Intergranular Corrosion
Intergranular Corrosion can be caused by poor processing during manufacture of the alloy. Usually it is caused by selecting the wrong chemical in attempting a conversion coating on a given alloy. It also can be caused by poor control of heat treatment processing. For example, over-etching aluminum can cause this type of corrosion.

Exfoliation Corrosion
Exfoliation Corrosion is similar to intergranular corrosion except the corrosion follows grain boundaries and large chunks fall out. Extrusions can be susceptible where grain boundaries are stretched and rough surfaces occur during the extruding process. If you have a corrosion critical part you may wish to use bar or rod material that has been wrought instead of extruded. In industry extruded stock is used a lot but it usually goes through several inspections before going on an aircraft.

Stress Corrosion
The high-strength heat-treatable wrought aluminum alloys in certain tempers are susceptible to stress-corrosion cracking, depending upon the product, section size, direction and magnitude of stress.

Stress corrosion cracking is where the internal stresses vary across a section so that when they are loaded with additional outside forces, the grain boundaries at the surface start to break. When a part (sheet, extrusion, etc) is quenched and the outside layer cools too quickly, tension stresses are set up on the outside and compression stresses in the middle. This is sometimes taken care of by stretching or shot peening. Local stresses can also be caused by selection of too small diameter of high shear fasteners (such as bolts), shrink or press fits, taper pins, and clevis joints in which tightening of the bolt imposes a sustained bending load on the clevis lugs.

Fretting Corrosion
Fretting corrosion is caused by two surfaces rubbing together at a very small amplitude. It can be eliminated by placing a very thin sheet of Nylon or Teflon between the surfaces.