Corrosion has always been the primary factor affecting the life of aluminum sheets. Although the aluminum plate processed by a certain process can be used in a complicated environment, there are many factors causing corrosion of the aluminum plate, and the perfect treatment process cannot ensure that the aluminum plate does not corrode. There are three main reasons for corrosion of aluminum sheets: environmental factors, metallurgical factors and stress factors.
1. Environmental factors
The environmental factors affecting the stress corrosion of aluminum sheets are: ion type, ion concentration, solution pH, oxygen and other gases, corrosion inhibitors, ambient temperature, and environmental pressure.
The stress corrosion of 2A12 and 7A04 aluminum plates in different atmospheric environments was studied. It was found that the aluminum plates have different stress corrosion sensitivities in different environments and are sensitive in the marine environment. The marine environment contains a large amount of salt, and Cl- will enter the interior through the maintenance film on the outer surface of the aluminum plate to cause corrosion.
The experiment indicates that when the mass concentration of HNO3 solution is between 20% and 40%, the corrosion of the aluminum plate is intensified, and the corrosion rate of the aluminum plate reaches the highest point when the concentration is about 35%. In the concentrated HNO3 solution, the stress corrosion of the aluminum plate is not obvious. The reason for this phenomenon is that a dense oxide film is formed on the surface of the aluminum plate, which prevents further corrosion of HNO3.
2. Metallurgical factors
The metallurgical factors mainly include the casting method, processing method and the influence of gold stress corrosion at the hot point. It is found that the cathodic polarization increases the stress corrosion sensitivity of the aluminum plate, and the stress corrosion sensitivity of the friction stir welding is lower than that of the fusion welding.
Corrosion does not occur with properly treated 6061-T6 and 3004 aluminum sheets. The difference of metallurgical factors changes the type of aluminum film and causes the internal structure of the aluminum plate and the crystal structure to change, which affects the electrochemical behavior and mechanical behavior of the aluminum plate, resulting in different stress corrosion resistance of the aluminum plate.
3. Stress factors
Stress factors mainly include load type, load size, loading direction, loading speed, and the like. In the case of corrosion, the direction of stress must be perpendicular to the grain boundary so that it can be separated. One of the key factors in the occurrence of stress corrosion is the need for stress. Different stress effects will produce different effects. Corrosion fatigue occurs when alternating stress and environment work together, and stress corrosion cracking which occurs with fixed stress is usually significantly different. Corrosion fatigue is usually more serious than stress corrosion. In addition, the difference in loading speed also affects the sensitivity of aluminum plate stress corrosion.