Effect of Mo content on microstructure and corrosion behavior in HCl of ultra-high strength maraging steels

Maraging steels are one of the highest strength steels commercially available. The new versions are known as ultra-high strength maraging (UHSM) steels and can offer yield strength values over 3 GPa. The aim of this investigation was evaluating the electrochemical behavior of three non-commercial/laboratory UHSM steels containing 13Ni–15Co and Mo content variations (7.5, 11 and 15 wt%). The UHSM steels were annealed at 1000 °C and studied at different conditions (non-aged and aged at 480 °C for 3 h and 6 h). As reference material, a commercial maraging steel (18Ni–9Co–5Mo) heat treated according to typical recommendations (annealed at 820 °C for 1 h and aged at 480 °C for 4 h) was used. The UHSM presented considerable higher hardness than commercial maraging. Comparing between the UHSM steels, the samples treated for 6 h do not contribute to increase the hardness when compared to those of 3 h. The electrochemical tests in hydrochloric aqueous solution showed that commercial maraging steel and the UHSM containing most Mo (11 wt%) aged by 6 h presented the worst corrosion resistance while the best performance was obtained by the samples with 7.5 and 11 wt% Mo aged for 3 h.