The appearance of Ni-W-P alloy plating from Ni-tungstate-citrate bath was examined by a Hull cell test as a function of the phosphorous acid concentration of the bath. The effects of P content on the corrosion resistance of bright Ni-W-P alloy plating ware examined by CASS and SST, and then their corrosion behaviors were investigated by measuring the rest potentials in CASS and SST solutions, and by XPS analysis. The main findings obtained were as follows :
(1) Mirror bright Ni-W-P alloy plating was obtained over the wide range of plating current density from the Ni-tungstate-citrate bath containing phosphorous acid of more than 0.05M.
(2) In the CASS test, bright Ni-W-P alloy plating of P content higher than 7at% performed good discoloration resistance; however, the plating of P content less than 3.5at% discolored to black. On the otherhand, bright Ni-W-P plating of any P content showed interference color on the surface of the plating by the SST of 2000hr exposure, resulting in good rust-resistive property of the Ni-W-P plating layered on Ni plated steel.
(3) The discoloration of Ni-W-P plating of P content less than 3.5at% from mirror bright to black by CASS test was revealed to be by conversion deposition of copper accompanied with oxidation of the W element in the Ni-W-P plating. On the other hand, good discoloration resistance achieved with the higher P content Ni-W-P plating was because of the existence of the concentrated phosphorus layer under the Ni oxide layer on top of the alloy surface.