(1) Spot corrosion: Spot corrosion, also known as pore corrosion, is a localized form of corrosion that produces needle like, dot like, and pore like structures on metal. Spot corrosion is a unique form of anodic reaction, which is an autocatalytic process. It refers to the conditions caused by the corrosion process inside the spot corrosion hole, such as the presence of corrosive media (CL -, F -, etc.) and substances that promote the reaction (CU2+, ZN2+, etc.), which not only promote but also are sufficient to maintain the continuation of corrosion.
(2) Uniform corrosion: When aluminum dissolves in solutions such as phosphoric acid and sodium hydroxide, the oxide film on it undergoes uniform corrosion, and the dissolution rate is also uniform. As the temperature of the solution increases, the concentration of the solution increases, promoting the corrosion of aluminum.
(3) Gap corrosion: Gap corrosion is a type of localized corrosion. The phenomenon of crevice corrosion in electrolytic solutions where metal components form gaps between metals or between metals and non-metals, with a width sufficient to immerse the medium while leaving it in a stagnant state, exacerbating internal corrosion within the gaps. Gap corrosion is particularly prone to occur at the joints of mechanical components, such as metal gaskets or riveting joints, and at the gaps between aluminum doors and windows and mortar. It belongs to a type of battery effect, but gaps generally occur within a specific range of size, such as having sufficient width to allow the solution to enter, narrow enough to allow the solution to stagnate, etc. Therefore, caution must be taken in applications or engineering to avoid environments that can cause crevice corrosion. The mechanism of crevice corrosion is similar to that of perforation corrosion. Firstly, it is uniform corrosion, and then due to the oxygen concentration and dilution of the battery, it can cause anode reaction (oxygen deficient zone) and cathode reaction (oxygen rich zone). Due to the inability to replenish oxygen in the gap, the anode reaction will continue to occur in the same position, resulting in serious corrosion results.
(4) Intergranular corrosion: refers to the phenomenon of localized corrosion occurring at metal boundaries. From an electrochemical perspective, due to the fact that the grains of the material are cathodes and the grain boundaries are generally anodes, the corrosiveness at the grain boundaries is still slightly greater than at the grain boundaries under uniform corrosion. If in special circumstances, the corrosion resistance elements at the grain boundaries of the material are relatively reduced, and intergranular corrosion will occur. AL-CU-MG and AL-ZN-MG aluminum alloys tend to exhibit intergranular corrosion.
(5) Stress corrosion cracking (SCC): SCC in aluminum alloys was discovered in the early 1930s. The failure of metals under the combined action of stress (tensile stress or internal stress) and corrosive media is called SCC. The characteristic of SCC is the formation of corrosion, a mechanical crack that can develop along grain boundaries or propagate through crystal decay. Due to the expansion of cracks inside the metal, the strength of the metal structure can be greatly reduced, and in severe cases, sudden failure may occur. When a material is subjected to local stress or uneven stress, the area subjected to high stress will form an anode, while the area subjected to lower stress will form a cathode. Therefore, the applied stress will accelerate the corrosion process, which is called a reactive battery. Stress corrosion occurs in materials that are cold worked, and areas that are highly cold worked are more anodized. Additionally, in the presence of cracks in the material, it can also cause stress corrosion.
