corrosion crack- ing (SCC), depending on concentrations, temperatures, and other variables. Some services have a risk of wet H 2 S damage and SCC if multiple corrodents are present. Desai, Murthy, and Shargay empha- size that adopting best engineering prac- tices at the engineering design and con- struction stages is an essential step for controlling corrosion and corrosion- related cracking in a ref inery. In a paper presented at CORROSION 2018, 1 they describe several cracking-related damage mechanisms and discuss control mea- sures to mitigate corrosion and cracking. SSC is def ined as metal cracking under the combined action of tensile stress and corrosion in the presence of water and H 2 S. SSC is a form of hydrogen stress cracking that results from the absorption of atomic hydrogen produced by the sulf ide corrosion reaction on the metal surface. SSC in ref ining equipment is affected by the complex interaction of factors such as steel composition, mate- rial strength (as indicated by the hard- ness and microstructure of the material exposed to the sour environment), weld material and heat treatment, total tensile stress (applied and residual), hydrogen f lux generated in the material due to sur- face corrosion, temperature (which pri- marily affects corrosion rates), and time. The primary step for avoiding SSC is to apply hardness limits to the base materi- als and welds. HIC can occur in steel exposed to wet H 2 S conditions. This form of attack ty pi- cally occurs in steel with relatively soft microstructures and commonly appears as blisters and/or stepwise cracking. HIC is caused by atomic hydrogen that dif- fuses into the steel and is trapped in internal defects such as laminations or nonmetallic inclusions (e.g., man- ganese(II) sulf ide [MnS]), where it recom- bines to form molecular hydrogen (H 2 ). Because H 2 has a considerably greater vol- ume than atomic hydrogen, it cannot dif- fuse through the steel, so pressure builds in the metal until cracking and deforma- tion (blistering) of the steel occur. Impor- tant factors for avoiding HIC are purity and homogeneity of the steel (i.e., mini- mizing laminations and elongated non- metallic inclusions). SOHIC is characterized by a stack of small HIC blisters that are connected by cracks that travel through the thickness of the metal. The cracks are driven by a high level of either applied or residual stress that is perpendicular to the metal surface. Since residual stresses are high- est in weld heat-affected zones (H AZ), one of the most important mitigation steps is postweld heat treatment (PW HT). Since SOHIC grows through the metal's thickness, it reduces the load bearing capacity of the component. Caustic SCC appears as intergranular cracking on carbon steel (CS) under the combined action of tensile stress and

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