Blackening failure mechanism of anti-corrosion Cd coating on 30CrMnSiA high-strength steel bolts

Given the susceptibility to surface blackening of electrodeposited Cd-coated 30CrMnSiA high-strength steel bolts in humid environments, elucidating the underlying darkening mechanism has become a pivotal research direction for enhancing their corrosion resistance and ensuring long-term service performance. This paper systematically investigates the blackening mechanism of Cd-coated 30CrMnSiA high-strength bolts, characterising both the surface blackening characteristics and corrosion products. The ammonia immersion method was employed to treat the 30CrMnSiA high-strength steel bolts to investigate the failure mechanism underlying surface blackening. Furthermore, the underlying corrosion mechanism within the cadmium coating was thoroughly investigated. The results indicate that localised stress concentration and residual stresses arising from the Cd electroplating and Cr passivation processes created surface defects, thereby promoting the oxidation of Cd and the formation of black cadmium oxide (CdO). Furthermore, corrosive media penetrate through micro-cracks into the coating-substrate interface, establishing galvanic corrosion cells that accelerate degradation and ultimately compromise the corrosion resistance of 30CrMnSiA high-strength bolts. This paper provides a fundamental scientific basis for exploring the corrosion mechanisms of Cd-coated, thereby offering considerable engineering implications for enhancing the corrosion resistance of Cd-coated fasteners in marine environments.