Thickness/Weight of Alodine/Iridite/Chemfilm on Aluminum

A. Hi Zander.
We appended your inquiry to a thread where several readers have offered their thoughts on values for coating weights.

I'd say that since the spec used to require 40 mg/sq.ft. for Class 1A, whereas more than 40 mg/sq.ft. is problematical as several people have stated, that one is already pretty tightly defined: 40 mg/sq.ft sounds like the right target.

A couple of experts have said that less than 15 mg/sq.ft can be difficult, so why not keep the numbers simple and rounded and say 20 mg/sq.ft. for Class 3, as long as you can meet conductivity specs.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. The question about coating weights is a very general question looking for a specific answer. Coating weights of chromates can vary according to the application, the alloy, and with the specific chromate you are using. We sell clear Alodines, for example, to automotive wheel manufacturers who want coating weights of 3 to 5 mg/ft2 for clear painted wheels.

Gold chromates usually run somewhat higher. 15 mg is not a bad coating weight, but one advantage of running a gold chromate is that you can see its color and can almost instantly tell whether you have a uniform coating. A range of 25-40 mg has a better gold color than 15. With accelerated chromates such as Alodine 1200S, 15 mg/ft2 might be difficult to achieve from the standpoint of requiring very short immersion times, and 40 mg would be more realistic.

It is more important to make sure the chromate is a good tight coating, never powdery or smutty. Incidentally, the trend these days is to use non-chrome treatments prior to paint. The latest generation of our non-chrome Alodines matches the performance of chromates or chrome-phosphates in most painted applications.

Phil Johnson
- Madison Heights, Michigan

A. Hi Mahabhadradas. Such coatings are conversion coatings rather than top coats. Therefore, some base metal is consumed, there may be intermediate layers of somewhat varying composition, and the surface is gel-like, and the question arises of exactly what do we mean by the thickness of the 'coating'. So people generally speak in terms of the 'weight' of the coating rather than its 'thickness'. We appended your inquiry to a thread where Phil explains typical coating weights, and there is a good deal of additional discussion of the topic on this page.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. You are quite right; it is not possible to measure such thicknesses without very specialised equipment - electron microscope viewing prepared cross sections I think. The customer may be worried about fine tolerances (?). Chromate coating weights are measured by the strip and weigh method. If the panels are not hot air dried you can strip the coating using 30-40% nitric acid cold. Prepaint coatings are typically 0.3 - 0.8 g/sq metre (c. 30-80 mg/sq ft. R

Roger Bridger
- Croydon UK

A. 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 °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.


Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics" [on Amazon or AbeBooks affil links]
           and "Plating ABS Plastics" [on Amazon or eBay or AbeBooks affil links])

A. Ralph,
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 chem treat (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, pre-cleaning 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 Experimenting

Tony Schmaltz
- Kent, Washington

A. Tony,
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

A. Ed,
If you are talking about an aluminum chromate coating, then yes there is a method of checking it. I really need more specifics on the product that you are using, but for an aluminum chromate coating, take test panels and run them through the bath at 3 different times, then weigh them, strip them in a 50% Nitric/Water solution and then weigh them again. Take your difference in weight and multiply it by 8000 and that will give you the coating weight in milligrams/foot squared, the units that most customers look for.

Good Luck.

Tony Schmaltz
- Kent, Washington

A. Hi Dean. Tony's posting was from 20 years ago, so he may not be reachable or may not respond. But the wetted surface area of a test panel is probably about 1/8 of a square foot in area, counting both sides (10 cm long x 58 cm wetted height x 2 sides = 116 cm2 = 0.125 ft2)

So if you multiply the weight difference in g by 1000 to get it in mg, and you then multiply by 8 because the area of a test panel is only 1/8 of a ft2, you'll get the coating weight in mg/ft2. Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. A method of chromate coating weight analysis that has become more popular recently is x-ray fluorescence. It provides quick results that the manufacturers claim are repeatable and accurate. It also doesn't require the use of hazardous chemicals in stripping solutions. It is doubtful that this process could be used for anodized aluminum.

Patrick Patton
Q-Panel Lab Products - Westlake, Ohio

The method you described for Alodine is very reliable.
Yours,

Moshe Yaakobv
IAI - Lod, Israel

A. Hi. Although I can't answer that question, I think it has become moot :-)
Unless I'm somehow missing something, there is no longer any reference at all to coating weights in MIL-DTL-5541 [on DLA] anymore :-)
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2022

A. The Military specification and the ASTM specifications show a procedure for measuring the weight gain of chromate coatings, then from that if you really desire the actual thickness you can compute. The industry does not consider thickness of chromate coatings, rather coating weight is measured. Any of the three NASA plating laboratories right there in Huntsville can show you how to measure coating weight. Robert H Probert Robert H Probert Technical Services Garner, North Carolina A. How thick of a coating are you expecting? Chromate coatings are generally extremely thin (typically on the order of tens of nanometers) and, unfortunately, I doubt dyes and a light microscope are capable of giving you any sort of valid data. Weight gain is a potential indicator. A quick google search found this paper on chromate coatings that you might find helpful. http://www.surtec.com/Publikationen/DeltacollE.pdf Regards, Thomas Hanlon, Materials Engineer aerospace finishing - East Hartford, Connecticut, USA

A. Coating weight analysis of chromium on metal substrates can be done easily on an x-ray analytical instrument conforming to ASTM D5723.

Tom Cronin
Pittsburgh, Pennsylvania