The production and properties of zinc-nickel and zinc-nickel-manganese electroplate
The aim of this research proj ect was to produce compositionally modulated zincnickel and zinc-nickel-manganese coatings usi ng a single bath process by variation of the applied electroplating current density. These could then be considered as possible replacements for electroplated cadmium. Zinc-nickel electrodeposits from both a simple bath and one containing the complexant tris(hydroxymethyl)methylamine (TRIS) were produced using either bath or by selective electroplating onto mild steel, 2014-T6 aluminium and 2000 series aluminium alloy connector shells. Zinc-nickelmanganese coatings were electroplated onto mild steel, using both DC and pulse electroplating from sulphate based baths containing either the complexant TRIS or sodium citrate. The coatings and the various electrodeposition processes were evaluated by thickness measurements, cun'ent efficiency calculations, composition detennination using energy dispersive X-ray anal ys is and SEM to examine the morphology. Corrosion characteristics were investigated by neutral salt fog exposure, alternate immersion, atmospheric trials, and electrochemical immersion tests including potential monitoring, galvanic and polarisation measurements. Other properties such as microhardness and surface electrical conductivity were also investigated. Compositionally modulated zinc-nickel and ziJlc-nickel-manganese were successfully deposited from single baths with nickel contents ranging from 5-20% Ni and manganese contents varying from 0-12% Mn. Current efficiency measurements showed the efficiency to be low especially for the zinc-nickel-manganese coatings, which exhibited a distinct morphology when electroplated from the citrate bath. Corrosion testing indicated that both coatings had a lower corrosion resistance than cadmium especially in accelerated tests, with a reduction in corrosion resistance being seen as the nickel content was increased; due to a decrease in density of the electroplate. This, and increasing through-thickness porosity promoted ennoblement of zinc-nickel and zinc-nickel-manganese leading to the coatings becoming nonsacri ficial to the substrate in accelerated tests. Zinc-nickel-manganese showed less white rust than zinc-nickel and exhibited lligher micro-hardness than zinc, cadmium and zinc-nickel. Electrical measurements indicated that both coatings were sufficiently conductive to meet the surface conductivity requirements for use on electrical connector shells.