brass deteration in w

[PAT SHEAFFER]

>
>The deterioration of some brass screws in water, as described as "br
>and mealy" is probably due to a phenomenon called dezincification. B
>is an alloy of primarily copper and zinc. Some brass alloys in some
>will be corroded such that the zinc is selectively corroded from the
>alloy. The remaining copper-rich material is porous and has very lit
>strength. This condition can be avoided by using brass alloys that a
>resistant to dezincification. Valve stems may be affected, but do no
>exhibit problems because the corrosion has not affected as much of t
>cross section as for the smaller screws. Dezincification of brasses
>easily recognized if the affected materials has a pink color rather
>the typical yellowish color of the brass.
>

Dezincification is still not thoroughly understood, and can be
related to contaminants, such as fingerprints. Higher zinc
alloys (yellow brass) are more prone to this than the lower
zinc alloys (red brass). It is often the case that the metal
has been "ruined" somehow, either by incorrect stoichiometry
or heat treatment.

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[Robert J Jordan]

I have seen bronzes that contain iron that "rust" because the melt was not
raised to a sufficiently high temperature. The iron forms a colloidal
dispersion within the copper/zinc mixture. Perhaps the same effect could
account for the dezincification in brass. Zinc would then act as the
sacrificial anode, and the salts dissolve, leaving a porous copper
matrix. The cure to the problem with the iron containing bronze is to
heat the mix to a high enough temperature and aggitate the mix so that a
true alloy is formed. I suggest that brass characteristics would be
improved by similar treatment.

Fingerprints leave an acidic residue, and would serve to promote corrosion
as postulated above.

These suggestions are theories, and someone would have to do some
experimental work to check them out. I would be interested in any
results.