pH 6のクエン酸浴によるNi-W合金めっきの組成と電流効率および皮膜特性

抄録

A high temperature bath of pH8 to 9, continuously supplied with ammonia water, has long been believed optimum for tungsten alloy plating. However, since ammonia is volatile and harmful to both working personnel and peripheral devices, we have formulated a citric acid-ammonia Ni-W alloy plating bath of pH6, which is free of ammonia smell even on boiling. On the basis of the data on alloy composition and current efficiency, we selected a bath temperature of 70°C as optimum. Baths of high nickel concentrations of pH6, 70°C yielded alloys of high tungsten content with relatively high current efficiencies, eg.44wt%W alloy at 49%CCE. The highest tungsten content obtained was 61wt%, which corresponds to Ni₂W stoichiometry. The tungsten content of the deposits did not show any abrupt change on variation of nickel concentration in the bath, and remained almost constant at current densities exceeding 8A/dm². The room temperature hardness of 44 and 51wt%W alloys was about Hv 1350 after heating at 650°C for 1h. Dilation measurements showed that the alloys were more densely deposited than those plated at high temperature/pH conditions. On the basis of these encouraging findings, studies on metal ion replenishment were also conducted. The methods recommended are the addition of ammonium para-tungstate solution for tungsten, and the use of a soluble nickel anode for nickel.