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Sensitization and Intergranular Corrosion of a Nozzle Pipe

Background: A nozzle pipe with welds to flanges at either end experienced a leak during a routine maintenance check before use. The nozzle was constructed of 304 stainless steel and was located on a tank that saw intermittent use and was then cleaned with water and left empty until it’s next use. Due to this practice, the client thought that the cause of the leak might be chloride SCC, and contracted KML to perform a failure analysis on the nozzle.

Figure 1: Photograph (Left) and Optical Micrograph (Right) of the Failed Nozzle Pipe Showing the Sensitization of the Microstructure, Pitting, and Intergranular Corrosion

Summary: The failure of the nozzle was the result of sensitization of the stainless steel microstructure and subsequent intergranular corrosion, resulting in deep pitting and through thickness corrosion of the nozzle pipe material. Sensitization of stainless steel refers to the precipitation of chromium carbides on the grain boundaries and occurs at temperatures between approximately 970 and 1470 oF (example shown in Figure 1). The presence of these carbides allows for intergranular corrosion of the steel, resulting in pitting and eventual failure of the material, as can be seen in Figure 1. This intergranular corrosion and cracking can occur in environments where stainless steel would not normally be subject to corrosion.

The sensitization of the microstructure in this case was caused by improper welding procedures. This can be seen by the location of the pitting, which occurred adjacent to the welds, and also by the microstructural changes and changes in the surface coloration of the steel near the weld areas, as highlighted in Figure 1.

Take Away: Improper welding or heat-treating procedures in stainless steels can cause sensitization of the microstructure that greatly increases the susceptibility of the steel to corrosion, resulting in corrosion failures in environments where stainless steel would not normally be subject to corrosion.