[mjb]

I'm wondering if anyne could help understanding a reaction
I've observed. We are pumping a liquid Gallium alloy (67% Ga,
20.5 In, 12.5 Sn by weight) through a cast iron diplacement pump.
Something is reacting and what we get out is a frothy cream/foam that
is darkened; blackened even more than the typical oxide layer/membrane
that forms at the top of the liquid normally in air.

One thought is that the sulfur content of the iron is reacting with the indium in the heated parts of the pump. If so, a stainless steel pump should do the trick, or so I think. Any other ideas?
Your suggestions or references would be appreciated, thanks.

[jdurg]

What you're probably seeing is iron oxide forming in the metal mixture.  Gallium is actually a very corrosive element to metals.  It will attack and "mix" with metals which it comes in contact with.  It is probably removing little particles of iron from the pump and forming a solution with them.  That would explain the blackness of the "foam."  As for why it's foaming up, that could be caused by some air being introduced into the mixture by the pump.  I'd have to take a look through some of my books when I get home from work today in order to give you a definitive answer.  But the darkness is most certainly being caused by little bits of iron oxide coming off of the cast iron, or pure iron coming off and immediately oxidizing.

[mjb]

I've found the gallium alloy (Ga/In/Sn) to not be as corrosive
as pure Ga (such as in the embrittlement of aluminum).

Iron oxidation seems plausible - so a stainless
steel pump could have the same problem? But we also observe
a darkening when it is wiped up off a surface of any material,
even from non-ferrous floor tiles....
Anyhow the darkening isn't the main problem for us, but rather
the changing of the fluid into a foamy souffle of sorts.
See attached picture (before and after shot), and thanks for any help.

[Mitch]

This sounds like a very interesting problem. I hope even if you don't find the solution here, you'll let us know what was causing the foam.

I've noticed a similar crud when I heat bismuth a couple hundred degrees pass its melting point. Perhaps your alloy was heated too much and your getting an other allotrope of the alloy? Would it be possible to do it at a lower temperature?

[mjb]

The material is now being sent out for formal analysis, but I found &
have spoken with someone with experinece using the liquid Ga. alloy
with cast iron. They confirm that the culprit is the indium component
of the alloy reacting with trace componets from the iron
(forming e.g. InS and InP). The role iron oxides and of air entainment remians unclear. Meanwhile it seems a plastic or stainless steel pump will be fine; probably plastic as Gallium does seem to eat away at other metals,
especially aluminum, but even stainless steel (to a lesser degree).
Anyhow thanks for your input.