Chrome conversion coating

A. Hi Alex,

Your question depends on the aluminum itself. Some alloys can be chromated effectively without etching the surface. In general, I would say that the etch stage is necessary because you have to remove the natural oxide layer from the aluminum surface in order to get a uniform chromate coating to form.

The best to determine whether the etch is necessary for your work is to perform a simple test. Run some parts/panels through your line as normal and then run another set with the etch stage turned off. If you have selective deposition of the chromate on that second set, you will see it with your eyes right away. You can then perform your regular testing on both sets and see whether the absence of the etch has any effect on the results of your tests.

A. All the etch does is fix a uniform surface, unless the metal has been heat treated, scaled, or work hardened, in which case, again, the etch prepares a uniform appearance.

I too am a supplier, and I always recommend an alkaline etch.

If your deox is in top quality it will remove the surface oxides and then you can chromate right over, but if a part comes thru with heat treat scale, you skip the etch, the deox does not quite remove all the scale, then you do not get a good chromate.

My general practice is (1) if the part is to be left in a heavy yellow chromate finish then you should etch to get a uniform finish. (2) if the part is to be painted, skip the etch and put the chromate on very thin