Q. Is zinc/iron alloy plating a good replacement for cadmium? Tests indicate that it performs well in salt spray tests. The lubricity of this deposit differs from that of cadmium, but could that not be overcome by adding a suitable additive? D.M.
A. I have discussed this question a number of times in this column and usually receive one or two questions every month regarding cadmium substitutes. The reality of the situation is this: Cadmium has a good set of properties relative to ease of application, corrosion resistance, lubricity, etc. The major drawbacks are environmental concerns and its toxicity. Surprisingly, not too many years ago the major concern seemed to be the cost of cadmium and not its toxicity. Times have changed, but the same question remains: What is a good replacement for cadmium?
When you start looking for a replacement, the first thing you discover is that none of the substitutes have the same set of properties as cadmium. You have to determine which components of the property set are critical to your particular application. Typically compromises have to be made. Various zinc-containing alloys (zinc/nickel, zinc/cobalt, zinc/iron and tin/zinc) have been suggested as replacements for cadmium.
As you suggested in your e-mail, electroplated zinc/iron does give very good re-sults in terms of corrosion resistance, but it does have problems in the area of lubricity. An alloy-plated deposit that seems to have better lubricity properties is the tin/zinc alloy.
The bottom line is this: One substitute cannot be used for every application in which cadmium has been used in the past. You can find out a lot more informa-tion about cadmium substitutes by Googling the term “cadmium plating substi-tutes.”