Study On The Exfoliation Corrosion Of Al-Li Alloys And The Application Of Fractal Dimension In Corrosion Research

First, the corroded images, EIS(electrochemical impedance spectroscopy) and EN(electrochemical noise) evolution of two kinds of high-strength aluminum-lithium alloys, AA2090 and AA2195, were studied. Both of the alloys were T6 tempered to peak status. The exfoliation behavior of the tow kinds of alloys was different from each other because of their different metallurgical structure caused by composition.Under the used temper parameters, the T₁ phase, which is anodic to the main grains, tended to extract itself at the grain boundaries, rather than within the grains. This kind of structure cannot be taken as effective anodic path for exfoliation propagation. As a result, 2195-T6 alloy only developed a shalow exfoliation degree with the corrosion image of small patches being peeled off the surface, showed good exfoliation behavior.Compared to that of 2195-T6 alloy, the T₁ phase in 2090-T6 alloy tended to evolve at the grain boundaries and expand there, comsumingθ'. A PFZ(precipitation free zone) developed itself along the grain boundaries as a result, providing an anodic path for exfoliation corrosion. Mean while, there were Fe-containing constute particulates in the structure. When the electrode was being immersed in EXCO solution, severe intergranular corrosion occurred along the boundaries because of the T₁ phase was comsumed. Large pittes developed around the constute particulates, providing initiation center for exfoliation corrosion. As long as initiated, exfoliation corrosion propagated along the anodic path which was parallel with the surface. 2090-T6 alloy developed a corroded image with large exfoliation bubbles, showed a very poor exfoliation behavior compared to 2195-T6 alloy.The EIS evolution of both two alloys shared some charactaristics: EIS both consisted of a high frequency capacitance arc and a low-middle frequency inductance arc. The inductance arc tended to dispear with the elongation of immersion time; EIS during the exfoliation propagation stage and post exfoliation corrosion stage consisted of two capacitance arcs. EIS can be taken as an efficient way to dipict exfoliation initiation and propagation.The EN data of two kinds of alloys were decomposed using wavelet method. Pits rupture and the afterward diffuse procedure were clearly manifested on the decomposed siganals. Mean while, time-depended evolution of the strenth of high frequency decomposition signals dipicted soundly the localized corrosion evolution of both the alloys.Second, the effect of the applied tensil stress on exfoliation corrosion evolution of two kinds of aluminum alloys, LC4 and 2090-T6, were studied, using EIS as a probe for different localized corrosion types. It was shown in the results that the applied stress didn't change the evolution mechanism of the exfoliation corrosion propagation for both the alloys, but did accelarate the initiation and propagatin of EC. The accelarating machanism lied in:The materials was elongated as the stress was apllied up on, the disfigurement density on the surface oxide film increased accordingly, so did the adsorption point density for Cl^- ions. Furthermore, the oxide film was more likely to react with the Cl^-ions, being resolved and enter the solution because of the falsity between the film and the base metal, the initiation of EC was promoted as a result. For LC4 alloy, which polarization resistance was bigger, the applied tensil would accelarate the corrosion in the EC inatiation stage. And the extent to which the materials was extended was in proportion with the stress levels in the flexibility of the materials. So, the polarization resistance was linear to the stress levels. For 2090-T6 alloy, which had a more active oxide film due to the high Li content because of the separation of Li in casting, rolling and tempering procedures, the effect of the stress levels on polarization resistance just immersed.On the other hand, the applied tensil stress can cause an additive stress index at the tip of EC. The lifted surface layer actually didn't bear any stress because there were large pitts or greaves that connect the EC area below the surface with the bulk solution. Therefore, there was a shearing strength between the tensiled base materials and the lifted surface layer. This shearing strength would cause an additive stress index at the tip, coeffecting with the wedging stress. It was showen in the calculating results that the total stress index was much lagger than that of the unstressed samples, under the applied stress levels. So, the applied tensil stress accelorates EC propagation.Last, the application of the image fractal dimension(Df) and the EIS dimension in dipicting localized corrosion in nutrual solution. For both of the aluminum alloys, 2195-T6 Al-Li and 2024-T3 Al-Cu, the image D_f and the EIS D_f evolved similarly. At the same time, the image D_f tended to over estimate the true D_f when the corrosion was much severe because of the innate shortage of the procedure, and the EIS D_f tended to less estimate the true D_f. D_f evolution of both the two kinds of aluminum alloys and both the two kinds of steels showed that, the D_f increased with the initiation of pitting corrosion; D_f would decrease because the corrosion pitts extand and joined with one another when the corrosion propagated; for same kinds of materials, larger D_f meant more severer corrosion. Therefore, D_f can be used as an effective corrosion evaluation method.