Types of Aluminum Corrosion

Electrolytic

• Water containing sodium chloride reacts with aluminum surfaces to produce a white residue. This is because it acts as an electrolyte. The ions produced when aluminum comes into contact with water become precipitated onto the surface as a result of chlorine's alkaline pH. Residues formed in this way are classified as "barely visible," "plainly visible," "intermediate," "thick" or "complete" in the food industry, according to a study done by North Carolina State University.

Pit-Type

• Related to electrolytic corrosion, pit-type or "pitting" is most commonly produced by halide ions, particularly chloride. In the presence of oxygen, aluminum is polarized to its "pitting potential." Aluminum tubing regularly succumbs to penetrating pit-type corrosion and rather than remain surface-bound, it penetrates through, creating small holes. This form of corrosion affects the process of aircraft manufacture. Pit-type corrosion is passive, localized and random, according to the website Key To Metals.

Inter-Granular

• Characteristic of some aluminum alloys, inter-granular attack occurs when the clearly defined boundaries between the different alloys succumb to corrosion. Each component of the alloy is attacked in a different way because of the variation in chemical properties between different elements. Inter-granular attack is commonly found in aluminum that has been over-etched (overworked or scored), poorly processed or coated with the wrong chemical, as described by the Experimental Aircraft Association.

Exfoliation

• A subset of the broader category of inter-granular corrosion, exfoliation follows grain boundaries and allows large "chunks" to fall out of the aluminum. If a grain boundary is stretched or compromised, attack will be more likely to occur. A rough-surfaced aluminum is also more susceptible than a smooth-surfaced type. Wrought aluminum is more suited to high-specification applications such as aircraft than extruded aluminum.

Stress-Associated

• A white-gray product is formed on the surface of aluminum that undergoes stress corrosion or fatigue cracking. As the name suggests, stress corrosion involves more than a chemical interaction--it happens when the aluminum is put under physical force. Cracks and warping reveal the aluminum's internal surface, which then succumbs to other forms of corrosion. High-strength, heat-treatable wrought aluminum alloys at certain temperatures are most susceptible to stress-associated corrosion.