We are yellow chromating aluminum alloy 360 die castings. After processing, a grey/black material appears on rags used to wipe the parts. What is this material, and can it be prevented?

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Q. We are yellow chromating aluminum alloy 360 die castings. After processing, a grey/black material appears on rags used to wipe the parts. What is this material, and can it be prevented? T.S.


A. As with any 300-series aluminum casting alloy, 360 is a mixture of aluminum and silicon. Nominally, 360 is 9.5% silicon and 0.5% magnesium, with the balance being aluminum. Although you do not describe it, I am assuming you perform a pretreatment process of the casting to remove mold release and generally provide an improved surface on which to apply the chromating. Also, while you are documenting the problem at the end of the process line, it would be useful to better understand where it is forming. The first appearance of the grey/black material is not likely to be in the final stages at the chromating steps.

To effectively remove the mold release and provide an optimal surface for chromating, it is likely that you are performing a chemical etching of the surface. Generally, there are two primary methods of doing this: acid or alkaline. If you’re etching with a product specifically designed for the job, the formulation would contain a large quantity of free caustic (if a base), typically in the form of either sodium hydroxide or potassium hydroxide. This product would also be specifically formulated without silicates in order to allow the high-pH process chemical to act as aggressively as possible on the aluminum.

In this case, the elevated pH generated by the free caustic is very effective at dissolving aluminum, but less effective a dissolving the silicon that is alloyed with the aluminum in the casting.

So the process is essentially dissolving aluminum from the silicon in the casting, creating a high-silicon residue that persists on the surface.

The alkaline cleaners I am aware of are not effective in the removal of this residue, also generically referred to as smut. This term can refer to any residual alloying elements remaining on the part surface following the alkaline etch steps of the process.
In many process lines, a nitric acid step is incorporated in order to remove the smut from the residual alloying elements on the part. This is generally effective at the dissolution of most alloying elements, including copper. However, in your case, if the residual element on the surface that is causing the smut is largely due to the silicon in the casting, nitric acid is only partially effective at its removal.

The primary method I am aware of to remove residual silicon from the surface is a mechanical action, as you describe when rubbing a casting following the chromating or a chemical means using hydrofluoric acid as your etching agent. A dilute mixture of this acid (1–2%) along with about 10% nitric acid at room temperature is very effective for etching aluminum without the issue of residual smut. The downside to this solution is the use of the acids for the process step, particularly the hydrofluoric acid, which can be especially dangerous.

I would urge you to further investigate your process prior to implementing a final fix. You need to identify exactly where the smut is being generated first. Then evaluate potential alternatives off-line, prior to full implementation in the current process line. If it is indeed in the etch tank upstream, your two solutions will focus on an alternate etch tank or an additional desmut step that would be added to the process. If you do identify the silicon as the source of your problem and further identify the need to go with an acidic desmut process step, it will be important to first consider the materials of construction of your process tanks. If designed for an alkaline cleaner, the acidic chemical will likely be more aggressive and the tank will not be compatible with the acid. It’s important to discuss this issue with the designer and builder of your process line prior to any implementation. 

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