Galvanic Corrosion of Duplex FeCr-10%Ni Alloys in Reducing Acids

The effect on corrosion of composition and microstructure in a series of FeCr-10%Ni alloys with increasing chromium content has been investigated in reducing acids. In the range of 25 to 35% Cr, a wide variation exists in the relative amounts of ferrite and austenite. In reducing acids (H2SO4 and HCl), preferential attack occurs 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 austenite (cathode). Previous investigators have derived equations for the effect of relative surface areas of anodes and cathodes on the galvanic corrosion of two dissimilar metals in the form of parallel sheets of constant composition. However, in the alloys of this investigation, simultaneous changes occur, 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 FeCr-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 understanding the action of duplex microstructures in casting, weld metal, and duplex stainless steels (SSs) when these are exposed to reducing acids during pickling and plant service.