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Aerospace shop asks about Nitric vs. Sulfuric Acid in deoxidizing

2003

Dear Readers,

I have been using Deoxidizer 7-17 with nitric acid as deoxidizing process for chemical conversion coating.

Recently, I have got very poor appearance quality after M.C coating (Alodine 1200S). In fact, I made some efforts to solve that problem (raise Alodine concentration, increase rinsing time between each pre treatment,...). I am afraid that I didn't get any improvements.

Strangely, Good MC coating appearance is obtained after I used "Sulfuric-dichromate etch" instead of my deoxidizer(7-17/nitric acid). But, "Sulfuric-dichromate etch" is only used for bonding pretreatment. And, I must use deoxidizer for MC coating. I was told that nitric acid does not react with Aluminum alloy elements.

Therefore, I am thinking about using deoxidizer 7-17 with sulfuric acid.

Is there anybody who tell me what result I would get or give me another solution ?

Thank you.

Dong-cheol Park
aerospace finishing shop - Busan, Kyungnam, South Korea

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Sulfuric is not a substitute for nitric in any deox chemistry that I have heard of. The appearance is different after the sulfuric etch because of the difference in material that is removed. The deox chemistry is not intended to etch aluminum, but just to remove the surface oxide. To address your problem, first look at how old the deox solution is. It may be time to start fresh, test the idea on a lab scale and see if you get an improvement. Of course it will be different in your shop, but in our shop, we get about a year out of the deox, before it is time to dump it. The problem could also be in your Alodine tank. Assuming your pH is good, there are certain anionic contaminants that can build up to the point where you need to at least do a bleed & feed. Again comparing to our shop, I expect to bleed 1/3 of the tank twice a year to keep the contaminant levels down. Of course you can also test this out on a small scale to see if it helps.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

2003

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Sulfuric acid/chromate mixtures are really the best way to Prepare Aluminum ("deoxiding solutions") for chromate conversion coatings such as Alodine or Iridite 14-2. However, the sulfuric acid dichromate mixture must be balanced so that etching does not occur. A short deoxidizing time is recommended. I recommend the proprietary deoxidizers to assure the correct ratio. The purpose of deoxidizing is to remove surface impurities such as copper from the Aluminum and thus improve corrosion protection of the conversion coating. There are also ferric sulphate deoxidizers that work well if chromium is a problem in your shop.

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])
2003