This looks like it could be corrosion possibly caused by the ASII. The following can be found at;
Galvanic Corrosion
This type of corrosion occurs when two conducting materials of different chemical composition are joined and exposed to conducting solution e.g. when iron is connected to copper. Iron passes into solution as Fe+2 ions and the residual electrons are conducted through to the copper and as a consequence hydroxyl ions, 0H- , are formed at the surface. Galvanic corrosion is very damaging because it concentrates on the less noble metal at the metal-metal junction, where deep attack occurs. At the junction a large corrosion current can pass because the electrical resistance of the short path through the electrolyte is low.
Common forms of metal joining, e.g. brazing, welding etc. provide junctions at which galvanic corrosion can develop. Galvanic corrosion on a microscopic scale can also occur between constituents of multi-phase alloys and of impure metals which contain foreign particles and intermetallic compounds.
When aluminium is coupled with copper or brass, corrosive attack upon the aluminium is accelerated by these materials in severe or modest atmospheres and conditions of immersion. Contact between aluminium and stainless steel (18/8, 18/8/2 and 13% Cr) will only slightly increase the corrosion of the aluminium in dry atmospheres. However, it is markedly increased in moist atmospheres, particularly in marine conditions.
A simple way of avoiding bimetallic corrosion is to isolate the two metals from one another. This can be achieved by good design and where necessary by inserting an insulating material such as neoprene between the metal/metal contact.
Exfoliation Corrosion
Exfoliation corrosion is a specific type of selective attack that proceeds along multiple narrow paths e.g. grain boundaries which run parallel to the surface of a metal. Generation of corrosion products forces the layers apart and causes the metal to ‘swell’. Metal flakes may be pushed up and even peel from the metal surface. Exfoliation corrosion is most common in the heat treatable Al-Mg-Cu and Al-Zn-Mg-Cu alloys. This form of corrosion is associated with a marked directionality of the grain structure. Applied stresses are not necessary for exfoliation to occur. However, in alloys susceptible to stress corrosion cracking, the corrosion product wedging action undoubtedly contributes to the propagation of the exfoliation attack. It is important to note tht some alloys not susceptible to stress corrosion cracking (Al-Mg-Si) may suffer exfoliation corrosion. However, if the grain structure is equiaxed, exfoliation corrosion does not usually occur.
