Conversion coating (Topic 11048)

The conversion coatings serve two purposes, corrosion protection and assuring good adhesion of paint to the aluminium surface. There are two main conversion treatments widely accepted, chromating and phosphochromating. When comparing the two treatments is it claimed that chromating normally gives the better corrosion protection and phosphochromating gives the better adhesion for paint films. If the aluminium is connected to other materials before surface treatment, the compatibility of the conversion chemical with these materials must be considered.

Before applying the conversion coating the surface must be degreased (acid or alkaline) and, after the application, rinsed and dried. The conversion coating can be applied either by immersion or spraying. Spraying equipment will result in higher investments and some parts of a complicated structure may be difficult to treat properly. On the other hand, the immersion treating of certain geometry's can cause dragout or carryover of the chemicals which can cause cross contamination of the process chemicals thereby reducing their active life. The choice between spray or immersion must be done after a total evaluation of the economy of the process and the quality of the coating.

The rinse should be done in a closed system, recirculating the rinse water through a de-ioniser in order to minimize the chrome containing effluent from the process. Because of the restrictions on disposal of chrome containing waste, there is a constant research effort to develop a conversion coatings without chrome. The rinsing should be done as fast as possible and not at too high temperatures, in order to avoid leaching out chromium compounds or degrading the protective properties of the coating.

The temperature limitations also applies to the drying stage, the breakdown temperature for the chromating coating is about 65ƒC and about 80ƒC for the phosphochromating coating.

Thin conversion coatings do not interfere appreciably with spot or seam welding, the chromate film has been reported to have an advantageous effect on fusion welding of aluminium. Thicker films will of course increase the contact resistance and may, at least, necessitate adjustments to the welding parameters.

It is very important that a conversion coating line is designed carefully and that the application of the conversion coating is done with high quality in mind.

Whether a conversion coating is sufficient as corrosion protection will have to be investigated thoroughly in accelerated and field tests. If the conversion coating is not sufficient, then some kind of additional protective system will have to be applied on top of this pre-treatment.

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