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Chemical Conversion Coatings
What is the nominal thickness (and potential range of thicknesses) for Iridite and or Alodine type conversion coatings? Are there processing / material problems which may lead to adhesion / cohesion issues with conversion coatings?Ralph J. LeBoeuf Jr.
aerospace mfgr - New Orleans, Louisiana
Since chromate conversion coatings are gels it is difficult to measure thickness directly. It is usually reported in milligrams per square foot. clear coatings will have form 12-20 mg/sq ft. Yellow chromates from 30-50 mg/sq ft. Above 40 mg/sq.ft the coatings are not as good. 40 is optimum for yellow; this equates to about .0001" or less. Generally thickness is not of interest because these coatings are too thin to have an effect on dimensions. Chromate coatings on Aluminum works well because if scratched the gel will migrate over and into the scratch and continue to protect. (168 or hrs 5% salt spray is required by most specs. They usually provide more if the solution is maintained, Impurities kept out and the finished coating is not exposed to temperatures above 150 degrees F for more than a few seconds. (The gel dehydrates) Chromates are excellent for promoting paint adhesion to Aluminum. Paint over chromate can be baked at the normal curing temperatures of high bake paints because the paint seals the chromate preventing dehydration. This may be more than you wanted to know; or if incomplete contact me.
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics" [affil link to the book on: Amazon or AbeBooks ])
Don is for the most part correct, and as far as thickness of the Alodine, it is in fact measured in milligrams per square foot. If you would like to check the coating of any color chemtreat (ex Alodine 1200) then use three test panels so you can test the best time to use all in one shot. Run the three panels at approximately 30 seconds, 1 min and 1 1/2 minutes. Then weigh your panels and strip them in a 50% 42 baumé Nitric strip for approx. 30-60 seconds or until the panels are stripped of the coating. Then weigh the panels again, get the difference of before and after, then multiply that number by 8000 and that will give you milligrams per square foot of the coating. Now, as with any conversion coating, precleaning is huge, as is temperature. Make sure that your cleaning is thorough and that your temperature is at the low end of the range, better adhesion at low temps. I have done thorough experimentation on this specific issue, if you want anymore info, post another response to this letter and I will help you out.
Have Fun ExperimentingTony Schmaltz
- Kent, Washington
That is the method I normally use, and while it works well I am looking for a method that does not have a time limit on it (the coating must be stripped within about 1 hour for full coating removal). As most small shops send the panels out for CW evaluation, there must be a method that can accurately be performed ex post facto.
Thanks!Benjamin J. Curto
- Ponderay, Idaho, USA