Problems Chromate Conversion Coating Aluminum Castings Q&A

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

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A. Hi Sanjay. I guess there's no harm in asking, but 400 hours SST for castings does not seem reasonable to me.

In real life, what environment will these castings we exposed to? Are any aesthetic requirement imposed (I hope not). Is there a reason you can't paint or powder coat them after chromating? Thanks.

Luck & Regards,

A. The main problem with deoxidizing castings is that the casting alloys are normally only 80-90% aluminum, with a large part of the remainder as silicon, which is a remarkably inert element to almost all chemicals except fluoride. When you treat a cast aluminum surface, at least 5-10 % of that surface is silicon. There are two choices: either you etch away the silicon with a strong mineral acid deoxidizer containing fluoride, or in many cases a non-etching dip in dilute nitric acid or proprietary nitric-containing deoxidizer is sufficient, particularly when parts are anodized. It is well known in the industry that cast parts should not be alkaline etched, as entrapment of alkaline materials are almost impossible to rinse out of the casting pores, resulting in the presence of neutralized aluminum salts when the parts are later immersed in acid solutions, and leaching out after the parts are dry. In other words, often a cleaning cycle of non-etching cleaner then deoxidize in dilute nitric is sufficient. Depending on the alloy and finishing requirements, the only other choice is a dip in a strong nitric-fluoride deoxidizer.

Phil Johnson
- Madison Heights, Michigan

A. The good solution for your problem is the Henkel Surface Technology Product Called Deoxalume 1000 System. This product set is ready for use bath, friendly to the user and very efficient for Cast Aluminium from my experience.

A. The Deoxalume 1000 system is a patented deoxidizer designed especially for castings. It contains additives which makes it highly efficient, and it turns the castings almost white. When followed by a suitable Alodine conversion coating, it gives remarkable corrosion resistance. It is friendly to the user in the sense that it is easy to control, but be advised that the bath is a very strong mixture of mineral acids and fluoride. It requires an efficient ventilation system with a corresponding fume scrubber. Safety issues involved with the process obviously need to be considered.

Phil Johnson
- Madison Heights, Michigan

A. I agree with Henkel concerning fluorides, but we are deoxidizing casting aluminium, even castings with 12% Si with a microetch. We are using it in a special way and you have to try it yourself if it works on your details. The benefit is, it doesn't contains any fluorides.

A. 1. Alodine is a trademark of Henkel Surface Technologies. Please use a search engine for the requested contact info; this site is made possible by our supporting advertisers, and we can't ask them to bear the cost of maintaining contact information here for their competitors :-)

2. I don't have a good answer for question #2, sorry. But it is not a coating on top of the aluminum, it is a conversion process.

3. Regarding substitutes, see www.jgpp.com/jtr-hugh.htm for a list of the most promising, and to learn the sad news that, when independently tested, no chromate substitute really holds up yet despite all the hype and the desperate search.

Ed. update Nov. 2022: The jgpp.com domain is now linked to a French real estate agent, but the good news is that some non hexavalent chromium conversion coatings are now approved under MIL-DTL-5541 [on DLA].

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A. Hi Fred.

Looks like nobody ran sample parts before the company committed to this drastic change :-(
It is far, far, more difficult to aesthetically finish cast parts than wrought parts.

As a first step, you'll probably need a fluoride-bearing acid in your pretreatment to dissolve the silicon in the alloy, which the anodizing shop did not previously need for your wrought parts. But it is hard to say whether just changing the pretreatment can improve the parts enough to meet your standards. You probably need a consultant, or chromate supplier, or shop experienced in A380, to do a quick development project for you to see how attractive the parts can be made.

A. Dear sir,

Please specify dimension of your part then we can transfer our experiences to you.

A. Hello , we were faced with this problem trying to get yellow color to show up on Aluminum die casts using a certain Macdermid product.

I achieved this by sandblasting the whole part. Depending on the dimensions on your part, this might as well be an alternate way to get your chromate to show up.

A. Blind holes and porosity in casting will trap and bleed with changing humidity and temperature and there is very little you can do about it. Keep the castings out of alkali. Clean only in acids. Use a post treatment on top on the chromate like some of the newer silicates, light oils, or one the newer organic "sealing" (not hydrating) type compounds . And finally, have the final chromate rinse "a little yellow", do not try to final rinse in high purity D.I. water, rather dry on a little yellow water. Let us know what happens.

A. Try 5% ammonium citrate solution(50 gm citric acid ⇦ this on eBay or Amazon [affil links] /1 lit water +some ammonia, pH must be 7).Hope it helps and good luck!