A fire occurred inside a three-story stainless steel chamber for capturing airborne particulates. To determine the extent of damage to the stainless steel material, an investigation was carried out to characterize the condition of the wall and ceiling materials. The goal of the evaluation was to determine what material required replacement or repair, and what material could remain in service. Thus, the evaluation could only be nondestructive.
The investigation included visual inspection and in situ microstructure evaluation. The chamber was examined on-site to select locations for in situ metallographic examination. At the selected locations of the chamber wall and ceiling, local areas of the surface were polished using in situ metallographic preparation methods. The prepared areas were examined on-site with a field microscope, and replicas were prepared for further examination in the laboratory.
The fire exposed the chamber to elevated temperatures in a carbon-rich environment. Austenitic stainless steels may become sensitized due to exposure to these conditions, so a test procedure was designed to examine the material microstructure for the effects of sensitization.
The test for intergranular corrosion susceptibility due to sensitization examines the microstructure of the material. Ditches at the material’s grain boundaries indicate potential increased susceptibility to intergranular corrosion. The microstructure of the stainless steel at several locations within the chamber had grain boundary ditches. Thus, the wall and ceiling material at these locations would have increased susceptibility to corrosion when the system was put back into service. The areas with sensitization were marked for replacement. The stainless steel surface in other areas of the chamber was refinished and remained in service.
Sensitization can occur in austenitic stainless steels from elevated-temperature exposure at temperatures between about 800 and 1600°F due to the formation of chromium-rich carbides at the material grain boundaries. The formation of chromium-rich carbides reduces the chromium content in the adjacent stainless steel material, which reduces the ability for stainless steel to form a protective chromium-rich oxide on the surface within a thin band along the grain boundary. Since a chromium-rich oxide is what gives stainless steel its corrosion-resistant properties, sensitized stainless steel is more susceptible to corrosion at the material grain boundaries.
Sensitization is more likely to occur if the elevated-temperature environment contains carbon, either as a gaseous constituent or a foreign material deposit on the metal surface. The carbon that forms the carbides can also be from the stainless steel itself if the carbon content of the metal is sufficiently high.
MEE’s evaluation notified the customer that several locations inside their chamber would have increased susceptibility to corrosion. The fire provided the perfect conditions (heat and carbon) for sensitization to occur. This assessment of the material’s condition allowed the customer to make an informed decision regarding the amount of material to be replaced, so that the chamber could be returned to service at minimal cost.