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The effect on corrosion of composition and microstructure in a series of Fe-Cr-10% Ni alloys with increasing chromium content has been investigated in reducing acids. In the range of 25 to 35% chromium there is a wide variation in the relative amounts of ferrite and austenite. In reducing acids (H2SO4 and HCl) there is preferential attack on the ferrite phases. However, the rate of attack on ferrite is considerably greater than expected on the basis of its chromium and nickel contents alone. This is because corrosion on the ferrite phase is significantly increased by galvanic action with the austenite in the microstructure. This galvanic effect is a function of the relative surface areas of the ferrite (anode) and the austenite (cathode).
Previous investigators have derived equations for the effect of relative surface areas of anodes and cathodes on galvanic corrosion of two dissimilar metals in the form of parallel sheets of constant composition. However, in the alloys of this investigation there are simultaneous changes, not only in the areas of ferrite and austenite, but also in the compositions of these two phases. In the analysis of the data on the Fe-Cr-10% Ni alloys, the mathematical treatment developed for relative areas with constant compositions has been used to separate this factor from that of the varying composition of ferrite and austenite. The results may contribute to an understanding of the action of duplex microstructures in castings, weldmetal, and duplex stainless steels when these are exposed to reducing acids during pickling and plant service.
galvanic corrosion, duplex alloys, austenite, ferrite, reducing acids
Bethlehem Steel Corp., Homer Research Laboratories, Bethlehem, PA
University of Delaware, Newark, DE