How it works
Rolling and extrusion flatten and stretch the grains of a wrought alloy into long, thin plates aligned with the working direction. When the grain boundaries are more anodic than the grain interiors — because precipitates or solute-depleted zones have formed along them during heat treatment — attack follows the boundaries, as in any intergranular corrosion. The difference is geometry: with pancake-shaped grains, the corroding paths run parallel to the surface instead of down into it. Aluminum corrosion products are bulkier than the parent metal, so as they build up between layers they pry the unattacked metal apart. The result is swelling, flaking and delamination that can remove far more sound metal than the corrosion itself consumed.
Where it shows up
Exfoliation is a classic problem of high-strength aluminum alloys, notably the 2xxx and 7xxx series, and of some 5xxx alloys with high magnesium content after prolonged warm service. It tends to start where end grain is exposed: sheared or machined edges, fastener holes, countersinks and cut ends of extrusions. Moisture, chlorides and contact with more noble metals accelerate it, since a galvanic couple drives the aluminum to corrode faster. In plants it appears on aluminum structures, enclosures, covers and transport equipment exposed to marine or de-icing salt atmospheres.
How susceptibility is tested
ASTM G34 is the accelerated immersion test for exfoliation susceptibility of 2xxx and 7xxx series aluminum alloys, known as the EXCO test. Specimens are immersed in an aggressive solution and rated visually against the standard's reference photographs. Exposure conditions, ratings and acceptance criteria come from the standard and the material specification, not from field judgment. Other ASTM methods cover 5xxx alloys.
What to do about it
| Approach | Principle |
|---|---|
| Temper selection | Overaged tempers are less susceptible than peak-aged tempers of the same alloy |
| Seal end grain | Coat or seal cut edges, holes and machined faces before assembly |
| Isolate dissimilar metals | Insulate fasteners and contact faces to break galvanic couples |
| Keep it dry | Drain, ventilate and avoid crevices that hold salty moisture |
Once layers have lifted, the affected metal has lost load-bearing section. Damage is removed to sound metal and the remaining thickness is checked against design requirements before the part is protected and returned to service.