P2 etch creating black "smut" residue on high-silicon aluminum casting alloy

[millimillenary]

P2 etch (ferrous sulfate, sulfuric acid, DI water, 160F, 25 minutes) is needed on aluminum at my facility prior to application of a structural primer paint on the aluminum. This is fine on wrought aerospace alloys like 2024, 6061, 7075. Today we tried an alloy similar to an A356 aluminum casting alloy, except this alloy has up to 10 wt% silicon. This time, the P2 etch created a brown/black residue on the alloy in question.

Does anyone know what is happening here? Can anyone recommend an etching process or solution (preferably to a MIL standard or ASTM or such)? Should I be doing FPL etch instead?

Thanks,
Patrick

 

[Ron]

Sounds like you are getting a sulphur residue. Suggest a change to nitric acid or combination of nitric and hydrofluoric acid. Try a small area first.

 

[millimillenary]

We put a sample in our SEM and ran EDS. Main elements present: aluminum (obviously), but also silicon, oxygen, and some magnesium, but no sulfur. The amount of silicon present on the surface is far beyond the 10wt% in the alloy. We thought that maybe our P2 etch bath had contaminants which may have reacted with the alloy, so we tried fresh P2 etch. But the fresh batch also produced the same dark residue.

 

[IRstuff]

Is it possible that you grew some silicon dioxide? What was the ratio of silicon to oxygen?

You might try a quick HF dip to see it the "residue" goes away.

TTFN
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[Compositepro]

Your analysis indicates that you etched away the surface aluminum and exposed silicon grains. Fine metal particles are black.

 

[Ron]

You're etching with ferrous sulphate and sulfuric acid, but found NO sulfur in surface analysis? How?

 

[millimillenary]

Hi, Ron. You're right to be surprised, I would be too.

The EDS and SEM are relatively new and are calibrated regularly. It is also used reliably every day for failure analysis. I have not seen a reason yet to doubt the EDS detector/analyzer. There may be sulfur present, but in trace amounts. There may simply be many more "counts" of other things like silicon, aluminum, oxygen, calcium, and magnesium, and these counts crowded out the trace elements.

As for why there would be so little sulphur on the surface after exposure to a solutions with sulfur-containing ions -- I can only speculate. I believe the P2 etch is used to "grow" a complex oxide layer that is ideal for adhesive bonding (see reference:

http://eagar.mit.edu/3.37/H-337-15.pdf).

I do not think the intent of the ferric sulfate and sulfuric acid is to impart sulfur as a coating or deposit. Perhaps it is there for oxidation and to attack certain elements in the aluminum. Plus the finished product was washed thoroughly with deionized water, so any sulfur-containing ions (which are water-soluble) should have washed away.

 

[TVP]

High-Silicon containing alloys require a different procedure for developing proper coating/paint adhesion. Acid etching will attack the Al preferentially and leave behind residue (Si, some O & Mg). Have you discussed this with your paint/pre-treatment supplier?