Corrosion Assessment of Sn-Ni Alloy Coatings using Neutral Salt Spray Tests and Electrochemical Methods

To elucidate the excellent anti-corrosion properties of a Sn-Ni alloy compared to those of a pure Sn coating, the accelerated corrosion behavior of an as-deposited Sn-Ni alloy coating from a pyrophosphate bath was investigated by salt spray tests and electrochemical methods in a neutral mist of 5% NaCl solution. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), potentiodynamic and electrochemical impedance spectroscopy (EIS) techniques were used to study the changes in the physical and electrochemical properties of the Sn-Ni alloy coating during salt spray testing. A thin compact passive film consisting of metal oxides (Sn and Ni) formed on the surface of the as-deposited Sn-Ni alloy coating, resulting in an improved corrosion resistance in the neutral mist compared to that of a pure Sn coating. The initial passive film was destroyed, and the metallic metals (Sn and Ni) were dissolved and formed oxides during the initial stage of salt spray testing. The composition of the oxide film on the surface of Sn-Ni alloy varied with the duration of the salt spraying. Finally, a compact passive film consisting of Ni(OH)(2), SnO and SnO2 was produced and remained stable in the neutral solution when the salt spray testing time reached 72 h. The regeneration of a compact passive film on the surface of Sn-Ni alloy demonstrated excellent resistance to electrochemical corrosion of the Sn-Ni alloy in the neutral chloride-containing solution. The good corrosion resistance of the Sn-Ni alloy supports the application of Sn-Ni alloys in neutral solutions.