Effect of different thermochemical treatments on theelectrochemical behaviour of a hot working steel

The X 37CrMoV5 1 KU hot working steel has been quenched and tempered, overcarburised, nitrided and nitrocarburised. The surface layers constituted after the thermochemical treatments were characterised by means of X-ray diffraction (XRD) analyses, optical and scanning electron microscope (SEM) observations, micro- and macrohardness indentations. The electrochemical behaviour was evaluated in two different aggressive environments: 0.1 M Na2SO4 and 0.1 M NaCl solutions. While the overcarburised and the nitrocarburised samples exhibit a poor electrochemical behaviour, the presence of a nitrided surface layer enhances the corrosion resistance. This is attributable to the protective action of the nitrided layer that hinders the anodic dissolution reaction of the matrix, whereas the presence of complex carbides in the overcarburised or nitrocarburised samples causes the preferential dissolution of the ferritic matrix due to galvanic coupling phenomena. Finally, the poor corrosion resistance exhibited by the nitrocarburised sample can be ascribed to the presence of a large amount of ferrite together with little e solid solution in the surface layer.