Influence of thermal treatment and shaping (Topic 11034)

Corrosion attacks that proceeds along grain boundaries, like intergranular, stress and exfoliation corrosion, may be enhanced by unfortunate heat-treatment. Particles that are cathodic or anodic compared to the matrix, can be precipitated on the grain boundaries and give rapid corrosion along these.

The alloys from the 2000- and 7000-groups are most vulnerable to these forms of corrosion. The 6000-group alloys are normally not regarded as particularly prone to these corrosion types. However, the susceptibility to intergranular corrosion has been found to be higher for specimens in the T6 temper compared to the T4 temper.

Experience indicates that a combination of unfortunate heat-treatment, unbalanced alloys and very corrosive environments can result in intergranular corrosion even on an AlMgSi-alloy.

Intentional deformation by bending, rolling, stretching and extruding and deformation by damages introduces changes in the microstructure. Grains will be stretched in the process direction. In extruded material, there will often be a recrystallized surface layer. Corrosion attacks in processed material will often progress in directions according to the process. Pitting will attack faster parallel to the direction of processing than perpendicular to the same.

A recrystallized material will often be more cathodic than a deformed structure. This can give a kind of galvanic corrosion, where for example a recrystallized surface layer will be cathodic to an underlying deformed structure. When the surface layer is penetrated, a rapid undermining will occur because of the large cathode/anode ratio.

References [35,38]