Coating Effect on High Strength Steels

Bare 4340 steel specimens were subjected to three coating systems, No. 1: cadmium electroplating, baking and chromate passivation, No. 2:  No. 1 and chromated epoxy primer application, No. 3:  No. 2 and gloss white polyurethane topcoating.  Bare M54 steel specimens were subjected to Zn-14% Ni  electroplating and baking.  Employing those bare and coated specimens of both steels, their mechanical properties and fatigue and stress corrosion cracking behaviors were evaluated.  For the both steels, the fatigue strength was lower for the coated specimens than for the bare ones in air, but the reverse was observable in 3.5% NaCl solution.  Especially, for the 4340 steel, the SEM fractograph of the coated specimen fatigue-tested in air showed intergranular cracking.  The lower fatigue strength and the intergranular cracking in coated specimen fatigue-tested in air showed evidence of coating-induced hydrogen embrittlement.  The results of SCC and fatigue tests indicated that the coating improved the SCC and fatigue resistance of those steels in 3.5% NaCl solution.