What are the characteristics of stress corrosion cracking in polydisulfate?
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Considering the background regarding the corrosion of austenitic stainless steel in refining units mentioned earlier, the characteristics of stress corrosion cracking due to polyoxysulfuric acid can be divided into macroscopic and microscopic categories. First, the macroscopic characteristics: it occurs frequently during the shutdown/maintenance phases of sulfur-containing refining units such as those involved in hydrogenation processes, and is most common in areas where residual stresses are concentrated, such as the heat-affected zones of welds, bent pipes, and heat exchanger tubes. The crack propagation rate is extremely fast; it can penetrate the component along the wall thickness within a few minutes to a few hours. There is no significant plastic deformation at the fracture surface, indicating a typical brittle fracture. Cracking originates from pits on the inner wall of the equipment; pits are densely distributed near the welds, and dark brown corrosion products often remain on the surface. II. Microscopic characteristics: The vast majority of cases involve grain-boundary (intergranular) cracking, with cracks propagating along the grain boundaries of sensitized stainless steel; mixed-type cracking, consisting of both grain-boundary and transgranular cracking, occurs only in special media containing chloride ions. The crack originates from an erosion pit on the inner wall, and its propagation path is step-like overall, with no distinct branching characteristics. It occurs only in sensitized 300 series austenitic stainless steels, alloys such as 600/800, etc.; low-carbon L-grade stainless steels have a significantly lower susceptibility to cracking.
Thank you to the original poster for organizing the characteristics of sulfated hydrogen stress corrosion cracking in such a systematic way; this is extremely useful for those who work on the maintenance of petrochemical equipment. I would like to add two points of experience from actual engineering projects: the shutdown period is indeed a time when cracks occur frequently, especially when the equipment is not cooled thoroughly or when no effective nitrogen protection is in place, as the reaction between water vapor and corrosion products of sulfides makes it easier for cracks to form. It is recommended to carefully inspect stress concentration areas such as welds and bent pipes before stopping work, and use PT (penetrant testing) if necessary to assist in the inspection. The “dark brown corrosion products” mentioned among the macroscopic characteristics are quite typical, but care should be taken to distinguish them from those resulting from high-temperature sulfidation corrosion (which are usually black or blue-black). If phase analysis is available on-site to confirm that it is intergranular cracking, then SCC caused by hydrosulfuric acid can be essentially identified. Additionally, it’s worth mentioning that preventive measures (such as alkaline washing for neutralization and controlling humidity during downtime) are also very important. If the original poster has any experience in this area, they’re welcome to share it as well~