How to judge whether equipment is corroded by polythionic acid
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Combined with the background information such as the occurrence conditions and corrosion characteristics of polythionic acid corrosion mentioned previously, the following methods can be used to determine whether the equipment is suffering from this type of corrosion.: 1. Inspection of macroscopic appearance characteristics Corrosion mostly occurs in the heat-affected zone of welds, elbows, pipes and other places where residual stress is concentrated. Dense corrosion holes can be seen on the inner wall of the equipment, and dark brown sulfide corrosion products remain. The crack propagates extremely fast and may penetrate the wall thickness in a short time. There is no obvious plastic deformation on the fracture surface, showing typical brittle cracking characteristics. 2. Professional testing means Penetration Testing (PT): The fine intergranular cracks with surface openings can be directly detected. Surface dirt needs to be ground and cleaned before detection to avoid dirt clogging the cracks and affecting detection sensitivity. Metallographic testing: By sampling and observing the microstructure, the intergranular cracking morphology of cracks extending along the grain boundaries can be clearly seen, which is a typical microscopic feature of polythionic acid corrosion. Media composition detection: Ion chromatography is used to detect polythionate in the sediment on the inner wall of the equipment or in the residual medium. If the target substance is detected, it can prove the existence of a corrosive environment. 3. Scenario conditions assist in determining whether the equipment is sensitized 300 series austenitic stainless steel, and has experienced shutdown maintenance, exposure to air and moisture, and meets the three necessary conditions of "sensitized material + polythionic acid generation + tensile stress", which can further assist in confirming the corrosion type.
The content of the post is very detailed, and the author’s method for judging polythionic acid corrosion is well summarized, especially the detail of cleaning up scale before penetrant testing, which many people tend to overlook. Add a little bit of your own experience: If there are no conditions for metallography or PT on site, you can first use magnetic particle testing (MT) to check for intergranular cracks on the surface of ferromagnetic materials. However, it should be noted that polythionic acid corrosion products may interfere with magnetic particle display, so it is best to clean them first. Before testing, it is recommended to confirm whether the equipment has been in contact with sulfur-containing media or experienced high-temperature working conditions. This is an important prerequisite for polythionic acid corrosion, otherwise it may be easily misjudged as other types of stress corrosion. In actual operation, it is recommended to focus on inspecting the heat-affected zone of the weld and the elbow of the nozzle, especially equipment that has been shut down for maintenance, which is more prone to such rapid cracking. If dark brown corrosion products are found, try to do a component analysis to confirm. However, the specific testing plan must be determined based on the equipment material, service history and on-site safety conditions. It is best to consult your unit’s corrosion engineer or testing agency. The risk of doing it yourself is higher.