Effect of Diffusion Coating on the Corrosion Resistance of Spring Steel

Abstract

Fatigue fracture of springs especially for automotive suspension is frequently caused by corrosion, from which it is difficult to prevent the spring surface by coating due to peeling off in an early stage. As a substitute for coating, the application of diffusion coating of Al and Ni-Al alloy was brought into experiment.
Diffusion coating layer of about 50μm in thickness was observed after heating the specimens with metal sprayed layer at a temperature of austenite region for 10 minutes. After that, the specimens were quenched into oil as usual, then they were tempered at 450°C to be shot-peened.
The diffusion coating layers were found to consist of intermetallic compounds such as Fe₂Al₅ (η phase) for Al coating and Ni₃Al (γ′ phase) for Ni-Al alloy coating. The hardness of the Al diffusion coating layer was HV 550 and for Ni-Al alloy coating it was HV 600.
When a corrosion test was made under cyclic exposures between salt spray and air for 20 days, the specimens with diffusion layers exhibit no corrosion pit on the surface, but etch-pits of about 0.13mm in depth were found on the surface with no diffusion coating layer. It was also found that the thickness of metal sprayed layers prior to heating is responsible for the amount of residual stress on the surface after shot peening.
But in case that the thickness of metal sprayed layer was restricted to a thickness within 50μm, the effect of shot peening was maintained, giving the specimens a fatigue strength equivalent to that with no diffusion layer. The thickness of 50μm of metal sprayed layer is just enough to be fully absorbed into steel, leaving no scale to interfere in shot peening after heat treatment.