Hydrogen Absorption into Electroplated Nickel Films and Formation of the Compressive Stress

We investigated the relation between the contents of hydrogen absorbed and compressive stress in nickel deposits during electrolysis of H2O corresponding to electric charges of 0.20 C at cathodic current density of 2 mA/cm2. Using an electric balance, the amounts of hydrogen absorbed in nickel deposits and evolved during electrolysis of H2O were evaluated quantitatively based on the buoyancy produced by the hydrogen gas bubble. Variation of the internal strain with the absorption and desorption of hydrogen was also measured in situ using a resistance wire-type strain gauge placed on the reverse side of the copper substrate. Compressive strain with absorption of hydrogen occurred immediately after electrolysis of H2O began. Then it relaxed spontaneously with desorption of hydrogen from the nickel deposits when current was off. However, the buoyancy caused by the hydrogen gas bubble started after the elapsed time of a quarter a minute or more from application of electrolysis of H2O. Gas bubble formation continued for a while after the current was turned off. The former resulted from hydrogen absorption into nickel deposits. The latter was associated with hydrogen leaking out of the deposits. The variation of compressive stress with increasing hydrogen contents was described as δσ/δn (Hab), which was ca. -28 kgfmm-2 mol%-1 in nickel deposits electroplated from a Watts-type bath containing brightener 2-butyne-1,4-diol at electric charges of 10 C/cm2 and current density of 30-120 mA/cm2.