Weak adhesion to Aluminum after Chromate Conversion Coating

A. Hi Rachith.

Although nothing lasts forever, chromate conversion coating is a permanent coating not a temporary one. It can be used as a base under paint or powder coating, but it can also be a free-standing final finish as well.

I don't think there is a last hour before paint/powder ... I think that's just a matter of not waiting for days because the chromate will get dirty and dusty. I presume that the chromate gel will eventually become dessicated after some number of weeks, months, or years, so you would want to paint within a reasonable time frame, not months later.

Chromate conversion coating is an excellent anti-corrosion coating so you'd probably want to apply it before painting anyway, but paint does stick to the conversion coating better than to raw aluminum, although the chemistry and topography of exactly why is beyond my pay grade.

Luck & Regards,


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

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A. Hi Tomaszewski ,

I suggest that you contact , Mr Dave Hunt of Stone Chemical, Naperville , Illinois and use my name as a means of introduction . You will find Dave a very knowledgeable fellow who networks very well. best regards John . C . Tenison - Woods

A. I'm not familiar with the P2 etch myself, but chromate conversion coating of aluminum is covered in specification MIL-C-5541.

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A. The P2-etch process was developed and patented by Picatinny Arsenal as an alternative to the FPL etch for adhesive bonding to aluminum alloys. Sulfuric Acid, conc (sp. gr. 1.84) 370 g Ferric sulphate, 75% 150 g Dilute to 1 liter with deionized water Temperature 150 °F Time 12 min

A. The P2 etch solution is described in ASTM=D2651.html"Standard Guide for Preparation of Metal Surfaces for Adhesive Bonding".

A. The original Alodine is a process that been invented some 70 years ago. The original company for it is Henkel Surface Technology.

A. At one of the companies I worked for, we applied an adhesive primer to a chem-filmed, (chromated) 1100 alloy 5 mil thick aluminum foil surface. We qualified the foils by lap shear testing at cold, ambient and elevated temperatures. The results were as good as FPL etched, adhesive primed lap shear coupons which were bonded as controls.

A. Dear Jeff Your statement show that DO NOT BELIEVE EVERY BOOK, especially books in surface finishing. The developments and innovation in this area is so fast that from the beginning time of writing the book to the end of period there are changes . This what is making our occupation so fascinating and interesting. You are doing the same as thousands of factories are doing and give step elevation to the quality of your products . Yours

A. It sounds like we're all saying, "if it works, don't try to fix it." We recently had a "think outside the box" training program in our R & D Dept., where we essentially learned not to assume that many of those "cast in stone" rules can't be broken. One thing I like about this business is that most successful finishers use very little exact "cast in stone" science and a lot of logic and common sense. Considering that there are probably a zillion types of adhesives out there with varying amounts of adhesive strength from Post-its to holding rocket parts together in outer space, making a blanket statement like the one in the handbook is naive and close-minded on the part of whoever made it. The statement may be true for some very high strength adhesives where anodize may be superior over conversion coating, but conversion coatings should work with many adhesives.

As long as your successful tests are representative of the actual real-life application of the product, then you've proven that it works, so why not go for it?

Phil Johnson
- Madison Heights, Michigan

A. You might want to look at a new process developed by The Welding Institute in England, friction stir welding. If your geometry isn't too complex, and you can find someone who can do it for you (it is new), it provides a welded junction without any heat distortion of the parts. There was an article on it in Advanced Materials and Processes early last year. There will be some presentations on the technology at this year's Aeromat meeting.

A. It could be an SAE standard (recommended practice): SAE Standards. Document Number: ARP1842 Date Published: July 1984 Title: Surface Preparation for Structural Adhesive Bonding, Aluminum Alloy and Low Alloy Steel Parts. This recommended practice describes the materials and procedures for cleaning aluminum allow detail parts and low-alloy steel parts (other than corrosion resistant steel) prior to structural adhesive bonding operations. The surface treatments described here have proven effective for producing a suitable surface for application of adhesive primers, films, or pastes prior to structural adhesive bonding operations. https://www.sae.org/servlets/productDetail?PROD_TYP=STD&PROD_CD=ARP1842

A. Hi Abdul. Chromate coatings are subject to dehydration if dried at too high a temperature or subjected to other operations which might disturb them before the 24-hour cure time. They are also subject to shearing, especially if too heavy.

Many people are doing it, but even still, chromate conversion coating might not be the best/only aluminum treatment before adhesive bonding. Your inquiry is attached to several others on this general subject, and you might or might not find a different process such as phosphoric acid anodizing for adhesive bonding to be better suited to the application :-)

Luck & Regards,

A. Alodine should help adhesion if applied properly, so you should look at how you apply the chem film. Do not heat over 140 °F or put it on very heavily. Heating over 140-160 °F will dry the film out and destroy adhesion. Putting on a very heavy dark film will also destroy adhesion. Just as a note a trivalent material will be less susceptible to drying out and should handle higher temperatures. Hope this helps