Metallurgical and Surface Influence on the Corrosion Resistance of Ni-Cr-Mo Alloys According to ASTM G 28

The influence of surface condition, chemical composition, and microstructure of Ni-Cr-Mo alloys has been investigated using the ASTM G 28 standard for intergranular corrosion testing. Experiments were carried out mainly using the Ni-based alloys C-22 (UNS N06022) and C-276 (UNS N10276). Repeatabilities of both testing procedures of ASTM G 28, Method A and B, were found to be in most cases ≤3%. A finer surface finish yields lower corrosion rates. The corrosion rate of electropolished specimens is about 5% to 20% lower than for ground specimens. Sandblasted and pickled specimens show a 1.3 to 4 times higher corrosion rate than ground specimens because of the crevice corrosion at laminations and crevices. Method B yields a better differentiation between specimens with different surface finishes than Method A because of the less-aggressive test conditions. In addition to the ratio Cr/(Mo+W), which has already been found to be an indicator of corrosion resistance of these materials, the elements Mo and W were found to segregate to a certain extent, and tended to form intermetallic phases and carbides resulting in a drop in corrosion resistance by a factor of up to 2. The effect of microstructure is distinguished better by Method A because of its higher uniform general corrosion rate. In this case, Method B exhibits too-mild testing conditions.