Influence of fabrication (Topic 11011)

There will be variations on the structure of extruded sections in both longitudinal and transverse directions and a recrystallized layer can often be formed at the surface of some alloys, e.g. alloy AW-6351. This recrystallized layer may act as a barrier layer to prevent formation of deep pits. Extrusions with peripheral coarse grains have performed better than equivalent plate material in sea-water corrosion tests.

If sections are highly cold worked, like after drawing or severe bending, and have elongated grains, they may experience exfoliation corrosion, but this corrosion form is usually mild and localized on AlMgSi alloys.

Graphite or carbon is a very efficient cathode. Graphite may be deposited on the aluminium surface and in weld zones internally in the profile and between billets because of the use of graphite containing lubrication fluids like dag, sooting of the billet or from graphite blocks on the run-out table. It is very important that remaining graphite is removed or it can act as an initiation point for corrosion by increasing the cathodic current.

References [35,38,45]