Fatigue Fracture Caused by Corrosion Crevices

This article reports on the failure analysis of a pump shaft made of Z30C13 martensitic stainless steel (equivalent to X30Cr13, Material No. 1.4028). It was shown by means of fractography and metallographic investigation that a fatigue crack had started at a corrosion scar, the corrosion being caused by crevices in the design and construction. In addition it was shown that the steel-maker has neglected to carry out the necessary heat treatment (hardening and annealing) required to achieve adequate corrosion resistance for this material. The shaft had been left in the fully annealed condition in which the chromium content of the matrix is reduced below the limit of 12% required to ensure corrosion resistance. As a result of this, the passive protective layer normally occurring on the surface of such steels was unable to form in a stable form, thus allowing localised corrosion attack to take place. This type of attack (crevice corrosion) results in a localised stress build-up, and finally to the failure of the component by a mechanism of fatigue.