| Experiments were conducted to determine the stress corrosion cracking (SCC) susceptibility of silver containing aluminum-lithium alloys in two different conditions T3 (naturally aged condition) and T8 (artificially aged condition) and another well known commercially used aluminum alloy, 2219-T87. The 2195 alloy was developed to replace the currently used aluminum alloys such as 2219 for space applications.; During the experiment, the load versus time and the applied potential were monitored. The main parameter which was used to quantitatively determine SCC susceptibility was the plastic strain ratio. Fractography using SEM and optical microscopy was conducted to confirm whether SCC occurred and, if so, to identify its mode (intergranular or transgranular or mixed). Limited potentiodynamic studies were also completed to evaluate the passivation behavior of these alloys. An attempt was also made to correlate alloy microstructure, slow strain rate test parameters and passivation behavior with SCC susceptibility.; The results of the polarization studies suggested that all the alloys exhibited anodic, cathodic, and passive regions. The polarization curves showed that the alloys passivate in the potential range of {dollar}-{dollar}1100 to {dollar}-{dollar}750 mV (SCE), but in both the cathodic and anodic regions the alloys undergo corrosive attack.; The monotonic tensile strength was the highest for alloy 2195-T8, and the ductility was the highest for 2195-T3. Values of these properties for the control alloy 2219-T87 are lower than for 2195 in either heat treatment.; The pre-exposure results of the specimens immersed in neutral 3.5% NaCl without applied load for 3, 7, and 14 days showed that the alloys were immune. The ductility ratio for all alloys under these conditions is approximately unity. The specimens pre-exposed for three days in 3.5% NaCl at pH 12 gave slightly reduced ductility ratio. For pH 3, however, the ductility ratio decreased significantly, a greater amount for alloy 2219 than for alloy 2195. The specimens of the three alloys showed shallow pits in pH 12, but in pH 3 they showed what appears to be intergranular attack.; Of all the alloys studied, SCC was observed only in 2195-T3 when tested in the potential range of {dollar}-{dollar}600 to {dollar}-{dollar}450 mV (SCE) and strain rates of {dollar}rm7.6times10sp{lcub}-5{rcub} ssp{lcub}-1{rcub}{dollar} and {dollar}rm7.6times10sp{lcub}-6{rcub} ssp{lcub}-1{rcub}{dollar} based on the ductility ratio criteria and metallography. However, the other alloys 2195-T8 and 2219-T87, and at the same range of potential showed directional attack (directional pitting). When the applied potential is in the range of {dollar}-{dollar}1100 and {dollar}-{dollar}750 mV (SCE), the alloys showed no SCC with a ductility ratio approximately unity. For the applied potential in the cathodic regime, more negative than {dollar}-{dollar}1350 mV (SCE), the alloys are subject to pitting corrosion. The ductility ratio in this region of the three alloys indicates that 2195 performed better than 2219. (Abstract shortened by UMI.)... |
