Improvement of Corrosion Fatigue Strength of High Strength Steel by Thermal-Sprayed (WC-Cr-Ni) Cermet Coating

In order to clarify the effect of thermal-sprayed (WC-Cr-Ni) cermet coating on the corrosion fatigue strength of high strength SKD11 steel, rotating bending corrosion fatigue tests were conducted in a 2 mass% ZnSO4 aqueous solution environment. It was found that the corrosion fatigue strength of the (WC-Cr-Ni) cermet thermal-sprayed specimen with 100μm thickness was 318.6MPa at 2×107 cycles in 2 mass% ZnSO4 aqueous solution and was about three times larger than that of the SKD11 base metal at the same number of cycles. The effective thickness of the (WC-Cr-Ni) cermet thermal-sprayed coating to improve the corrosion fatigue strength of SKD11 steel was about 100μm. The principal cause of improvement of the corrosion fatigue strength of steel by thermal-sprayed (WC-Cr-Ni) cermet coating is the insulation of SKD11 steel surface from the permeation of ZnSO4 aqueous soultion. Impregnation of fluorine-contained resin into the thermal-sprayed (WC-Cr-Ni) cermet was ineffective to improve the corrosion fatigue life of SKD11 steel at a high stress region. However, the corrosion fatigue strength at 2×107 cycles was improved by about 22%. In the high stress region of S-N curves, the insulation effect was not enough due to an exfoliation of fluorine-contained resin from the thermal-sprayed (WC-Cr-Ni) cermet coating. Corrosion fatigue crack initiated at a corrosion pit in the SKD11 base metal surface and propagated through the transcrystalline path.