AlSi8Cu3(Fe) and steam

Unless your steam is produced from deionized water in a glass boiler, it is not "water, nothing else". Practical steam sources always carry over traces of boiler treatment chemicals and other contaminants (including oxygen) to the steam, in concentrations which can affect corrosion. You could analyze your steam condensate to find out what is in it, or find out what boiler treatment chemicals are used in your steam source and assume that small amounts are carried over with the steam. One boiler water oxygen scavenger can break down to produce ammonia in the steam also, but the name escapes me at the moment (nasty stuff only used in very large power boilers).

Could you protect the surface by Anodizing before putting the casting to work in steam? Elia Levi

A copper-containing Al-alloy is always much more sensible to corrosion than a copper-free one.
Michael Dahms

: have you tried using de-ionized water? You can get a de-ionizer unit fairly : cheap ( as far as industrial equipment goes). It is especially helpful for : parts that will be heat treated after production.
Sorry, I can't change environment; meaning steam is what it is. Thank you any way.
tville

My guess is that the steam is reacting with the aluminium and thickening the aluminium oxide layer, hence the colour change.
Neil
Metallurgist NAMTEC Q Block Swinden Technology Centre Moorgate Road Rotherham S60 3AR tel. 44 (0)1709 722463 fax. 44 (0)1709 724999

I neglected to read to read the previous posts, so I don't know the hue of the discoloration. Steam on the material would produce oxides for which I note that aluminium oxide and silicon oxide are colorless (looks white). But copper oxide could be blueish or greenish depending on the valence of the copper in the oxide. Of course, plain old crude might also be present from steam corrosion of other materials around. So the discoloration might look blackish.

What temperature is the steam? Aluminium oxidation tends to change from a thin amorphous layer to a thicker gamma layer at about 450C (maybe lower when Cu is present, I don't know?). Surface roughness may play a part in determining the observed color, even when the oxide is white. You may also be getting some segregation of the silicon to the oxide/Al interface, due to the increased mobility at higher temperatures, if it is supersaturated. Finally, is there any carbon in the iron or CO2 in the steam? This may have potential to accumilate aluminium carbide on the surface. Because there is a multiplicity of answers, it looks like there is no getting past surface analysis, at least SEM for starters.
Brendan Hall

I would like to Thank You All who have ansvered to my questions.
Answer was copper-rich-phases at the surface of these castings. There (at the surface) was also more impurity-rich-phases than in the middle of castings.
if there is still something to add I really like to hear it.
tville ville . koskenniska tut dot fi