What Does Stratified Corrosion Mean?
Stratified corrosion is a type of corrosion that progresses parallel to the metal surface in such a manner that underlying layers are gradually separated. It is a form of intergranular corrosion that occurs on the elongated grain boundaries.
Aluminum alloys that have been extruded or otherwise worked heavily, with a microstructure of elongated, flattened grains, are particularly prone to this type of damage. It often initiates at end grains encountered in machined edges, holes or grooves and can subsequently progress through an entire section.
Stratified corrosion is also known as lamellar corrosion, layer corrosion or layer exfoliation.
Corrosionpedia Explains Stratified Corrosion
Stratified corrosion is a form of intergranular corrosion that proceeds laterally from the sites of initiations along planes parallel to the surface, generally at grain boundaries, forming corrosion products that force away metal products from the body of the material, giving rise to a layered appearance.
The corrosion product has a greater volume than the volume of parent metal. This increased volume tends to force the grains apart and leads to stratified corrosion. In extreme cases, the edges of the affected area are leaf-like and resemble the separated pages of a wetted book that has become swollen and begun to spread apart. Corrosion products building up along these grain boundaries exert pressure between the grains and the end result is a lifting or leafing effect.
This type of corrosion is visually recognizable if the attack is severe, otherwise microstructure examination under a microscope is needed. Controlled shot peening can be very effective in the process of both identifying and repairing this corrosion damage.
This type of corrosion is often found next to fasteners in aircraft where an electrically insulating sealant or a sacrificial cadmium plating has broken down, permitting a galvanic action between the dissimilar metals.
This corrosion can be prevented through:
- Use of proper coatings
- Selecting a more stratified corrosion-resistant aluminum alloy
- Using heat treatment to control precipitate distribution