Zinc alloy plating

• Published in: Metal finishing Vol. 101; no. 1A; pp. 279 - 284
• Main Authors: Budman, E, Sizelove, R R
• Format: Journal Article
• Language: English
• Published: 01-01-2003
• ISSN: 0026-0576

The application of sacrificial coatings onto steel and other ferrous substrates has long been established as an effective and reliable standard of the industry for corrosion protection. Due to its low cost, zinc has been the predominant coating, although cadmium has also been widely used where zinc fails to provide the necessary corrosion protection for certain applications. Recent demands for higher quality finishes, and, more specifically, longer lasting finishes, have prompted a move to alloy zinc deposits. This has been especially true in the automotive industry, but is also true in the aerospace, fastener, and electrical component fields, among others. Additionally, cadmium users are under increased pressure to stop using it due to its toxic nature. Several different alloy zinc systems have been introduced, giving deposits of somewhat different properties. The differences come not only from the choice of alloying metal, but from the electrolyte system used as well. Much of the recent research work on alloy zinc electroplating processes was done in Europe and in Japan, where cadmium was effectively outlawed during the 1970s. The alloying elements successfully used with zinc have been iron, cobalt, nickel, and tin. Except for the tin, which is typically an alloy of 70% tin and 30% zinc alloy, zinc comprises from 85 to 99% of the alloy deposit. At these compositions, the deposit maintains an anodic potential to steel, yet remains less active than pure zinc. Analogous to conventional zinc, each of the alloys requires a chromate conversion coating to obtain improved corrosion resistance. Indeed, the chromate in this case is more effective on the alloy deposits than on the pure metal.