Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2009

Phosphate Conversion Coating Apply on Aluminum Diecasting

Q. We have got a request from our customer to make a coating on diecasting aluminium (A360). Their drawing said it was chromate conversion coating. There is no top coating but it appears as white colour.

We are thinking whether it is not chromate conversion coating. However, it is a bit strange to have phosphate conversion coating on its own without a covering top coating.

A. First, the word "phosphate" in your question might be inaccurate. Chromate conversion coatings for aluminum generally do not contain phosphates. Some companies do use dedicated phosphate pretreatments for steel and they often also run aluminum parts through the same process. However, phosphate pretreatments intended for steel are generally not recommended for aluminum.

Chromate conversion coatings (CCC) as sold by companies such as Henkel Surface Technologies, Chemetall, DuBois and more are excellent, but you are correct in saying that they are not white. In fact, they have a yellowish bronze color.

In my opinion your customer probably expects you to apply a white topcoat over the CCC. Frankly, I would prefer to apply a primer followed by the topcoat.

Regards,

Ron Joseph