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Metal Dusting of Bolts

Background: A client contacted KnightHawk Materials Lab requesting an urgent analysis of failed bolts found during a turnaround. The bolts had relatively flat fracture surfaces that appeared to have been caused by fatigue cracking, but were severely corroded (shown in Figure 1). In addition, the process was known to have problems with metal dusting.

Figure 1: Photograph of the Fracture Surfaces of the Bolts


Figure 2: SEM Micrograph of Pitting of the Bolt Threads

Summary: The failure of the bolts was the result of metal dusting corrosion which led to the formation of long pits in the threads (Figure 2). Extreme carburization and small surface and sub-surface layers of nearly pure C, as shown by EDS, confirmed the presence of metal dusting. The resulting pits served as stress risers which likely spawned a progressive cracking mechanism (e.g. fatigue). However, due to the extreme corrosion damage to the fracture surface, the presence of such a mechanism could not be definitively confirmed.

Take Away: Metal dusting is a highly damaging corrosion mechanism that occurs in certain extreme environments. Metallurgical analysis of the failed parts can identify the presence of early stages of metal dusting in the material.