Hydrogen induced surface cracking of two orthopedic implant alloys

Electrolytic charging of hydrogen, at room temperature and in the absence of externally applied stress, induced surface cracking in 316 stainless steel and cobalt based ZIMALOY. Hot Isostatic Pressed (H.I.P.) ZIMALOY showed less susceptibility to surface cracking than 316 stainless steel samples. The susceptibility of 316 stainless steel to surface cracking was determined with samples in the High Energy Rate Forged (HERF), the sensitized, the annealed, and the annealed and sensitized conditions. Investigations showed that surface cracking typically occurred at specific microstructural features. Hence, the relative susceptibilities of twin boundaries, slip bands, grain boundaries, and heavily sensitized regions was established.

It was observed that twin boundaries crack most readily in non-sensitized samples, and that both grain boundaries and twin boundaries crack easily in sensitized structures. These observations, coupled with the similarity between hydrogen embrittlement and failure of orthopedic implants, suggest that orthopedic applications should use H.I.P. ZIMALOY in preference to 316 stainless steel whenever possible, and that when the use of 316 stainless steel is unavoidable, HERFed parts should be used. Further investigations are recommended to better assess the hydrogen compatibility of sensitized 316 stainless steel, and to determine the influence of sensitization on the suitability of 316 stainless steel for orthopedic application.