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topic 42739

Dark smut on conversion coated cast aluminum parts


Q. I just had some cast aluminum parts (alloy 380) chromate-conversion coated. The result is totally unexpected. We are somehow new to cast aluminum parts. The majority of our products are made out of extruded aluminum, employing alloys with a lower silicone content. In the past, conversion coating has not presented any problems for these machined parts. However, when we conversion-coated the cast parts, I noticed a very uneven coat (patchy yellow spots) and some sections presented a dark smut film. The purpose for the conversion coating is not only to protect the aluminum parts against corrosion but mainly for powder-coating preparation. Besides specifying ASTM B449-93 [link by ed. to spec at TechStreet]-67, is there any other criteria I should specify to obtain satisfactory results with conversion-coated cast aluminum parts? Should I specify any other type of coating more suitable for this kind of process? I would greatly appreciate any insight on this issue.


Rolando Toledo
Electronics Enclosures Engineer - North Hills, California, USA


A. You chromate aluminum metal. You do not chromate the 15.4 % of non-aluminum alloying ingredients of 380. Many chromating job shops do not know how to reduce the silicon (and other stuff) from the surface. Some so-called 380 die cast will plainly not uniformly chromate. Thin walls cool faster than thick sections and more silicon comes to the top. The best advice to give your chromater is to Clean, DO NOT ALKALI ETCH, deox for 20 to 30 seconds in 50%/vol Nitric Acid with one pound per gallon ammonium bifluoride.
This will not match your 6061, but it will be as good as you can get on that particular casting.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide

October 16, 2017

Q. Hello, We are having an issue with some smut/dirtiness making it through our Chem Film line. This smut is a dark powder that you can rub off with your finger. We thought it had to do with our pH and concentration levels of the Alodine at first, but we made some adjustments, (now running the pH around 1.75 and concentration about 1.05 oz/gal.) When the problem continued, we saw that the smut first was visible after the etch tank. We are thinking there is a problem in our etch or deox tanks. However, we are uncertain. See the details of our process below. We have a few questions that directly relate to this issue as well as ones that may indirectly lead us to a solution.

1. What are some recommendations or suggestions?
2. Are our chemicals for the etch and deox sufficient and correct?
3. Is the amount of air agitation a large factor to removing smut?
  a. We do have air agitation but have lost some air pressure recently and are considering the lower agitation a possible cause.
4. We perform water break test after our cleaner in Tank 1. Should we be performing this test after the deox? If the water break test fails, should the job be re-started?

Thank you in advance for your time and suggestions.

Our Line is as follows.
MATERIAL: 6061, 2024 aluminum

Process Chemical & Concentration Time
Degrease SC-78K 4-10 oz/gal 2-10 min, air agitation
Dip Rinse in & out, air agitation
Spray Rinse in & out
Etch SC-608B 4-10 oz/gal 30 secs, air agitation
Dip Rinse in & out, air agitation
Spray Rinse in & out
Desmut Deoxidize SC-592 6-15% 1-3 min, air agitation
Dip Rinse in & out, air agitation
Spray Rinse in & out
Chem Film ALODINE 1200s 15 sec - 3 min air agitation
Dip Rinse in & out, air agitation

Joseph Lonigro
- Magnolia, Arkansas, USA

simultaneous October 18, 2017

A. Joseph, try a 10-15 minute final rinse and letting the parts dry before handling them. Sounds to me like residual residue from the Alodine.

Ryan Underwood

October 17, 2017

A. Joseph

I would not etch the parts unless absolutely necessary i.e. the surface has a heavy mill skin. If you must etch the parts, deox the parts prior to etching so that you can reduce the etch time. Less etch, less smut. Solution agitation accelerates the rate at which the deox will remove smut. If the deox isn't up to par on chemistry, a longer immersion would likely help.

Your rinse times are on the short side for me. Very short. If you don't get the soap off the surface of the parts, the water break test isn't worth much. Yes, start over if the WB test fails. If you perform the WB test after the soap, you know you won't have contaminants that could interfere with subsequent steps.

Willie Alexander
- Green Mountain Falls, Colorado

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