Intergranular corrosion of TiNb-microalloyed martensitic stainless steels processed by quenching and partitioning

This study investigates the localised corrosion mechanisms in laboratory-processed Q&P-treated martensitic stainless steels. Two steel variants, one NbTi-free (alloy B) and the other micro-alloyed with Nb and Ti (alloy M) were investigated to elucidate the influence of microalloying on corrosion behaviour. Both NbTi-free and NbTi-micro alloyed martensitic stainless steels were examined using a combination of electrochemical methods (potentiodynamic polarisation and double-loop electrochemical potentiokinetic reactivation) and microstructural analysis (Transmission Electron Microscopy and scanning Kelvin probe force microscopy). Potentiodynamic polarisation results showed no significant differences between the alloys and no clear evidence of pitting corrosion. Optical analysis of the specimens showed preferential attack at grain boundaries. Double-loop electrochemical potentiokinetic reactivation measurements revealed a higher degree of sensitisation to intergranular corrosion in the microalloyed steel compared to the NbTi-free variant. Transmission Electron Microscopy showed that intergranular corrosion in both steels originated from chromium depletion zones adjacent to chromium carbides along grain boundaries. The increased susceptibility in the microalloyed steel was linked to the presence of TiN(Nb) particles. Scanning Kelvin probe force microscopy further revealed variations in surface potential at grain boundary precipitates and depleted zones, emphasising their role in intergranular corrosion initiation. These findings emphasise the critical influence of processing routes on the corrosion mechanisms of Q&P-treated martensitic stainless steel.