Hexavalent chrome

There is often a degree of confusion over exactly what chromium containing products and components will actually be proscribed by the RoHS Directive.

The Directive specifically refers to hexavalent chromium and thus it does not include chromium in other oxidation sates. Consequently, chromium which is plated, or deposited in some other way, as a metallic layer or coating e.g. a bright chrome finish, is not included.

Chromium can also exist in the trivalent state and again its use in this form is not proscribed. It is purely chromium in its hexavalent state that must be replaced.

• Conversion coatings
  The main area where hexavalent chromium is likely to be found is in conversion coatings applied to another substrate to impart corrosion resistance. Chromate finishes are often applied directly to zinc and aluminium substrates as well as to cadmium, tin, tin-zinc alloy and silver plated finishes.

  Typical applications include the coatings used for the protection of steel chassis parts and aluminium heatsinks.

  It is possible to form trivalent chromate conversion coatings and a number of other alternative techniques such as anodizing may be suitable alternatives. To achieve similar corrosion resistance performance to the hexavalent chromate coatings it may be necessary to use additional layers of coating.

  Recommendations for a suitable alternative to hexavalent chromate can only be made when the part, the application and the degree of corrosion resistance are considered collectively. Hexavalent chromium is used because it is one of the best corrosion inhibitors known and it is effective for long periods, even as a very thin transparent film. There is no single alternative conversion coating that can replace chromate. Although it is possible to use chromate-free coatings for most metals when the metal is subsequently painted, hexavalent chromate coatings are typically used for protecting bare metal because they offer the best performance and, in many cases, the alternatives will be inferior.

Summary - hexavalent chromium

It is important to note that hexavalent chromium plating solutions are used for the deposition of metallic chromium by electroplating. In this process the hexavalent chromium ions are converted to the metallic form in the plating process and thus the deposited coating will not contain any hexavalent chromium.

The RoHS Directive does not proscribe the use of hexavalent chromium in plating processes. Also, there are currently some specific exemptions for chromium where it is used in the carbon based steel cooling systems of absorption refrigerators.