The Electrochemical Behavior of Some Nickel Base and Cobalt Base Alloys

Potentiostatic anodic polarization curves are provided for Hastelloy Alloys B, C, D, F, and G, Haynes Alloy No. 25 and Haynes Stellite Alloy No. 6B in a variety of media. These data demonstrate that, if oxidizers are present in environments in contact with low chromium alloys, the dissolution rate will increase with increasing oxidizer level. The polarization data also show that copper markedly decreases the dissolution of alloys in reducing media. Also, the presence of grain-boundary precipitates increases the dissolution rate of Hastelloy Alloy C in oxidizing, pitting, and hydrochloric acid media while not affecting the dissolution rate in unaerated sulfuric acid. The effective alloy content of an alloy is altered by the precipitation of new phases, according to the data obtained. A 30% chromium alloy, Haynes Stellite Alloy No. 6B, had higher current densities, hence lower corrosion resistance under oxidizing conditions, than a 20% chromium alloy, Haynes Alloy No. 25. This was causes by the removel of much of the matrix chromium content by the precipitation of chromium containing carbides in the Haynes Stellite Alloy No. 6B.