Effect of the pH value of the hydrogenation solution upon the phase transformation of nickel into nickel hydride

The effect of pH value of the hydrogenation solution, containing H2SeO3 as a hydrogenation-enhancing additive upon the phase transformation of nickel into P-nickel hydride in electrodeposited bright nickel coatings during cathodic charging was studied using X-ray diffraction. The nickel hydride formation terminates at pH value over 2,1, At this pH value in the bulk of hydrogenation solution, the pH Value within the layer in the vicinity of the nickel electrode shift during the cathodic process to the alkaline region as a result of hydrogen evolution. An indication of the alkalization is the shift of the nickel electrode potential towards more negative values (- 1.3 V NHE) within the range of pH values where phase transformation terminates. The hydrogenation enhancing effect of H2SeO3 appears to occur when the deposition of selenium on nickel surface begins. Strong hydrogenation of samples with co-deposited selenium proceeds during cathodic polarization in an acidic medium without an enhancing additive. (C) 2001 Elsevier Science Ltd. All rights reserved.