The aluminum industry has long recognized that stress corrosion cracking (SCC) in metal materials requires three essential conditions: first, the material must be susceptible to SCC; second, it must be exposed to a specific corrosive environment—such as a saline medium or a corrosive atmosphere for aluminum alloys; and third, tensile stress must be present. These factors are interrelated and influence each other significantly.
Environmental factors play a crucial role in stress corrosion of aluminum alloys. They include ion species, ion concentration, pH levels, the presence of oxygen and other gases, corrosion inhibitors, temperature, and pressure. For example, studies on 2A12 and 7A04 aluminum alloys have shown that their susceptibility to SCC varies depending on the atmospheric conditions. Marine environments, which contain high concentrations of salt and chloride ions, are particularly aggressive. The chloride ions can penetrate the protective oxide layer on the alloy surface, leading to localized corrosion and eventual cracking.
Experiments also reveal that when the mass concentration of nitric acid (HNO3) is between 20% and 40%, the corrosion of aluminum alloys increases significantly. The corrosion rate peaks around 35% concentration. However, in concentrated HNO3 solutions, SCC is less pronounced. This is because a dense oxide layer forms on the aluminum surface, acting as a barrier against further corrosion.
Metallurgical factors, such as casting methods, processing techniques, and thermal treatments, also influence SCC. It has been observed that cathodic polarization can increase the susceptibility of aluminum alloys to SCC. Additionally, friction stir welding tends to produce lower SCC sensitivity compared to fusion welding. Properly treated alloys like 6061-T6 and 3004 are generally resistant to SCC. However, metallurgical changes can alter the composition of the surface film and the internal structure, affecting the electrochemical and mechanical properties of the alloy, thereby influencing its SCC resistance.
Stress factors, including the type of load, magnitude, direction, and loading speed, are equally important. For SCC to occur, the stress direction should be perpendicular to the grain boundaries. Stress effects are a key element in the process, with alternating stresses often leading to corrosion fatigue. Unlike SCC caused by static stress, corrosion fatigue typically results in more severe damage and faster crack propagation. Moreover, the loading speed can significantly affect the SCC sensitivity of aluminum alloys, making it an essential consideration in material selection and application design.
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