The role of microstructure and microchemistry on intergranular corrosion of aluminium alloy AA7085-T7452

Microstructure and corrosion evolution of additively manufactured aluminium alloy AA7075 as a function of ageing

“…Following ageing at 120°C for 24 h, nanoscale η′-phase (MgZn2) precipitates evolved in the Al-matrix, mostly along cellular structure boundaries, as shown by the dotted arrow in Figure 2c′, indicating that the solute segregated along the cellular structure boundaries aid in nucleating η′-phase precipitates via solute diffusion and redistribution. In fact, η′-phase precipitates were also found to contain Cu, as previously reported in wrought Cu-containing 7xxx Al-alloys [38,42,61].…”

“…Whilst at anodic sites, 'self-dissolution' of the second phase particle occurs, most typically in the form of incongruent particle dissolution / dealloying of Mg from such anodic second phase particles (Mg2Si, Al2CuMg and MgZn2) resulting in remnants of either Si-, Al2Cu-and Zn-, respectively [38,[46][47][48][49][50][51][52]. As has been documented previously, following its dealloying, the remnant of Al2CuMg has been determined to behave as a local cathode, dissolving the Al-matrix [50][51][52], the role of Mg2Si and MgZn2 (the latter of which may be Cu enriched [42]) remnants is not clear. Pitting and localised corrosion associated with second phase particles evolves with time, and serves as the precursor to intergranular corrosion in wrought AA7075-T6, which is an alloy that can also suffer stress corrosion cracking in the presence of tensile stress [31,[33][34][35][36][37][38][39][40][41].…”

“…It is also noted that the SLM process annihilated coarse Fe-containing constituent particles in as-SLMed AA7075. The annihilation of coarse Fe-containing constituent particles in as-SLMed AA7075 is beneficial in the context of localised corrosion susceptibility, since localised corrosion predominantly associated with such particles in wrought AA7075-T6 [26,[29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45]. In addition, the loss of Zn and Mg may also influence the artificial aging of SLMed AA7075, as Figure 1c, were determined to be Al-Cu-Fe(-Si) particles, which do not dissolve upon solutionising [19].…”

“…The localised corrosion, and its morphology, of wrought 7xxx Al-alloys is principally associated with second phase particles, as the particles exhibit electrochemical characteristics different from those of the Al-matrix [26,[29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45]. Electrochemical characteristics of several second phase particles that populate in the Al-matrix of wrought AA7075 have been previously determined in solutions of various chloride concentrations and pH, either by the production of bulk intermetallic phases, by the electrochemical microcell technique, or by electron microscopy [30,38,[44][45][46][47][48][49][50][51][52] providing a deterministic rationale for the of each second phases in the localised corrosion of AA7075.…”

Unravelling the characteristics of Al-alloy corrosion at the atomic to nanometre scale by transmission electron microscopy

“…However, such microstructures, which are intentionally generated for increasing strength, notably deteriorate the localised corrosion resistance of the alloys [1][2][3][4][5]. Nanoscale precipitates exhibit electrochemical characteristics that are different from the Al-matrix, resulting in the selective dissolution of precipitates or the surrounding Al-matrix, leading to pitting and/or intergranular corrosion (IGC) [3][4][5][6][7]. The extent of localised corrosion experienced by different Al-alloys is dependent on the type (and composition) of precipitates within the alloy, and therefore, a definitive understanding of the role of specific nanoscale precipitates in the localised corrosion of Alalloys is essential.…”