Pretreat for Magnesium AZ91-d

Question:

I have some questions around proper pre-treatment of Magnesium AZ-91D. We have suppliers casting our parts in China and are experiencing blistering. Is there a standard pretreatment process and chemical to condition the substrate to accept paint? We are using a primer and 1K high-bake paint for the finish appearance. Also, is there some type of tinting agent that we could add to the non-chromate conversion to show visually that the parts have been pretreated? S.W.

Answer:

Regarding your first question, I am assuming you are not performing any sort of cleaning process. You may want to talk to your chemical vendor about first validating a process prior to investing in a new line. It could be that even with the best of cleaning, there will still be outgassing that occurs from your cast parts that leads to blistering of your paint.

I would suggest starting with a simple alkaline cleaning step to remove shop oils, mold release compounds and any lubricants that may come in contact with the casting through later machining. The alkaline cleaner needs to be a fairly high pH (greater than 11) so as not to attack the magnesium. Also, most magnesium cleaners have a fairly simple builder system consisting of hydroxides and carbonates. Complex phosphates (pyrophosphate and polyphosphates) are generally avoid when cleaning aluminum. This could be done conceivably in as little as two steps (clean and rinse). More steps would be required for more effective rinsing or lowering the amount of overflow water used.

You will need to allow the parts to dry, or you could install a dry off oven. The dry off oven would have the benefit of increasing throughput significantly and would act as a "pre-bake" of the part. If there is residual outgassing taking place from within the casting, it is better to take care of that in the dry off oven than the paint cure oven.

Regarding your second question, I am not aware of a colorant or tinting agent that could be added to the conversion coating so it shows up. It would have to be an inert chemical, possibly a dye of some sort. I would talk to your conversion coating chemical supplier. They should be able to help you with this.