Abstract
The surface of commercially pure aluminum and aluminum alloy were darkened to dark gray or brownish dark gray by boiling well-water with a pH of 6.8, but in distilled water the surface colo rremained in light gray.
Surface oxide layeres, naturally formed in room atmosphere, does not prevent the darkening.
Blackening occurs in neutral or alkaline water.
In distilled water containing magnesium, calcium, sodium, or iron salt, aluminum was not darkened, but in some cases the degree of the luster decreased. The mutual action of the salts may play an important role in darkening.
Blackening seems to be caused by adsorption of some substances contained in water.
