Products Failing Salt Spray Test

Q. We have repeatedly failed a 336-hour salt spray test on aluminum casting. Our pretreat system and coating have met the same requirement on steel and a shorter salt spray test on aluminum castings. We use an immersion bath iron phosphate conversion coating (soap, rinse, phosphate, rinse) and urethane powder coating. The results have consistently been blisters and significant creepback from scribe. How can we improve our adhesion and weather resistance on these die-castings? What is causing the blistering? We could instead use a TGIC Polyester or a Polyester. Would these powders perform better? M. S.

 

A. I called you to discuss this issue personally and to obtain more information. It is a good thing that I did since there were several additional facts that are missing from your question. First of all, you have no Fluoride additive in the cleaning or phosphate tanks to help etch the aluminum surface. Second, you are using A380 alloy aluminum as your die cast metal, which has a high copper content. This makes a natural galvanic reaction in the salt spray chamber causing rapid and catastrophic failure of the coating, as indicated by blisters and creepage. Other aluminum alloys like A369 will perform much better in this application.

You can always add a chromate, or non-chrome substitute, conversion coating to your pretreatment process for your aluminum products to increase the corrosion resistance up to 5,000 hours but this may be over-kill for your application. So let’s keep it simple. Just use the correct aluminum alloy, add an etchant to your bath (Fluoride) and fully cure the powder coating to obtain your desired performance.