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Letter 39015
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?
Keith Moses
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.
Willie Alexander
- Colorado Springs, CO
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