What are the preventive and control measures against polyoxysulfate corrosion?
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Considering the application context of austenitic stainless steel equipment in sulfur-containing refining processes mentioned earlier, the prevention and control of peroxymonosulfuric acid corrosion can be achieved through three aspects: material selection, process operation, and maintenance management. The specific measures are as follows: First, optimize material selection by giving priority to sensitization-resistant austenitic stainless steels such as 304L and 316L, which have ultra-low carbon content (C≤0.03%), or stainless steel grades from the 321 and 347 series that contain stabilizing elements like Ti and Nb. Dual-phase stainless steels and modified versions such as 347AP, which are specifically designed for corrosion resistance, can also be used; these materials help to prevent the formation of chromium-deficient regions at grain boundaries, thereby reducing susceptibility to corrosion cracking. II. Shutdown and Operational Control: After the unit is shut down, the austenitic stainless steel equipment/pipelines that do not require maintenance are sealed with blind flanges, and a slight positive pressure of nitrogen is applied to exclude air ； If the system temperature is below 38°C, anhydrous ammonia at a concentration of about 5000 ppm must be injected to prevent the formation of liquid water. During operation and shutdown phases, maintain the equipment temperature above 149°C, always keeping it above the dew point temperature to keep the equipment surface dry and disrupt the water environment required for the formation of polyoxysulfates. III. Special protection during maintenance: For equipment that needs to be disassembled for maintenance, it should be rinsed with a soda ash (sodium carbonate) solution before being exposed to air, in order to completely neutralize the polyoxosulfates that have formed ； After rinsing, wash with clean water containing low levels of chlorides and dry it, to avoid alkali residue causing alkali embrittlement and to prevent the introduction of chlorides from affecting the activity of subsequent catalysts. Furthermore, during the manufacturing phase, welding residual stresses can be eliminated through solution treatment and stabilization heat treatment; by avoiding structural designs with stress concentration, the risk of corrosion cracking can be further reduced.
The original poster has summarized it very well, covering the key points from three aspects: material, operation, and maintenance. I would like to add two points based on my experience in practical work: First, during the washing process after shutting down the equipment, if the alkaline cleaning is not thorough, residual sulfides can create an acidic environment on the surface of the equipment, which can also lead to polysulfate stress corrosion. It is recommended to maintain a pH level of greater than 9 during the washing process, and to monitor the wastewater until it reaches a neutral pH before concluding the process ； Secondly, during maintenance, if cracks are detected in the welds or heat-affected zones, PT (penetrant testing) should be used first to confirm their presence, in order to avoid exacerbating the damage through grinding. Welding for repair should also be used with caution, as the heat input may cause re-sensitization. These are the lessons we learned on site; everyone can use them as a reference, but the actual implementation still needs to take into account the operating conditions of the equipment and the material grades.