The answer, it turns out, is similar to why iron rusts; if iron is left unprotected in open air, it will corrode and form a flaky orange-red outer layer. "When copper metal corrodes, it forms what is called an oxide layer," Paul Frail, an advanced senior engineer in corrosion ...
The pref. alloys contain less than 0.8% Al and less than 0.3% Mn, and pref. also 0.5-1.5% Cu, and/or 0.01-0.06% Mg. An esp. pref. compsn. (I) is 0.75% Al, 0.25% Mn, 1% Cu, 0.04% Mg, remainder zinc. The high creep resistance is combined with the low corroding effect of...