1987 Volume 73 Issue 14 Pages 1730-1737
The corrosion behavior of electroplatings of Zn-Co, Zn-Fe and Zn-Ni alloys containing about 10% alloying elements in pH 6 solutions was investigated. At the biginning of immersion, corrosion rate of the Zn alloy platings was larger than that of pure Zn plating, because the alloying elements (Co, Fe and Ni) promoted cathodic reaction. As corrosion proceeded, corrosion potential of the Zn alloy platings in pH 6 buffer solutions shifted to noble potential in a complex manner. The shift of the potential is characterized by the following corrosion process. In the early stage of the corrosion, a corrosion product with low electrical conductivity (Zn(OH)2) is formed on the surface of the alloy platings by the dissolution of Zn. The development of corrosion induces the formation of corrosion groove reaching to the underlying steel and the increase of relative content of the alloying elements to Zn in the corrosion products and/or on the alloy plating layers. As the area of exposed underlying steel and the relative content of the alloying elements increase, the power of galvanic protection of the plating decreases, leading to corrosion of the underlying steel.
When the solution contains NaCl, a compound ZnCl2·4Zn(OH)2 is formed as a corrosion product. The qualitative nature of corrosion process, however, does not change by the addition of NaCl to the solution.