Removing chem-film from aluminum

A. Kathy,

My response may be disappointing, but I believe there is no such "easy" method: only very fresh (a few hours after deposition) may be softened in nitric acid (typically 50%). You'll have to wipe it off after such operation. Any old, and therefore sufficiently cured coating is only removable by caustic etch: approx. 100 -150 g/L of NaOH at 40 - 60 °C with subsequent desmut in 50% nitric acid.

I would like to add that properly performed, this operation is harmless even to precision machined parts: in 5-15 seconds (time usually sufficient to remove all chem. film) not more than .0001" of base material will be removed.

A. If it is only one or a few pieces, Try common bicarb of soda in hot water. One lb/gal gives a pH of about 9. Will work slowly. You might try a very thin paste with a fine scotchbrite. This will avoid having to do the acid dip and have a lot less waste to get rid of. If you are going to do a lot of it, your lab exclusion might not satisfy EPA.

A. Chromate conversion coatings are easily stripped with a caustic etchant with minimal material loss.

Any reputable plater should be able to help you.

A. MIL-STD-871 [link is to spec at dla.mil / Defense Logistics Agency] provides instructions for "Electro-Chemical Stripping of Inorganic Finishes". In this document you will find process parameters and chemical solutions. Also specific instructions for removing MIL-C-5541 coating.

A. You might try using a spent zincating solution.

A. You might try a mixture of sodium carbonate ⇦ this on eBay or Amazon] and sodium bicarbonate solutions, mixed such that the final pH is something like 9 - 9.5. It has to be high enough to dissolve the chromate film, but not high enough to dissolve any Al.

Castings can present a real problem in reworks. Once that surface "skin" is broken/etched away, exposed pits and consequent bleed out can make your life a misery.

A. Fresh Alodine 1200 coatings can be stripped by immersion in 50% nitric acid for up to 5 minutes. This will not attack the base material.

Older coatings are harder to remove. I have successfully used a couple of the proprietary deoxidiser solutions, but it is very dependent on age of the coating. You can also use a quick dip in caustic. Be very careful with caustic dipping of aluminum as the solution will attack the base material aggressively.

A. Yes, chemical conversion coatings can be removed - most commercial etches will do it fine. If it is an old coating, boiling the parts for 15-30 minutes will make it come off a little easier. The coating itself most likely contains hex-chrome, so make sure that you are either sending it to someone that knows how to handle this, or make sure you are capturing all waste products and treating them appropriately.

Also, you may get much more specific guidance by contacting Henkel - the manufacturer of the Alodine chemicals.

Good luck!

A. Hi Anthony. Alchromate seems to be a PMD trade name for chem-film/chromate conversion coating on aluminum. You might inquire of them if there is any reason the suggestions on this page might be inapplicable, but I doubt it. Good luck.

Regards,