Bubbles in the Coating
What causes small bubbles to appear in cured coating on aluminum?
Q. We recently have had some trouble with an area of the part where small bubbles will appear in the film. The parts look all right after they are coated and cured but the bubbles start popping up a day or so later. The bubble can be popped and the aluminum surface is bare under the film. We cannot figure out what the cause is and we need advice on what to look for. –A.P.
A. You could have a surface contaminant but from your description I think that what you have is humidity blisters. The aluminum is not treated properly and an oxide layer forms on the surface prior to coating. The oxide layer creates a low pressure zone under the film. After the parts have sat around for a while the low pressure zone begins to draw moisture through the film though osmotic pressure. The small areas of moisture swell over time and lift the coating into the bubble you see.
You should examine every detail of your pretreatment system. Do you have the right chemistry for treating aluminum? The process starts with a good acidic or alkaline cleaner. Since aluminum rarely has heavy oils or grease you may want to consider an acid cleaner to help etch the aluminum. Rinsing is very critical. All residual chemistry and soils must be removed before the part enters the next stage.
You cannot allow the part to dry-down in the drain zones. If you are in the drain zone too long and the part dries it will begin to form an oxide layer again. Wetting nozzles are sometimes helpful to make sure the part stays wet. Never stop the line with parts in the washer. This will almost certainly cause problems of the sort you describe.
The conversion coating should be designed for aluminum only. Many companies use iron phosphate with a fluoride additive for the aluminum. This may be adequate for some indoor use products but it is hard to control the parts per million of fluoride and not reliable for sensitive products, especially products that will be used outdoors. A chrome or non-chrome conversion coating for aluminum is best. If you have to run multi-metals (ferrous and non-ferrous) through the washer you should see if you are a good candidate for a transitional metal treatment such as zirconium oxide. These products can work well for steel and aluminum in the same system.
The final rinse is critical. You need deionized or reverse osmosis quality water to ensure that no residuals are left on the surface to dry in place and leave minerals on the surface.
If you do have a surface contaminant that the washer cannot remove you will need to do a thorough soil audit and identify the type of material causing the blisters and find the source.
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