Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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electroplating solutions ZINC ALLOY PLATING BY EDWARD BUDMAN, AESF FELLOW (RET.), BENSALEM, PA., TOSHIAKI MURAI, PRESIDENT, DIPSOL OF AMERICA, LIVONIA, MICH., AND JOSEPH CAHILL, VICE PRESIDENT, DIPSOL OF AMERICA, LIVONIA, MICH. 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 cor- rosion protection. Due to its lower 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 result- ed in a move toward alloy zinc electrodeposits. This has been especially true in the automotive industry, but is also true in the aerospace, fastener, and electrical com- ponent fields, among others. Additionally, cadmium users are under increased pres- sure to stop using it due to its toxic nature. Several different alloy zinc systems are available, giving deposits of some- what different properties (Fig. 1). 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 83 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 require a conversion coat- ing to obtain improved corrosion resistance. Indeed, the passivate in this case is more effective on the alloy deposits than on the pure metal. ZINC-NICKEL Several electroplated processes have been invented since the zinc-copper alloy was developed in 1841. The merits of alloy plating are as follows: 1. New phases that did not exist on metallography phase diagrams can be achieved. 2. Homogeneous alloy compositions not attainable through standard melting methods, because low melting point metal vaporizes at the higher melting point temperature metal. 3. Thin film coating deposits can provide high performance Features • Corrodes sacrificially to steel Figure 1: Corrosion performance in neutral salt spray (NSS). 261 • Stability of corrosion by-products

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