Chromate Conversion Coating of Cast Aluminum: Problems & Solutions

A. 355 - 1.3 copper, 5 Si, 0.5 Mg. 356 - 7 Si,0.3 Mg. These alloying constituents create difficulty in chromating, and may reduce the protection, and affect the appearance of the chromate. Proper cleaning and pretreatment can improve the effect of a final chromate conversion coating. See www.finishing.com/chemicals/ for chemicals.

A. Degrease using non-etch alkali cleaner. Warm phosphoric acid based etch cleaners may be used with care, possibly fluoride accelerated. Hexavalent chromate processes are the only ones that will give satisfactory performance bare or painted.

Use high pointage chrome content and maximise coating weight without getting loose coatings. Aim for a rich golden yellow colour. Do not use hot water rinsing and keep dry-off below 100 deg C Roger

A. The only limitation I know of surface wise is the limitation on corrosion resistance associated with castings, its around 1/4 to 1/6 the corrosion resistance associated with wrought material. Other than that, there's no downfalls per se with chromating casting material, it does provide increased resistance, but to what extent is questionable.

A. Casting aluminium is a problematic because of the hig silicone content in the alloy . Special treatment for removing the silicone obstacle with proprietary formulation can remove the obstacle. In my career I developed and sold product for that problem.

A. Essentially you need a hot cleaner (140 °F) acid activator, process tank and at least 1 preferably more rinses after each step and a final hot water (110 °F) rinse.

A. The processing parameters change fairly dramatically dependent upon the method of application (spray, dip, or brush) and chemical. If you are trying to evaluate a current supplier you may want to find out which chemical their using and contact the manufacturer of the chemical. If you are contemplating adding the capability to your facility, you may want to decide which of those application methods you prefer and then refer to the QPL for MIL-C-5541 to find which manufacturers make which chemicals for that application method. Then, again, contact the manufacturer and ask for the process parameters for their product.

Good luck,

A. Do you want to know the parameters of chromate solution?
Now some of product are treated by chromate.The parameters of chromate is:
pH=1.5-2.2
Cr(VI): 6-9
Temperature: 25-35 °C
Time: 1-3 minutes
Good luck!

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.

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.

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.

A. Hi Tom, black smut on Aluminum castings is a very common issue and nearly always stems from problems surrounding the DeOx step: insufficient time in the DeOx (should pass a white glove smear test before proceeding), the wrong kind of DeOx (Nitric-Sulfuric-Ferric combination is great), insufficient concentration in the DeOx (run to Manufacturer's recommended levels), too much contaminant buildup (Silicon is an easy red flag to test for and maintain below 30ppm- your alloy has a LOT of it, up to almost 10%, which makes the Alodine process more difficult even in a clean DeOx tank), insufficient rinsing, or some combination of these. Furthermore, if the tank is being used for a two step process on castings- yours or others'- such as Alodine->Mask->Strip->Hardcoat (this is very very common) that requires time in a caustic etch prior to DeOx to remove the unwanted portion of the Alodine, these problems may become more serious due to a much faster rate of contaminant buildup.
And then the conversion coating simply does not work, and you are not getting back parts processed per your PO.
I've never checked resistivity of the smut itself as it's not supposed to be there to begin with. It is a deposit of alloying constituents, and should have been removed by the time the part leaves the DeOx tank.

A. I preach that the ferric based deox is not the best for most diecast. Clean in non-silicated, skip the alkaline etch, deox in 50%/vol commercial nitric acid with one pound per gallon ammonium bifluoride, watch the part turn frothy white all over, 'bout 20 seconds in a fresh bath, rinse twice and chromate. But, again, all diecast metal is different, the recipe has wide ranges.

A. Hi cousin B. We appended your inquiry to an existing thread where it seems to have already been answered. Please ask clarification questions as required. Good luck.

Regards,