Material characteristics of Ni–P–B electrodeposits obtained from a sulfamate solution

Material properties of Ni–P–B alloy electrodeposits obtained from a Ni sulfamate bath were investigated as a function of the contents of the P and B sources (H3PO3 and the dimethyl amine borane complex (DMAB), respectively) with/without additives of 35 mg/L and the duty cycle by analyzing the chemical composition, microstructure, microhardness results, residual stress and stress–strain curves. SCC (stress corrosion cracking) resistance for a Ni–P–B electrodeposit was assessed by using the C-ring test. It was found that P and B are incorporated competitively during an electrodeposition and the sulfur from an additive is codeposited into the electrodeposit. As the contents of the alloying element sources of P and B increased, the crystallinity and the grain size of the electrodeposit decreased. The effect of boron on the crystallinity and grain size was also relatively larger than the phosphorus. It was explained by a restraining force against a grain growth and an adsorption of DMAB. Introduction of an additive into the bath retarded the crystallization and grain growth, which may be attributed to a change of the grain growth kinetics by the additive adsorbed on the substrate and electrodeposit surfaces during an electrodeposition. Hardness change with the heat treatment temperature was explained by a NiPx and NiBy precipitation which effectively impeded a grain growth and a precipitate coarsening leading to a recrystallization and grain growth. Moreover it was found that grain size was refined leading to a hardness increase with an increase of the P and B contents, which is in good agreement with the Hall–Petch relation. With an increase of the duty cycle, the grain size increased leading to a decrease in the yield strength and tensile strength. This could be described by the relatively faster kinetics of the hydrogen reduction reaction. A Ni–P–B electrodeposit formed inside a tube by using an anode assembly showed an excellent SCC resistance suggesting that an electrodeposition inside a tube can be used as a SCC mitigation method.