CORROSION RATE MEASUREMENTS OF ELECTRODEPOSITED ZINC-NICKEL ALLOY COATINGS

The corrosion rates of detached electrodeposited zinc-nickel alloys co ntaining up to 30% nickel by weight were determined under total immers ion conditions in NaCl solutions and on exposure to neutral salt fog. Weight loss measurements and linear polarisation resistance (LPR) tech niques were employed to determine corrosion rates under both experimen tal conditions. The corrosion rates of zinc-nickel alloys were found t o decrease with increasing alloy nickel content under total immersion conditions in NaCl solutions, indicating that the barrier corrosion re sistance of zinc alloys increases with increasing nickel content. This effect was attributed to both a reduction in the rate of zinc dissolu tion due to nickel enrichment at the metal surface and an increase in the stability of the corrosion product layer. Pure zinc was found to e xhibit a higher corrosion rate in the salt fog environment than in qui escent NaCl solutions and this was thought to result from the higher l evel of available oxygen in the salt fog environment. In contrast, the corrosion Tate of a zinc-14 wt% nickel alloy was lower in the salt fo g environment than under quiescent conditions due it is suggested to a more rapid ennoblement of the surface and the formation of a protecti ve corrosion product layer.