Is Zinc Plating with Chromate Conversion Coating RoHS compliant?

Hi, Sudhir. Dichromate is Cr₂O₇^-2. The chromium is in hexavalent state and is forbidden by RoHS. However, RoHS-compliant trivalent chromium conversion coatings are widely available.

Regards,

A. Hi John. I should probably take it from the top because common usage in this area unfortunately isn't scientifically accurate :-)

In actuality 'chromate' is CrO₄^-2 and 'dichromate' is Cr₂O₇^-2. But a natural equilibrium takes place between the two anions: in an alkaline solution you will predominantly have chromate anions, recognizable by their bright yellow color; and in an acid solution you will predominantly have dichromate anions, recognizable by their orange color. When you appreciate that the valence of oxygen is -2, it should become apparent that in what we've discussed so far, the valance of the chromium ion has to be +6, or hexavalent.

Now into the semantics and (wrong) common usages ...

First off, chromate conversion coatings are always acidic, and would therefore be predominantly dichromate, and should probably actually be called dichromate conversion coatings, rather than chromate conversion coatings. Enter common usage #1: many if not most zinc plating lines long tried to accommodate customer demand by offering the choice of a 'clear' vs. a 'colored chromate' ... and plating shop employees & managers often grew into the habit of calling the clear or pale tank the chromate tank, and the darker one the dichromate despite there being little basis for it.

Then came RoHS pressures, requiring the elimination of these hexavalent dichromate coatings. What was generally substituted was conversion coatings where the chrome ions are in +3, or trivalent, oxidation state. Since the chromium is at a +3 oxidation state, the formula obviously cannot be CrO₄^-2 nor Cr₂O₇^-2, and therefore is not actually 'chromate' nor 'dichromate'. These formulations are proprietary, and I'm not sure whether they are predominately CrO₂^-2 and should be called chromous acid or hydroxy(oxo)chromium, or exactly what they are ... but people call them chromates out of their similarity to the conventional hexavalent dichromates they are replacing. A better name is probably trivalent chromium conversion coatings.

Since the early days of trivalent chromium conversion coatings there has been discussion of whether they can or do produce hexavalent chromium in use and I'm not scientist enough to offer a good answer; rather, I'll say that you will comply with RoHS standards if you use trivalent chromium conversion coatings and you won't if you use hexavalent conversion coatings.

Luck & Regards,