How to Chromate Conversion Coat ADC12, 380, Castings, and Secondary Aluminum Alloys

A. Hi Pravin. I think that if aluminum is properly chromate conversion coated and properly painted/powder coated, it should last that 336 hours.

I suggest that you investigate whether the chromate conversion coating process is being done really right. You may need to replace the contents of the tank, or adjust your de-smutting process as discussed in this thread. What do the parts look like after they have been chromate conversion coated but haven't yet been painted?
Salt spray testing is meant to help to keep your process on track; it should not be used to mislead you towards replacement processes, but if you want alternate process recommendations anyway, please tell us about the component and its requirements.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. 380 has a lot of copper in it, about 3.5 percent, so I'd certainly assume that the copper is coming from the alloy, and immersion depositing. At this point there is probably dissolved copper in the chromating bath. Whatever you are doing in terms of rinsing after cleaning / etching /desmutting probably needs improvement; but I don't know if that alone can fix the problem, or if the longevity of your chromating solution will be limited by its picking up copper this way.

A. My experience has shown that chromating solutions for use with aluminum alloys have to be discarded quite frequently. It usually comes concentrated and takes little to make up. Some good hand scrubbing, caustic etch, rinse, Preposit® 748, rinse and then chromate. This works great for me.

Hope this helps.

A. Alodine is a trademark of Henkel Surface Technologies. Find them on the web or ask your local plating process distributor if they handle Henkel. This site is made possible by supporting advertisers, and we can't bill to maintain the contact info for their competitors :-)

I don't have a good answer for question #2, sorry, but I do have an excuse: This is a conversion process not a coating process, so it's usually measured by weight rather than thickness. But it is very thin (millionths of an inch, not thousandths).

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.

Update Nov. 2006: The jgpp.com domain is still functional but the referenced page link is now broken. Progress has been made on this subject though, and there is now a MIL-DTL-5541 [on DLA] approved hex-free substitute called TCP (trivalent conversion process) licensed by the navy to CST-Surtec, Henkel, Luster-On, and Metalast.
Update Feb. 2020: www.jgpp.com is now broken and leads to a French site about house building, but there are probably additional TCP licensees by now.

A. Hi Peter. To my knowledge there is no product called "Alodize" but a technical dictionary claims --

So I think this thread about "Alodining" ADC12 covers your topic. Good luck.

Regards,

A. Don, perhaps whoever told you this is not familiar with the diecasting business. We probably Alodine 90-95% of the aluminum castings we run and that would include A380, A360, ADC12, 413.2, etc. Now if you had mentioned anodize, your source of information would be correct.

A. Alodine and other chromates and also anodizing may be applied to the casting alloys you listed. Only the surface preparation is different. In the case of the cast alloys the quality of the finish (either chromate or anodize) is a function of how much non-aluminum alloying ingredients we can remove from the surface, and different preparation for different alloys. Many job shops are only set-up for wrought alloys and when they try to process the casting alloys thru the same deoxidizer they leave alloying ingredients on the surface; then since we can only anodize aluminum, the presence of the silicon, copper, etc, interferes with the cosmetic appearance and the protection of the chromate or anodize oxide.

A. Hi Pritam. We appended your inquiry to an existing thread which answers it. Diecastings contain silicon, copper, and other materials which will cause these problems if not removed from the surface with proper pretreatment chemistry. The etch, and especially the desmut process that you are using, are probably not adequate for castings. Please talk to your supplier about new chemistry for these steps. Good luck.

Regards,

A. Hi Abhishek. Alodine is Henkel's trade name for their line of chromate conversion coating chemistry.
Yes, it is confusing because there are many types depending on whether you need a light, low coating with good electrical conductivity vs. a heavier more corrosion resistant coating where conductivity doesn't matter; whether you intend to apply it by spray or immersion dip; whether you want to buy it as a liquid or a powder, whether rinsing is do-able or you need drying in place, whether this is primarily a touch-up operation where you want a single-part concentrate for simplicity vs. a system where you can add replenishment chemicals to maintain a balanced system at lower cost; and whether you can use a hexavalent process or need to use trivalent. Hopefully you can understand why people cannot answer your question without more info. Please tell Henkel or another supplier these additional details about your needs, and they will tell you which of their products to use.

Good luck and Regards,

A. The issue is with your precleaning. Each of those alloys have either high Copper or Silicon, which when etched with a caustic solution will form a smut of black powder. You will need to use a more aggressive deoxidizer after caustic etch, one formulated to remove those elements. There are proprietary deoxidizers on the market, your Chromate supplier can help, or you could try a Nitric Acid/ Nitric-Sodium Bi-Fluoride Etch.

A. Hi Varun. We added your inquiry to this thread since your interest may be in chromate conversion coating. You say you've tried different processes and you've tried Surtec brand chemicals, but it's not really clear to me what those processes were.

What cleaning regimen did you use to remove the graphite? And did you try chromate conversion coating after that cleaning? Was the chromate the final surface, or did you paint or powder coat it? How many hours of salt spray are you obtaining? Is your desmut step removing all the copper & silicon, or is the surface black? Thanks.

Regards,

A. The problem with ADC12 is the high copper and silicon content, which don't chromate and can form a smut during etching or acid pretreatment. Clear chromate or anodize will not be cosmetic on copper containing castings.

Appearance can be improved by using a fluoride based etch with nitric and HF or ammonium bifluoride to remove the copper and silicon, but cosmetic can still be an issue as the composition is different everywhere in the casting due to cooling rate. Avoid alkaline etching, this will only remove aluminum and concentrate the impurities.

I have also found better results switching to A360 grade casting material that does not contain copper.

A. Hi Sourabhi. We added your question to a thread on chromating of ADC12, which (as you will see) contains some hints about improving the appearance. But I'm a bit confused about why, aside from appearance, the chromating was okay on machined parts but not on castings. It certainly would not have had much abrasion resistance even on machined surfaces.

Yes, as an alternative, the component can be anodized for greater corrosion & abrasion resistance (topic 43432 is "Anodizing On Cast Aluminum ADC12"), but (as you will read) you will still likely find the appearance unsatisfactory.

Another alternative is plating the castings -- see topic 33883, "Problems in Plating on ADC-12".

This is a WWW site for the world so I don't like to get overly nationalistic or jingoistic, but attempting a development projects working with a shop half way around the world instead of right in town is doing things the hard way :-)

Regards,

  Thank you for your response. I should have mentioned previous generation parts were stainless steel machined vs. current die cast aluminum.

I agree with you on your reasoning about parts coming in from half way around the world but unfortunately that was not my decision to make. :)

Again, I appreciate the time you took to respond to this.

Best,