Effect of Cr on the corrosion resistance of a V plus Nb-microalloyed medium-carbon Cr-Mo high-strength bolt steel

The atmospheric corrosion behavior of V+Nb-microalloyed medium-carbon high-strength bolt steels with different Cr contents (0.54-2.09 wt%) was explored by dry/wet cyclic corrosion test combined with corrosion product characterization and electrochemical measurement. The results show that increasing Cr content decreases the corrosion rate and the corrosion pit depth. The decrease of corrosion rate is more significant with 2.09 wt% Cr and at the early corrosion stage, which is mainly attributed to the shift of the self-corrosion open potential of the bare steel to the positive direction. At a later corrosion stage, the increase of Cr content promoted the transformation of gamma-FeOOH to alpha-FeOOH, the formation of Cr-doped alpha-(Fe,Cr)OOH and FeCr2O4 as well as the generation of Cr oxides (Cr2O3 and Cr(OH)3) in the rust layer and Cr-enrichment in the inner rust layer. All these are beneficial to generate a more stable, compact and protective corrosion product layer, and thus enhance the corrosion resistance of the tested steels.