The aluminum industry has long recognized that stress corrosion cracking (SCC) in metal materials requires three key 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 chemically aggressive atmosphere for aluminum alloys; and third, there must be an applied tensile stress. These factors are interrelated and influence each other in complex ways.
Environmental factors play a significant role in stress corrosion. They include the type and concentration of ions in the surrounding medium, pH levels, the presence of oxygen or other gases, temperature, pressure, and the use of corrosion inhibitors. 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 are rich in salt and chloride ions, are particularly harmful. The chloride ions can penetrate the protective oxide layer on the surface of the alloy, leading to internal corrosion.
In acidic environments, such as nitric acid solutions, the effect of SCC is also notable. Experiments have shown that when the mass concentration of HNO3 is between 20% and 40%, the corrosion rate of aluminum alloys increases significantly, peaking around 35%. However, in highly concentrated HNO3, SCC is less pronounced because a dense oxide layer forms on the alloy’s surface, offering protection against further corrosion.
Metallurgical factors also contribute to SCC. These include casting methods, processing techniques, and the effects of thermal stresses during manufacturing. It has been observed that cathodic polarization can increase the SCC sensitivity of aluminum alloys. Additionally, friction stir welding tends to produce lower SCC susceptibility compared to fusion welding. Properly treated alloys like 6061-T6 and 3004 are generally resistant to SCC. However, metallurgical changes can alter the surface film, internal structure, and crystallographic orientation of the alloy, affecting its electrochemical and mechanical behavior, and thus its resistance to stress corrosion.
Stress factors, such as the type of load, magnitude, direction, and loading speed, also play a crucial role. For SCC to occur, the stress must act perpendicular to the grain boundaries. Stress effects vary depending on the type of stress applied. Alternating stress combined with a corrosive environment leads to corrosion fatigue, which is typically more severe than SCC caused by static stress. Furthermore, the rate at which the load is applied can influence the alloy’s susceptibility to SCC, making this a critical factor in material selection and design.
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