How to determine whether a device has been corroded by polyoxysulfuric acid
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Given the background of austenitic stainless steel equipment used in sulfur-containing refining processes mentioned earlier, it is possible to determine whether such equipment has suffered from peroxysulfuric acid corrosion by considering the following factors: investigation of the scenario and conditions – determining whether the equipment is part of sulfur-containing refining units such as hydrodesulfurization units, whether it has been exposed to air and water vapor during shutdown and maintenance periods, and whether the material of the equipment is a sensitized 300 series austenitic stainless steel with residual welding stress; these factors constitute the three necessary conditions for peroxysulfuric acid corrosion to occur. During the macroscopic visual inspection, special attention is paid to areas prone to stress concentration such as the heat-affected zone of welds, bent pipes, and fittings. It is checked for the presence of numerous pitting lesions and brittle cracks without significant plastic deformation; some of these cracks can be through-going, and dark brown corrosion products often remain on the surface. Professional testing and verification: Penetrant testing can quickly detect surface open cracks, while metallographic examination allows the identification of typical crack propagation along grain boundaries. Scanning electron microscopy analysis of the fracture surface helps confirm the characteristics of grain-boundary corrosion fracture, thereby ruling out the influence of other types of stress corrosion.
The content shared by the original poster is highly professional; it explains in detail everything from the operating conditions of the equipment and its visual characteristics to the testing methods, making it very useful as a reference for those working in the refining industry. I would like to add a few details that are easily overlooked during actual inspections: the identification of corrosion products on site – dark brown products are generally Fe₂O₃·xH₂O, but if sulfides are present, they may appear black or gray-black. It can be simply tested with a magnet: the corrosion products of polysulfuric acid are usually weakly magnetic, whereas ordinary rust is more magnetic. Of course, the most accurate method is to take samples for spectroscopic analysis or XRD. Limitations of penetrant testing: Although PT can quickly detect open cracks, it is prone to missing fine cracks or closed cracks (such as those in compressive stress areas). It is recommended to use ultrasonic testing (UT) or phased array testing on high-risk areas such as the heat-affected zone of welds, especially in pipes with greater thicknesses, to avoid missed defects. Protection reminders during shutdown: Corrosion is highly likely to occur when three conditions are met (sensitized stainless steel + sulfides + oxygen/water). During maintenance, it is essential to control the dew point, use nitrogen protection, or apply alkaline cleaning for neutralization. If the corrosion products are accidentally removed, the cracks may be covered up; therefore, it is best to perform a PT inspection before cleaning. The above is merely based on experience; a final judgment should take into account the material of the equipment, its service history, as well as third-party inspection reports. If necessary, it is advisable to consult an expert in material failure analysis for further review.