Corrosion Mechanism of Zn-Ni Alloy Electrodeposited Coatings

Structure of Electrodeposited Zn–Mn Alloy Coatings

“…[1][2][3][4][5][6][7] For electrodeposited Zn-Fe alloy, single h Zn phase is observed in the range below 17 mass% Fe, which is much higher than the solid solubility of Fe in thermally equilibrium h Zn phase. G like phase is observed over about 20 mass% Fe and a Fe phase which contains Zn atom exist in Fe rich region.…”

“…2) For electrodeposited Zn-Ni alloy, h phase and g phase are found in the Zn rich region but d phase in the thermal equilibrium Zn-Ni binary alloy are not observed. [3][4][5] The g phase of the electrodeposited ZnNi alloy exists as a nonequilibrium phase having a similar crystal structure to the thermal equilibrium phase and the g phase of the electrodeposited Zn-Ni alloy is transformed to the thermal equilibrium phase by the heating in the temperature range above 180°C. 6) Moreover, gЈ phase which does not exist as a thermal equilibrium phase is found in electrodeposited Zn-Cr alloy.…”

The new look of sheet steels.

“…It has been suggested that the corrosion products from dezincification become trapped in these microcracks, preventing the transfer of the corrodant to the base steel. 17,21,22) It has also been proposed that the corrosion product that forms in zinc-nickel alloys is zinc hydroxide, Zn(OH)2, which in itself is effective in suppressing the oxygen reduction rate. 22) Thus, although zinc-l3 % nickel electrodeposited alloy initially has a simple, single-phase structure, the corrosion protection mechanism is complex, involving the reduced activity of the coating, the gradual change of the coating from sacrificial to barrier protection and secondary barrier protection provided by the corrosion product, possibly by entrapment within microcracks in the coating that form during corrosion.…”

“…Thus as corrosion proceeds in NaCI solutions the coating gradually becomes richer in nickel and, hence, more electropositive. 17,21) An additional protection mechanism is that the residual stresses in the coating become strongly tensile during dezincification, leading to the formation of microcracks in the coating. It has been suggested that the corrosion products from dezincification become trapped in these microcracks, preventing the transfer of the corrodant to the base steel.…”

“…[17][18][19][20]23) At the optimum composition of about 13 % nickel, these coatings consist only of a single phase-the gamma phase of the zinc-nickel system. In a NaCI solution, this phase is anodic to steel, but to a lesser degree than zinc19)--in other words it is less active than zinc.…”