I was just handed an article from the July 2001 issue concerning a question of outgassing. The question was as follows:
“We are having problems powder coating over clear zinc chromate plating. It looks like bubbles form under the powder after curing. Is there any solution to powder coating mild steel that has been zinc plated?” M.K.
You offered some ideas and I would like to offer some as well. We have had similar problems with powder coating over the years and have recently done some DOE’s to help alleviate some of the outgassing problems.
First, the chromate application over the zinc will cause outgassing/bubbles. We have the zinc applied and no chromate (GM4345 6U48/0), and then send to the powder coating process.
Second, we have gone to a longer bake time. Standard is 4 hr, but we have increased it to 8 hr. After a 4 hr bake, when the parts were dried post cleaning but prior to the powder coating application, it was discovered that heating the part was bringing residual gasses to the surface, causing bubbles. One of our powder coat vendors did an experiment where they actually pre-heated the part for 10 min at 550F, and it produced a better finish. This is what prompted us to try the 8 hr bake.
Third, light or inconsistent plating seems to be a factor. When the iron phosphate is applied, it etches the zinc plating. If there is insufficient plating, the iron phosphate could etch enough plating off to expose bare metal, with the result of poor adhesion or bubbles.
Fourth, make sure the zinc plate has time to “cure.” We have found that when there has been a need to expedite parts and they have been rushed through both processes (zinc and powder coat) in a time frame of 48 hr, the results were not good. Someone told me that the zinc must be allowed to cure for a minimum of 24 hr. I cannot substantiate that with anything other than the large bubbles I have seen after parts have been rushed through. I truly do not know if it was because the zinc was not cured or some other failure in the process, but I use the 24 hr as a rule of thumb now.
Last, zinc and powder are just a bad combination. They do not complement each other. However, customers specify the zinc because of its corrosion protection.
Some of this information may be things that you already know, but I wanted to pass it along just in case. We have suffered with this problem for years and this is the first time that we have had significant results. We still have some cosmetic issues, but they have been reduced from a 50% fallout to less than 5%.
Thanks for allowing me to contribute some of my experiences. R.S.
I am happy to pass your information along, although I question some of your cure times.