What are the typical crack characteristics of polyoxysulfuric acid corrosion?
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Considering the application context of austenitic stainless steel equipment in sulfur-containing refining processes mentioned earlier, the typical crack characteristics caused by polyphosphoric acid corrosion are as follows: In terms of propagation mode, sensitized austenitic stainless steel is characterized primarily by intergranular cracking; whereas in the case of un-sensitized materials or when chloride ions are present in the medium, a mixed type of cracking involving both intergranular and transgranular patterns occurs. The cracks can penetrate the wall thickness rapidly, within a period of minutes to hours. Macroscopically, cracks generally originate from pits on the inner wall; there is no significant plastic deformation, and the cracks are distributed in a dendritic/radiating pattern. They are concentrated in the heat-affected zone of the weld and in areas with high stress. Some of these cracks are through-going, and dark brown sulfide corrosion products often remain around them. Microscopic characteristics: The cracks extend along the grain boundaries, with minimal uniform corrosion loss in the thickness direction. Sulfur elements become significantly concentrated at the crack tips and grain boundaries, forming active corrosion pathways.
The original poster has provided a very accurate summary; the grain-edge cracking characteristic caused by poly-sulfuric acid corrosion is indeed typical in sensitized stainless steels, especially in the heat-affected zones of welds and in areas under high stress. I encountered a similar case before; I’d like to share a few of my personal experiences for reference: This type of corrosion is highly sensitive to the pH and temperature of the environment. It is recommended to pay attention to temperature fluctuations during periods when the equipment is shut down or operating at low load, as the slightly acidic environment created by condensate water can accelerate crack propagation. If branch-like cracks are observed in the macroscopic morphology, it is advisable to first check whether the passivation film on the inner wall of the equipment has been damaged, such as due to hammering during maintenance or residual welding slag. Microscopically, a distinct sulfur-rich characteristic is evident; the composition of sulfides can be confirmed using EDS or XRD, but care must be taken to distinguish it from mixed cracks caused by chloride stress corrosion. Additionally, the above is merely based on experience; a precise diagnosis requires taking into account material testing and site conditions. It is recommended to consult a corrosion engineer or refer to NACE standards.