Brass changing colors
Brass can have a wide variety of compositions, some found on the
Leaded yellow brass contains approx. 67% Copper, 29% Zinc, 3% Lead,
and 1% Ti
Red Brass 85% Copper, 5% Lead, 5% Tin, and 5% Zinc.
Silicon Bronze approximately 95% Copper, 4% Silicon, and 1%
alpha brasses have 37% of zinc
beta-brasses have 40-45% zinc
Admiralty brass contains 30% zinc and 1% tin
Aluminum brass contains aluminum.
The common thing here is the copper, nice red copper.
So which is yours
Lets assume it is a simple brass, in the range of 60% Cu 40% Zn. Zinc
is much more active than copper. Put the brass in a environment that
attacks metal, and the Zn goes first. This leaves a top coat of nice
pink / red copper.
See wikipedia on Selective leaching, where they state: Selective
leaching, also called dealloying, demetalification and parting, is a
corrosion type in some solid solution alloys, when in suitable
conditions a component of the alloys is preferentially leached from
the material. The less noble metal is removed from the alloy by
microscopic-scale galvanic corrosion mechanism. The most susceptible
alloys are the ones containing metals with high distance between
each other in the galvanic series, eg. copper and zinc in brass.
I think the answer is so simple that everyone thought someone else
would answer. I think we all have put brass in the pickle and
watched it turn red - same thing. But corrosion does not need a
overtly obvious acid to work, most electrolytic solutions will
promote some corrosion. Heat up the solution or agitate it (or both
in an ultrasonic) and the reaction is faster.