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Color variation on conversion coated aluminum
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I have a problem with color vaiation of conversion coated surface of aluminum fabricated parts showing through the subsequent primer coating. Due to the thin primer coating requirements some customers find the appearance objectionable as the primed parts appear to have
"staining" in the areas where the conversion coat is dark.
We process mainly 2024, 6013 and 7075 aluminum either bare or clad. Our process involves vapour degreasing (Trichloethylene), Alkaline Cleaning (Turco 4215 NCLT), Rinse, Deoxidize (Amchem 6-16 made up with Nitic Acid), Rinse, Conversion Coat (Alodine 1200S), Rinse and oven dry. Racks used depend on part size but are made of aluminum and/or stainless steel.
Processed parts can (not alway) have color variation (non uniform) from very light iredesent to dark golden within the same part. The darker areas are usually along edges and near holes and often appears as streaks or striping. The very light areas appear throughout the body of the parts and are more blotchy in appearance. These conditions are more prevalent on 7075 clad materials.
We have run various test panels trough the process changing variables to try to isolate the cause with no difinitive solution as yet. Tank solution chemistries, times, temperatures are all within customer specification requirements.
Anyone with similar problem/solution or suggestions?
parts manufacturer - Amherst, Nova Scotia, Canada
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Keith
Good agitation ( or even agitation) in the chem film tank can help prevent the uneven appearance. The dark areas around the edges/holes seem to indicate preferential agitation in those areas.
- Colorado Springs, Colorado
May 11, 2011
I have encountered a similar problem with some parts that appear to have no coating on them at all. The problem was contamination of the solution caused by parts treated with small amounts of steel. The only way we found to solve the problem was to empty the tank, clean it thoroughly and refill with new solution.
Peter FerrisQ.A. Engineer. - Sunbury-on-Thames, UK