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February 20, 2009

Performance of Plated Aluminum

Because of its position in the electromotive force series, aluminum trends to protect electrolytically most of the other metals that may be used as electroplates. Therefore, once a break develops in the coating, conditions are favorable for corrosive attack. The amount and extent of attack will depend not only on the difference in potential but also on the amount of current that flows. One of the more important characteristics of chromium plating when applied over a nickel deposit on aluminum is its ability to reduce greatly the amount of current flowing from any exposed aluminum areas.

In addition to this, any process involving a layer of zinc between the aluminum and the electrodeposit introduces a specific corrosion problem. In many environment zinc is anodic to aluminum and consequently tends to protect both the aluminum and the electrodeposit. Because this sacrificial corrosion of the zinc tends to undermine the electrodeposit, the best corrosion resistance is obtained with the thinnest practical zinc films on aluminum alloys having the lowest solution potentials.

For this reason, zincate solution containing heavy metals can produce improved corrosion resistance by altering the potential of the immersion film. Other things being equipped the Alstan stannate process should have the capacity for producing excellent corrosion resistance through the complete elimination of the anodic zinc from the immersion deposit.

Although salt spray testing is often considered of questionable value, it does have a definite role as applied to plated aluminum. Grooved samples exposed to the salt spray demonstrate the effect of the zinc immersion film on the resistance to corrosion of the plated part. Atmospheric exposure, of course, is the ultimate controlled test for obtaining data on the outdoor life expectancy of plated aluminum. With conventional copper-nickel-chromium-plated aluminum, applied according to the best practice, a thickness of 38 μm of nickel under the chromium should prevent blistering for at least three years in an industrial atmosphere such as that of Pittsburgh, Pensylvania.

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