Electrochemical formation and disintegration of the NiH phase in bright nickel coatings

Abstract

An unstable hydride phase is formed during the cathodic polarization of nickel in acid media containing hydrogenation-enhancing additives. The influence of the hydrogenation conditions in 2 N H 2SO 4 on the formation and decomposition of the hydride phase in electrodeposited bright nickel coatings was investigated by X-ray techniques. Maximum hydrogenation of the nickel layer was achieved at H 2SeO 3 concentrations of about 10–20 mg l −1, a cathodic current density of 15 mA cm −2 and a hydrogenation time of 30 min. The hydrogenation of bright nickel electrodeposits is easier and more pronounced than matt coatings and the conversion of nickel into a nickel hydride phase is almost complete. Bright nickel coatings probably occlude hydrogen in amounts in excess of those needed for the formation of the hydride phase which causes the decomposition of the hydride phase to slow down after interruption of the cathodic polarization. After the reverse diffusion of the occluded hydrogen, the decomposition of the hydride phase follows a first-order equation. The decomposition constant acquires values of the order of 10 −4 – 10 −3 s −1 depending on the hydrogenation conditions.