Resolving Blisters/Peeling of Powder Coating on Zinc Plating

A. John:

Are you sure you're getting all the grinding swarf off the surface of the weldments?

This may be off the wall, but I have seen some unusual things. When you grind steel, you generate a lot of very small particles. Now small particles of a particular size have an electrostatic attraction for the steel surface. If you do not have spray pressure or in the case of immersion ultrasonic waves with enough energy to overcome the electrostatic attraction of the fines, they may not be removed from the surface of the weldment.

What could then happen is that during the application of the powder coat the fines stand up (They are iron, by the way and are magnetic) and get trapped by the powder and cause the defect you are describing.

Again that may sound off the wall, but I will also say that the way to figure out what the defect is is fairly simple. You need only two things: a microscope and a razor blade.

Here's how:

1. View the defect from above to determine if anything protruding. Make notes.
2. Slice a little of the paint away from the defect and view again. Make Notes.
3. Repeat #2 until you can see the substrate.

You'll find either something or nothing. Nothing generally means solvent pop or degassing of some sort. If you find something, then the hunt is on to determine where it came from. Here's a quasi-diagram of the process:

              Bubble Bubble Bubble
      Paint Film Paint Film Paint Film Paint Film
 Slice 1 Slice 1 Slice 1 Slice 1 Slice 1 Slice 1 Slice 1   
Slice 2 Slice 2 Slice 2 Defect Slice 2 Slice 2 Slice 2 Slice
Slice 3 Slice 3 Defect Defect Defect Slice 3 Slice 3 Slice 3 
SubstrateSubstrateSubstrateSubstrateSubstrateSubstrateSubstrate

Good Luck,

A. It is possible that when using coarse grinding discs, some of the abrasive becomes embedded in the steel surface. This may not occur with finer discs. What is the coating adhesion like? If it is satisfactory, then try "casting" grade powders as these are more forgiving to popping.

Another possibility is that the zinc phosphate deposited on the coarse ground surfaces may itself be coarse and crumbly (does the phosphated coating sparkle in the light?). If this occurs then the coating will exhibit popping and roughness.

A. First thought could be that you are not waiting 24 hours for the chromate to cure before painting. Second, are you heating the part during the painting operation? Excessive heat (above 140 °F) can cause the chromate to powder resulting in a lack of adhesion of subsequent organic coatings. Third, check with the plater! Are they putting on any post chromate dips which might affect paint receptivity?

A. The problem could be from bad plating. Make sure the plating process is operating under properly controlled conditions. Two of the major causes of poor plating adhesion would be incomplete cleaning and/or too much brightener additive in the zinc plating tank. Make sure your plating supplier is following good process control procedures and is maintaining their equipment properly. There could be other reasons, but I would start there.

A. I think you are using pu paint over plated surface. Clean the plated surface with trichloroethylene - LR grade and heat it at 130 °c for 10 mts, then clean the surface with cotton waste. Surface should be free from oils and solvent. Then apply paint (P.U) and cure it at 70-80 °C for 10-15 mts. Check the adhesion test after 3 hours.

A. David,

Assuming that you previously did not have adhesion problems when you used your chrome seal, I would suspect that the seal has been contaminated. You could process a test panel through the seal, and have it analyzed. SEM EDS should let you know if there are any residual salts on the surface. I once saw a problem when the water supply (wasn't using DI, RO or distilled) flucuated so badly with chlorine, that soluble salts were left on the surface of the parts.

A. David, chromate coatings have structures that contain water. If you dry your parts at too high of a temperature or for too long a period of time, you can dehydrate the chromate coating. The amount of water in the chromate coating has an impact on whether paints can wet out the surface and adhere. To determine if changes in your drying practices are responsible for your adhesion failure, you should design an experiment that simulates the two extremes in your drying process; shortest time/lowest drying temperature and longest time/highest drying temperature. If the longest time/highest temperature parts have poor adhesion, then you need to control you drying process better.

A. It is probable that the chromate is too heavy, try putting on a lighter less thick film of chromate. The chromate film, when too thick, can shear under paint. Also as the above said, the water in the gel explodes through the powder coat at curing temperature, but if the chromate film is thinner you can bond the powder at normal curing temperature without problem.

Another possibility is that this is "acid" zinc and the surfactant film and co-deposited non-zinc junk will outgas right through the powder. Have the plater QC his acid dip ahead of the chromate to be sure he gets rid of any tenacious zinc plating ingredients.

A. I suspect that a good number of shops are experiencing a very similar problem, Ryan. Please also see topic 42213. In the rush to develop trivalent chromates to meet RoHS requirements, corrosion resistance was priority number one, and whatever had to be done was done. Compatibility with powder coating wasn't a high priority at the time, so only some trivalent chromates are compatible. This sometimes has to be fixed with a totally different trivalent chromating system.

Hopefully you are on the right track that yours can be fixed with some tweaking, but I think the supplier's tech service people may have to work with you on site.

A. Hi, Elnaz. I think the main problem is pretreatment. Aluminum requires chromate conversion coating or equivalent; steel requires phosphatization; galvanizing (it's not really clear if you are galvanizing or zinc plating) requires phosphatization. Zinc plating requires chromate conversion coating.

Regards,

A. Hi, Elnaz. Hot dip galvanizing is different than zinc electroplating. It must be phosphatized for good paint adhesion.

Regards,

A. Hi Elnaz and Ted.
We had the same problem, but we changed into a different Al Pretreatment with a really good salt fog chamber performance, since then we don't have adhesion problems.
And sorry Elnaz, but I didn't understand your Fe pretreatment process.

Regards from Mexico!