Corrosion–fatigue properties of a 4340 steel coated with Colmonoy 88 alloy, applied by HVOF thermal spray

The corrosion–fatigue behavior of a quenched and tempered AISI 4340 steel has been evaluated under two different conditions: (a) uncoated; and (b) grit-blasted with alumina and coated with a thermal-sprayed Colmonoy 88 alloy (220 μm in thickness), employing a high-velocity oxygen fuel (HVOF) gun. The tests were conducted under rotating bending conditions employing a 4-wt.% NaCl solution. The results indicated that the fatigue behavior of the coated material under this condition is very similar to that previously reported after testing in air. The fatigue cracks were nucleated at the alumina particles deposited in the matrix of the substrate steel during blasting rather than at corrosion pits formed during testing. Therefore, the corrosion–fatigue strength of the coated substrate has been found to be controlled by the same mechanism that governs the fatigue behavior of the material in air. The fatigue strength of the uncoated substrate tested in the NaCl solution has also been found to be significantly less than that in air and that, if the substrate steel is coated with the Colmonoy alloy, its corrosion–fatigue life increases substantially. The microscopic observation of the fracture surfaces also showed that under some alternating stress conditions, the substrate–deposit interface can be severely cracked giving rise to the detachment of the deposit from the substrate steel. The fatigue performance of the material under the two conditions analyzed has been quantified by determining the Basquin parameters from the fatigue life curves obtained.