Development of a New Electrodeposition Process for Plating of Zn-Ni-X (X = Cd, P) Alloys: I. Corrosion Characteristics of Zn-Ni-Cd Ternary Alloys

Cadmium has been used extensively as a corrosion resistant coating in aerospace, electrical, and fastener industries owing to its excellent corrosion resistance and engineering attributes. 1 Cadmium deposition is done from cyanide baths, which are subject to stringent regulations. 2 Alternate baths for cadmium plating are also undesirable due to the toxicity of the metal and its salts. 3 Further during cadmium deposition, large amounts of hydrogen are introduced into the underlying metal. 4 This increases the risk of hydrogen embrittlement failure in the structure. Hence environmental concerns and performance criteria mandate the search for alternatives to cadmium coatings. 5 Various zinc and zinc alloy coatings show promise in this regard. Electrodeposited zinc has been widely used for the protection of steel from corrosion. These electrogalvanized coatings have been shown to be more effective when alloyed with metals such as nickel, iron, and cobalt. Zinc-nickel alloy coatings have been suggested in the literature as replacement for cadmium coating because this alloy provides good corrosion protection on steel, 6-8 superior formability, and improved weldability. 9-11 Zn-Ni alloys containing 15-20 wt % nickel has been shown to possess four times more corrosion resistance than cadmium-titanium deposit. 12 However, due to the high zinc content in the deposit, these alloys are more negative than cadmium and hence dissolve rapidly in corrosive environments. Although Ni is a more noble metal than Zn, the codeposition of Zn-Ni is anomalous and a higher percent of Zn is present in the final deposit. The mechanism for this preferential deposition has been discussed extensively in the literature. 13,14 Typical nickel composition in the alloy is approximately 510%, and any further increase in nickel composition is based on using a higher-than-predicted Ni/Zn ratio in the bath. 15,16 An enhancement in the nickel composition would lead to more anodic open-circuit potential, which in turn will reduce the driving force for the galvanic corrosion. Also the barrier properties associated with nickel-rich deposits are superior compared to other coatings. Several researchers have attempted to decrease the anomaly and increase the nickel content by either introducing inert species in the bath or by developing a ternary alloy. 17-21 Nonyl phenyl polyethylene oxide (NPPO) has been used to reduce this anomaly and to produce uniform deposits. 20,21 NPPO inhibited zinc electrodeposition