Influence of boric acid on the electrodepositing process and structures of Ni–W alloy coating

Effects of boric acid introduced into an ammonia-citric plating bath were studied both on the plating process and on the deposit characteristics. It was found that boric acid increased not only current efficiency but also tungsten content of the deposits, implying that boric acid may act as a surfactant to impede the proton reduction and also form some complex with tungstate to be beneficial for tungsten co-deposition. The deposit hardness increased firstly with increasing tungsten content but then decreased as tungsten content was beyond 42 wt.%. The reasons lie mainly in both the Hall–Petch and the reverse Hall–Petch effects rather than solid–solution strengthening when the grain (or domain) size of the Ni–W alloy was in the nanometer scale.