Exfoliation Corrosion

Background

Exfoliation corrosion is a somewhat rarer type of intergranular corrosion that tends to be associated with aluminum alloys that have been rolled or extruded. Sometimes this type of corrosion has been called layer corrosion or lamellar corrosion. Highly cold-worked aluminum alloys, especially those containing copper (2XXX alloys) or zinc-magnesium-copper (7XXX) as alloying constituents have been reported to be the most susceptible. Thin gauge products forms can be most affected.

Mechanism

Rolled or extruded aluminum can have flattened grains that are elongated in a parallel arrangement. The result is the formation of sheet-like layers. Corrosion attack often initiates at edges that have been sheared laterally, perpendicular to the direction of elongation. These edges are exposed to the environment. Attack tends to proceed either along grain boundaries (more common) or along planes of insoluble constituents stretched out in the direction of working. The resulting corrosion is parallel to the surface. A thicker stratum of corrosion products can be adjacent to thinner strata of non corroded metal. The corrosion products cause a swelling of the metal. This swelling can be extremely large. In thin gauge products the swelling can result in a several-fold increase in thickness. A micrograph of this type of corrosion can be found in ASTM Standard G-34, "Standard Test Method for Exfoliation Corrosion Susceptibility in 2XXX and 7XXX Series Aluminum Alloys (EXCO Test)". ASTM Standard G66 "Standard Test Method for Visual Assessment of Exfoliation Corrosion Susceptibility of 5XXX Series Aluminum Alloys (ASSET Test)" provides further information on testing. Pictures in these two standards show how various degrees of exfoliation corrosion might appear.

Additional Characteristics

  • Slightly acidic environments may enhance the degree of exfoliation corrosion.
  • Commercially pure 1XXX grades and the manganese bearing 3XXX grades of aluminum alloys seem to be very resistant to exfoliation corrosion.
  • Aging to certain tempers has been reported to improve the resistance of the 2XXX and 7XXX alloys. Some loss in strength might occur.
  • Accelerated tests may not accurately predict longer term behavior with respect to exfoliation corrosion. Such tests, though, might provide reasonable screening prior to longer term testing such as atmospheric corrosion tests.
  • The web site http://www.argentumsolutions.com/tutorials/alloy_tutorialpg5 provides information on the various grades and tempers of aluminum alloys.

Return to the index page for CORWIKI