Conversion Coatings

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Chromate conversion coatings are obtained by chemical reaction with the metal being treated to form a complex chromate film over the entire surface.

The characteristics of the resulting surface offer the following benefits:

Corrosion Resistance:
The coatings are particularly useful in protecting against oxidation even under humid conditions.
Key for paint, lacquer and powder coatings:
In addition to promoting good initial adhesion they also prevent loss of adhesion due to under-film corrosion.
Low electrical resistance:
There is only a minor effect upon contact resistance and the value will generally be much lower than that of an unprotected part with an oxidised or corroded surface.
Minimal build-up:
The coatings are very thin and can be used for threaded parts without having to make allowance for dimensional changes.
Flexibility:
Being integral with the metal surface, the films can withstand bending without adhesive failure
Easy Repair:
Damaged coatings can be easily renewed.

The coatings are suitable for use on all types of aluminium alloys including high-silicon pressure die-castings, which are difficult to anodise. In addition, they can be used to treat zinc and magnesium alloys – particularly to promote good paint adhesion.

The Process

Chromate conversion is either carried out by immersion or by brush application.

A number of different coatings are available but the most commonly used products in the U.K. are alacrom1200 and alacrom 1000. In the USA the most commonly known is ‘alodine’.

Alacrom 1000 gives a clear coating and alacrom 1200 a colouration from champagne through to orange depending upon exposure time. The latter is by far the more common.