Red colored residue in enclosure, is that clay or just corrosion?

[PatrickStar]

I have an aluminum enclosure returned from the field that had red colored water in it. The enclosure should be rain tight and the gasket looks good. I have dried the enclosure in an oven and now have a thin red residue in the bottom.
I assumed that the residue came from the corroded electronics.
A college thinks it might be clay from the enclosure being submerged. The exterior of the box is clean with no sign of being submerged. However it would take little effort for the customer to clean it up.
Is there away to tell if it is clay as oppose to corroded copper and aluminum.

 

[Ron]

Neither copper corrosion nor aluminum corrosion produce red residue unless there is iron involved. Copper generally produces green and black residues, while aluminium corrosion products are generally white.

Clay residue is usually an orange-red to deeper red depending on the clay source. Rust is usually brownish red.

Get as much of the residue as you can scrape and put it on a glass plate. Drop one drop of phosphoric acid or hydrochloric acid on it. If it fizzes or turns black, it is likely rust. If no reaction, it is likely clay.