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Chromic anodizing and fatigue

May 29, 2008

I need to know a MIL- specification for applying chromic anodizing that involves fatigue test. I'm studying the factability of a specification of Bombardier where, among the controls that we need to meet, there is a fatigue test. I already have seen the MIL-A-8625 but it does not call for any fatigue test.
Another question: what is the effect of the chromic acid? I mean if I have the following solutions which one would have better fatigue resitance, corrosion resistance, adhesion for paint, film continuity or hydratation? or there is no affect (same results)?.

Solution 1:
Temperature: 35°± 2.5°C (95°F ± 5°F)
FREE CrO3: 35-45 g/l
TOTAL CrO3: 100 g/l max.
ALUMINA (Al2O3): 10 g/l max
sulphate: 0.5 g/l as S04 max
Chlorides: 0.2 g/l as NaCl max.

Solution 2: The same except for the Free CrO3: 50 - 60 g/L

Isaac Ruiz
Process Planes in plating shop - Querétaro, Querétaro., México

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May 30, 2008

My take on it is you want to use the lowest amount of chromic acid that will meet the spec that you are certifying to, with a bit of a pad for when the anodizer puts a bit too much water in the tank when topping up. The reason for this is the more concentrated it is, the more your dragout is. If you plowback the rinses, your aluminum content will build up faster and require an earlier "dump".

Your sulphate and chloride content are far, far too high for many OEM certifications and lead to pitting and orher problems.

Make sure that you seal the parts in a dilute dichromate solution at near boiling for the best coating. Note that the first few loads will come out a green color on 2xxx series until the anodizing tank gets a bit of aluminum in it. The parts are good, but QC goes ape when they see the "wrong" color parts. Takes about 3 full loads to go away, or you add some scrap aluminum to start with.

James Watts
- Navarre, Florida

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