Electrochemical corrosion behaviour of innovative mould steels in a chloride-containing environment

Investigations concerning the electrochemical corrosion behaviour of mould steels are scarce and limited. In the present study the corrosion resistance of an innovative mould steel (a microalloyed steel, steel 2), devoted to the fabrication of large size moulds, and a steel having the same composition without microalloying elements (B, Zr, Nb, steel 3), proposed for small size moulds, was assessed in a chloride-containing environment and compared with that of the traditional 1.2738 steel. Potentiodynamic polarization and electrochemical impedance spectroscopy measurements were carried out on the investigated steel samples in a 0.1 M NaCl solution at 25 °C. Scanning electron microscopy and electron probe microanalysis were used to characterize the specimens surface after the electrochemical tests. Despite having a bainitic microstructure, the electrochemical corrosion behaviour of the investigated steels is quite different. Results showed that the steel 3 and the 1.2738 steel samples exhibit the best and the worst electrochemical corrosion behaviours compared to the steel 2 sample. The presence of pearlite is considered as the main reason for the poor corrosion behaviour showed by the 1.2738 steel sample, due to galvanic coupling phenomena between this phase and the bainitic matrix which experiences preferential dissolution. The coarse bainitic microstructure characterizing the steel 2 sample is responsible for its lower corrosion resistance as compared with the steel 3 sample exhibiting a finer bainitic microstructure.