Localized corrosion behaviour of powder metallurgy processed cobalt base alloy Stellite-6 in chloride environments

In the present investigation, electrochemical corrosion techniques (cyclic potentiodynamic and potentiostatic) were used to study the localized corros ion (pitting/crevice) behaviour of a cobalt-base alloy, Stellite-6. This al loy was produced by two different powder metallurgy (P/M) processing routes , namely, hot isostatic pressing (HIP) and wet powder pouring (WPP). The be haviour of the wrought alloy was also investigated for comparison. Corrosion tests were performed in neutral 3 % NaCl solution at ambient temp erature. The results showed that the HIP material possessed the highest res istance to localized corrosion. This was explained in relation to the proce ssing parameters and the microstructure of the alloy. The results of runs conducted in neutral test solutions showed that the cri tical crevice temperature, CCT, is certainly above the test temperature (ro om temperature). However, lowering the test solution pH (to 3 and 1.5) led to the onset of crevice corrosion. It was clear that the severity of the at tack depends on the electrochemical technique applied as well as on the pH of the test solution. The electrochemical findings were further supplemente d by surface examinations (SEM and EDX) of the corroded specimens.