Coatings to reduce hydrogen environment embrittlement of 304 austenitic stainless steel

Electroplated Zn, Ni, Cu, Al, PVD-Ti-DLC and electroless NiP coatings as well as carbon, nitrogen and oxygen diffusion layers were investigated for their suitability to reduce hydrogen environment embrittlement (HEE) of 304 austenitic stainless steel. The mechanical stability of the coatings was evaluated by interrupted slow strain rate tensile testing. The Zn, Ni, Ti-DLC and NiP coatings as well as the oxygen diffusion layer cracked or delaminated at very low strains. The Cu coating was too thin and showed poorly coated areas. HEE of the underlying bulk material was not improved by any of these coatings because embrittlement always started at such coating imperfections. Al coatings showed a high ductility but could not reduce HEE of the underlying material due to a columnar structure at which the H2 gas could get in direct contact to the substrate material. The carbon and nitrogen diffusion layers could not eliminate HEE of the 304 steel entirely but crack propagation was reduced in these layers.