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Poor Powder Coat Adhesion with Dissimilar Metals.

Does anyone have experience with cleaning and powder coating dissimilar metals that are in contact with one another? We have an intermittent powder coat adhesion problem at the steal and aluminum interface of our product. We clean cold rolled steel doors with attached aluminum hinges through a 5-stage iron phosphate wash system. Stage 1 is an alkaline detergent; stage 3 is a multi-metal acid phosphate; stage 5 is a mild phosphoric acid sealer. After cleaning, the steel surface around the aluminum looks silver compared to the blue of the rest of the door. The powder coat occasionally flakes off of the steel in this area. Powder coat adhesion to the rest of the door and the aluminum is good.

Why is there a color difference on the steel around the aluminum? Do dissimilar metals in contact with one another interfere with the conversion coating and reduce powder coat adhesion properties?

Mary DiGiambattista
- Rochester, New York
2000

It is a poor combination to treat at the same time. Steel cleans better at a higher ph and a higher temp. those same factors produce hydroxides and possibly other nasties that give a bad surface preparation. Drop it down to aluminum values and the steel sometimes does not get clean.

Steel and aluminum together forms a wonderful battery. Most chemistry is trading ions or electrons.

There is a very narrow range of your cleaner and phosphate that will suffice for both.

James Watts
- Navarre, Florida
2000

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I suspect that the aluminum parts are galvanically protecting the nearby steel surfaces from reacting properly with the acid processes in the surface preparation. The electrochemical action of the acid with the steel will have two components - an anodic reaction (etching) and a cathodic component (observed primarily as gas evolution).

The etching reaction that you want to occur with the steel is occurring preferentially at the aluminum surface, while the cathodic reaction is occurring exclusively the steel. That is why the surface of the steel around the hinge remains bright. If the steel is bright, the surface condition that you want for adhesion has not been obtained.

Larry Hanke
Minneapolis, Minnesota
2000

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2000

Frankly,

powder should adhere perfectly to any CLEAN metal surface irrespective of the presence of a phosphate coating.

1. Use a non-etch alkali cleaner
2. Use a fluoride free iron phosphate at pH 5.5 approx.

Add separate degreasing additives to the phosphate to ensure min. entrapped oil behind the hinges will bleed out during powder cure. Check area around hinges after dry off for evidence of oil bleed. Production of a phosphate coating on Al actually reduces corrosion resistance due to the destruction of the oxide film unless a final rinse additive is used based on chrome or zirconium compounds.

Roger Bridger
- Croydon, UK

From the way you described the problem, I would have to say that it is probably not a problem with your cleaning chemicals, but more a problem with your manufacturing flow.

Most cutting and forming operations require lubricants of some kind, not to mention there are usually residual oils and films on the raw substrates. When the two pieces are placed together, it is very likely that you are trapping oils between the parts. Even a state-of-the-art cleaning system will have problems with this kind of product. So, the oil that is trapped between the two stays hidden until you apply heat.(i.e. curing your powder) then the oils move out of their recess and under your coating, thus causing lack of adhesion, discoloration and blistering. You need to wash the parts prior to assembly or make sure there are no oils on them another way.

I hope this helps.

Jim Cummings
powder coating - Buffalo, New York
2000

If the parts look silver around the hinges when they exit the washer then you have no phosphate coating in that area, Roger is correct though, that immediate adhesion should be fine in those areas. If the parts are OK coming out of the washer, then have the silver mark after drying, you have an oil bleed-out problem that you can only solve 100% by precleaning the parts. This is a common problem, if the washer can't spray it and the parts are touching each other, then you'll not get clean efficiently in the wash.

Galvanic corrosion will show up later, and will only be a problem in the areas where corrosion product builds up under the coating and ruins adhesion. It is possible that the galvanic potential is keeping a phosphate from forming in those areas, all you can do to improve long term corrosion in that area short of major changes would be to run a good reactive non-chrome final rinse material. The phos acid product you describe is most likely based on old technology.

If your chemical vendor can't fix that, there are some good ones that will help you take care of this.

Jeff Watson
- Pearland, Texas
2000