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Alkaline treatment during Alodine process

Hi

Coating AL: The Alodine process sometimes uses Alkaline treatment after the alkaline soap-wash processes. Then comes the acid etching followed by wash and chromate conversion.
Is the basic (NaOH) treatment after the soap cleaning and rinsing necessary?

What does the alkaline treatment contribute in this case?
Thanks
Shay

Shay Ben-Menachem
chemist - Israel
January 2, 2011

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First of three simultaneous responses -- January 4, 2011

Shay,happy new year to you all.
I believe if your coating is not rubbing off and the colour is o.k . then it is o.k.

Khair Shishani
aircraft maintenance - Al Ain, UAE

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Theoretically, it is not necessary to alkaline etch before aluminum chromate. Whether I recommend it or not depends on the surface condition. Authorities in our industry are split, controversially, on this subject. One school says "do not etch" because it opens up tiny pits, makes them larger, causes corrosion sites. The other school, my leaning, says, "yes,do etch" because it smooths rolling mill lines, removes tiny folds, so that the chromate takes more uniformly. The people that wrote the original MIL-C-5541 say do not etch. I have passed and failed both ways, depending the original surface,good panels yield high percentage of pass if not etched. Bad panels yield high percentage pass if etched. Use your 10 X loop, and decide each case individually. If you see pits with the 10X loop, neither way will work.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Second of three simultaneous responses -- January 4, 2011

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After cleaning and before Alodine you need to etch typically to remove at least 0.0002" and then desmut the part. An acid etch is often used to both etch and desmut in one step. Otherwise you etch in the alkaline bath then desmut in the acid bath. The alkaline etch is faster, but obviously your line requires more tanks. Either way is okay assuming that you are using appropriate chemistry.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

Third of three simultaneous responses -- January 4, 2011

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January 5, 2011

I like Robert's answer as far as the decision to etch or not to etch.

If you do decide to etch, Robert, do you feel that it makes any difference if you use an acid that both etches and desmuts in one step compared to an alkaline etch that is followed by a desmut? My feeling is that it doesn't matter at all unless the alloy is high in silicon, but I would like to hear your comment.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

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In small machine shops that do not have to pass salt spray, I have used a combination phosphoric acid/detergent type product to clean and etch in one step. However, (1)for cosmetic reasons we never want to etch before the soil is off because that causes a step pattern and (2) plain acid etching does not do the same thing as a deoxidizer and if you want to be sure to pass salt spray you need the deox (with acid or alkali etching). You have a good idea, I have been there and done that, hope I did not hurt your feelings by negating it.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

January 6, 2011

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Thanks, Robert. I certainly wasn't suggesting an etch before or in combination with a clean, just wanted your opinion between

1) clean, 2) alk. etch, 3) acid desmut, then 4) chem film

vs. 1) clean, 2) acid etch/desmtt combo, then 3) chem film.

Without mentioning trade names, the acid etch/desmut variety I am thinking of usually contains a fluoride for the etching and perhaps nitric acid for the simultaneous desmut. The etch rates are slow compared to alkaline etching, but I haven't felt that there was any other disadvantage for non-castings if a shop didn't have the space for the extra tank.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

January 7, 2011