The Basic Types of Corrosion of Aluminum and Aluminum Alloys
When comparing the corrosion characteristics of aluminum and aluminum alloys to other materials, it is important to first understand the basic types of corrosion that this metallic element is susceptible to. For example, aluminum has a higher natural resistance to corrosion than steel and yet is still not immune to it. There are five distinct types of corrosion affected by aluminum and its alloys; general corrosion, pitting corrosion, galvanic corrosion, stress corrosion, and intergranular corrosion. All five types of corrosion are classified under a general concept known as anodic oxidation.
General Corrosion
General corrosion is the most common type of corrosion that aluminum and its alloys experience. Caused by oxidation, general corrosion is an electrochemical process that is accelerated by the presence of dissolved salts in water or airborne particles in the atmosphere. Aluminums resistance to general corrosion depends mainly on its alloying elements, including magnesium and zinc, with higher contents of each metal providing greater corrosion resistance. The general corrosion rate of aluminum and its alloys can be further decreased by the addition of an effective protective coating.
Pitting Corrosion
Pitting corrosion, also known as crevice corrosion, is a localized form of corrosion that occurs on the surface of metal components. Unlike general corrosion, which progresses gradually across the entire surface, pitting corrosion is focused on a specific area and opens a small hole or crevice in the metal. The metals affected by pitting corrosion are typically those that possess a passive surface film, such as anodized aluminum alloys. The presence of chloride ions, especially sodium chloride, further increases the risk of pitting corrosion since their powerful electrochemical activity can create an acid environment prone to localized attack.
Galvanic Corrosion
Galvanic corrosion occurs when two different metals with different electrical potentials are connected in an electrolytic system in the presence of an electrolyte solution. The difference in electrical potential causes a reaction between the two dissimilar metals and accelerates corrosion, which is most often found around electrical connections or other wet areas. When aluminum and one of its alloys are connected, the more active alloy will be more susceptible to galvanic corrosion. However, this type of corrosion can usually be avoided by using a dielectric material to prevent direct electrical contact between the two different metals.
Stress Corrosion
Stress corrosion is a special type of corrosion that occurs when a metal component is subject to both tensile stress and a corrosive environment. This typically happens when a metal component is under sustained non-moving or non-cyclic stresses from vibration, fatigue, or creep. Unlike galvanic corrosion, stress corrosion is not caused by differences in electrical potential but rather the presence of corrosive Stress elements such as chloride compounds, certain acids, and hydrogen in the atmosphere.
Intergranular Corrosion
Intergranular corrosion, sometimes referred to as intergranular attack, is an electrochemical process that corrodes the grain boundaries of aluminum and its alloys. This occurs when the grain structure of the alloy is penetrated by a corrosive element, such as chloride, creating cracks and other openings in the grain boundaries. Intergranular corrosion is commonly associated with high temperature conditions or exposure to aggressive chemicals, but can be mitigated by controlling grain boundary chemistry or adjusting the manufacturing process.
Conclusion
The corrosion of aluminum and aluminum alloys can be divided into five basic types, each with its own unique characteristics and causes. General corrosion is caused by oxidation and is ameliorated by the addition of corrosion-resistant alloying elements. Pitting corrosion and galvanic corrosion are localized forms of corrosion that can occur if two dissimilar metals are connected in contact with an electrolyte solution. Stress corrosion can occur when a metal component is under sustained non-moving or non-cyclic stresses and finally, intergranular corrosion is caused when corrosive elements penetrate the alloy grain structure.
