Tailorable deposition of Sn Ni alloy from a pyrophosphate bath with an adjustable Sn:Ni molar ratio

Tailorable deposition was carried out by varying the Sn:Ni molar ratio of a pyrophosphate bath to adjust the deposition rate as well as the phase composition, morphology and corrosion resistance of the as-deposited SnNi alloy. The nickel content of the resultant SnNi alloy decreases gradually from 50 wt% to 28 wt% when the Sn:Ni molar ratio of electrolytes increases from 0.2 to 2.0 and remains reasonably constant at 31–37 wt% when the ratio is in the range of 0.6–1.8. Codeposition of a reasonable content of Ni with tin refines the crystal grains of the alloy and produces a compact coating. The phase composition of the SnNi alloy deposition changes along with the Sn:Ni molar ratio, and alloys with low nickel content are composed of Ni4Sn, Ni3Sn2 or Ni3Sn4 phases. A single-phase homogeneous intermetallic compound of Ni4Sn appears in alloys containing 31–37 wt% nickel. Compared to pure deposited Sn, the electrochemical measurements reveal that the SnNi alloys containing single-phase Ni4Sn have a smaller corrosion current density, smaller blunt current and larger electrochemical charge transfer resistance in 10% H2SO4 solution, indicating that the nearly equiatomic alloy SnNi exhibits excellent corrosion resistance in sulfuric acid solution.