THE EFFECT OF MICROSTRUCTURE ON THE LOCALIZED CORROSION BEHAVIOR OF AHIGH-STRENGTH MN-CR AUSTENITIC STEEL

Potentiostatic etch tests in chloride and nitrate solutions and in pur e water were performed to examine the mode of localised corrosion in a 18Mn-4Cr-0.6C steel under different microstructural conditions. Adver se stress relief conditions resulted in massive carbide precipitation in the microstructure. Localised corrosion was found to initiate at gr ain boundary triple points and at other heterogeneous sites. The thres hold potential for the prevention of localised corrosion shifted in th e active direction in the presence of carbide precipitates. Carbides w ere found to dissolve in oxygen containing water, uninfluenced by the applied cathodic potential. With massive carbide precipitation, a path is provided by the carbide network for localised corrosion to occur. It is then no longer possible to prevent localised corrosion by cathod ic polarization.