| Stress corrosion cracking (SCC) is a type of subcritical cracking of materials that occurs when a SCC susceptible material is simultaneously stressed in tension (applied or residual) and exposed to a specific corrosive environment. Failure of materials due to SCC could occur at stress levels much lower than the allowable service stress, causing catastrophic consequences. Decades of efforts to investigate the SCC phenomena have established the general behaviors of different materials during SCC and it is widely accepted that a susceptible material, tensile stress, and a specific corrosive environment are the prerequisites for the occurrence of SCC. However, the fundamental mechanisms behind the apparent SCC behaviors remain unclear mainly due to contradictory experimental data from different researchers, the intrinsic difficulties associated with material characterization within the restricted geometry of cracks, and the complexity of the interactions between different chemical species. In this thesis, attention is focused on a single material system, AA5083 aluminum alloy, where the SCC can be made to happen quickly so that the effects of various aspects on SCC can be examined within reasonable amounts of time, for the purpose of mechanistic study. To clear the controversies about the actual SCC behaviors and to better understand the basic mechanisms of SCC, all of the three prerequisites of SCC, i.e. susceptible material, tensile stress, and corrosive environment, have been carefully examined using various materials characterization techniques. For the metallurgical aspect (susceptible material), AA5083 aluminum alloy (Al-4.4Mg-0.7Mg-0.15Cr) has been intentionally annealed at 175 °C, a process called sensitization, for a series of progressively longer treatment times. The SCC behaviors, microstructures, mechanical properties, and electrochemical properties of these differently heat treated AA5083 specimens have been characterized. It is found that the SCC incubation time decreases for longer sensitization time while the SCC initial crack growth rate increases as sensitization time becomes longer. This phenomenon is explained as a result of the development of continuous films of anodic intermetailic, Mg2Al 3, known as beta-phase, on the grain boundaries as sensitization time increases, based on the microstructural, electrochemical, and mechanical characterizations. For the environmental aspect (corrosive environment), the SCC behaviors, as well as microstructures, of AA5083 sensitized for both 120 and 240 hours have been examined in sodium chloride (NaCl) solutions with different concentrations and pH. It has been found that both higher NaCl concentration and lower pH values lead to shorter incubation time, higher initial crack growth rate and higher total crack growth, and the SCC behaviors of the specimens that have been sensitized for 240 hours are more sensitive to environmental factors, compared with their codnterparts with 120 hours sensitization condition. The fractographic analysis demonstrates the cracking mechanism is independent of the environmental factors and is still anodic dissolution based intergranular separation. The interactions between mechanical driving force (stress intensity) and chemical driving force (NaCl concentration) are also discovered. For the mechanical aspect (tensile stress), the SCC behaviors of sensitized AA5083 with different initially applied stress intensity levels have been investigated. It is found that the incubation time is a chemical process while the initial crack growth rate and total crack propagation are determined by both mechanical and chemical driving forces. The interactions between mechanical and chemical driving forces are described as a process that is dominated by chemical driving force and only assisted by mechanical driving force. Additionally, the crack mechanism is found to be anodic dissolution as well, for all different starting stress intensities. All of the above experimental efforts indicate that anodic dissolution is the dominant mechanism for SCC in sensitized AA5083 alloy. |
