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-----What type of plating is this?
Quickstart:
Most metals are silvery colored, including aluminum, cadmium, chrome, magnesium, nickel, silver, steel, stainless steel, tin, and zinc (gold is of course yellow, and copper is orangish). When we see metal of another color, it is almost always because something else was put onto it.
Zinc and cadmium plating were commonly used for corrosion-proofing back in the day, but cadmium is a biocumulative poison similar to mercury and lead, and is used only in exceptional cases these days. Both zinc & cadmium are prone to rapidly suffering "white rust" so they are invariably "chromate conversion coated". These chromate conversion coatings may be clear, slightly bluish, yellow, green, or black.
Also today most chromating of zinc is done with trivalent chromates rather than the toxic hexavalent chromates of the old days.
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Q. What type of plating is this? This is a spring from c. 1939. Guessing either cad or zinc dichromate.
I don't know if they had Type II cad in 39, or if it would have been used on springs. Though I don't know if they had zinc dichromate either.
William Novalany
- Pittsburgh, PA
April 16, 2026
A. Hi William,
My guess is that it's cadmium plus green chromate because there was no concern about cadmium's environmental effects back then, little difference in cost, and cadmium probably would have been specified because it is more resistant to salt water, is easier to plate onto hardened steel, offers better ductility, and its corrosion products aren't gummy. Still, it's only a guess.
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Ted Mooney, P.E. RET
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finishing.com - Pine Beach, New Jersey
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Q. Hi Ted, thanks for the insight. I would agree.
What is the difference between having a green chromate finish and the regular Type II finish with the faint gold cast? Any practical difference to have the green vs. regular?
Engineer - Pittsburgh
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A. Hi again,
In the old days of hexavalent chromating, olive drab was the heaviest and most corrosion resistant. With most of today's trivalent chromates any coloration is just dye. Most chromates are proprietary, with a trade secret ingredient list, so it's hard to say exactly what made it green rather than just the natural golden color of chromic acid, perhaps a percentage of trivalent chromium, perhaps dye.
I don't know whether this spring is supposedly 80 years old or a brand new replacement. If it's new it's probably zinc plating with trivalent chromate dyed green or even a CED coating
⇦ huh? rather than plating.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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Thanks Ted, very helpful response. That spring is original; I don't believe they are reproduced.
Side note, when we reproduce metal parts, we almost never get them plated as it's expensive for a small production lot, and adds little value for cars that never see the elements.
If reproduced, that would be made from spring steel, but for general hardware items we often use stainless steel as the material cost is insignificant compared to set-up/tooling costs for low production. Of course, no need to plate stainless for a consumer application anyway.
- Pittsburgh
April 21, 2026
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