Chromic-phosphoric acid treatment of thermally formed oxide on aluminium alloys

The ASTM chromic-phosphoric stripping process is widely used for the removal of oxide and corrosion products from aluminium surface. The method has also been used for the heat-treated surface with the assumption that it will remove both the amorphous and crystalline γ-Al 2 O 3 formed. GD-OES measurements on alloy AA 3102, which was heat-treated at 600oC, indicated that the oxygen depth profile was not affected by the stripping process. TEM observation of oxide cross sections verified that the stripping process failed to remove the thermally formed oxide layer. A compact oxide layer, consisting mainly of elongated γ-Al 2 O 3 crystals adjacent to the metal surface, was not affected, while the amorphous part of the oxide was effectively stripped.