Mechanisms of localized aqueous corrosion in aluminum-lithium-copper alloys

The objective of this dissertation was to determine the role of microstructural heterogeneity, the localized environment chemistry and surface films in localized corrosion and stress corrosion cracking of an Al-3Cu-2Li alloy 2090.; The influence of microstructural heterogeneity was studied by identifying the phases important to local corrosion and preparing these in bulk form for characterization of their corrosion behavior by standard electrochemical techniques. Based on this characterization, preferential boundary attack was determined to occur via selective dissolution of the T{dollar}sb1{dollar} (Al{dollar}sb2{dollar}CuLi) intermetallic phase on subgrain boundaries. A gross form of pitting associated with impurity Al-Cu-Fe particles was shown to initiate by the local galvanic attack of the matrix by the noble particles.; The short-transverse stress corrosion cracking (SCC) behavior of the alloy was studied using a static load, smooth bar SCC technique in Cl{dollar}sp{lcub}-{rcub}{dollar},Cl{dollar}sp{lcub}-{rcub}{dollar}/CrO{dollar}sbsp{lcub}4{rcub}{lcub}2-{rcub}{dollar} and Cl{dollar}sp{lcub}-{rcub}{dollar}/CO{dollar}sbsp{lcub}3{rcub}{lcub}2-{rcub}{dollar} environments. Results showed that anodic dissolution SCC along subgrain boundaries was to be expected when applied potentials were more positive than the breakaway potential (E{dollar}sb{lcub}rm br{rcub}){dollar} of T{dollar}sb1{dollar} but were less positive than E{dollar}sb{lcub}rm br{rcub}{dollar} of {dollar}alpha{dollar}-Al. It was shown that this criterion was not satisfied in aerated Cl{dollar}sp{lcub}-{rcub}{dollar} solutions and S-T SCC resistance was good. This criterion was satisfied in Cl{dollar}sp{lcub}-{rcub}{dollar}/CO{dollar}sbsp{lcub}3{rcub}{lcub}2-{rcub}{dollar} environments and in alkaline isolated fissures exposed to a CO{dollar}sb2{dollar}-containing atmosphere.; Anodic polarization showed that the corrosion behavior of T{dollar}sb1{dollar} was unaffected in alkaline CO{dollar}sbsp{lcub}3{rcub}{lcub}2-{rcub}{dollar} environments but the {dollar}alpha{dollar}-Al phase was passivated. X-ray diffraction of crevice walls from artificial crevices suggested that passivation of {dollar}alpha{dollar}-Al occurred by the formation of Li{dollar}sb2{dollar}(Al{dollar}sb2{dollar}(OH){dollar}sb6{dollar}){dollar}sbsp{lcub}2{rcub}{lcub}+{rcub} cdot{dollar}Co{dollar}sbsp{lcub}3{rcub}{lcub}2-{rcub} cdot{dollar} nH{dollar}sb2{dollar}O.; Simulated crevice experiments were performed with various model alloys to determine the effects of Al{dollar}sp{lcub}3+{rcub}{dollar}, Li{dollar}sp{lcub}+{rcub}{dollar} and Cu{dollar}sp{lcub}rm 2+{rcub}{dollar} hydrolysis on steady state crevice pH. The effect of an external cathode on crevice chemistry was also studied by changing bulk solution aeration and specimen potential. Results showed that for 2090, crevice acidification occurred when the bulk solution was aerated. When the crevice was coupled to a deaerated solution, subjected to a net cathodic polarization or isolated from a bulk solution, an alkaline crevice developed. Based on predictions from hydrolysis product distribution diagrams, this chemistry evolves due to lithium dissolution combined with hydrogen reduction.