The effect of microstructure on corrosion resistance and anodising behaviour of a 3rd generation lithium-containing aluminium alloy, AA 2099-T8, has been studied.Microstructural characterisation of the alloy has shown elongated grains with high angle grain boundaries and approximately equiaxed subgrains with low angle grain boundaries. Two types of constituent particles of high and reduced contents of copper were found, which are present individually or in the form of multiphase particles. The alloy also contains rod-like Al-Cu-Mn-Li and spherical (Al3Zr) dispersoids, and fine age hardening precipitates of T1 (Al2CuLi), (Al3Li) and (Al2Cu).Immersion testing in a chloride-containing solution revealed two types of localised corrosion sites: one was initiated from intermetallics and quickly became passivated; the other lasted for the course of testing, with extensive boundary attack. The latter has been correlated to grains and subgrains with relatively high stored energy.Alkaline etching removed intermetallics from alloy surface; generated a copper-enriched layer in the alloy matrix immediately beneath the residual alumina film; and gradually developed a surface nanotexture. The surface nanotexture has been correlated to the formation of copper-rich nanoparticles within the copper-enriched layer.Anodising the alloy in tartaric-sulphuric acid disclosed that copper in the alloy matrix could be occluded in the anodic film material as copper-rich nanoparticles or be oxidized and incorporated into the film material as copper ions, depending on the anodising voltage. In the latter case, the process was accompanied by oxygen gas generation within the film material, forming anodic films with lateral porosity. Intermetallics of different compositions behaved differently during anodising, leading to modification of the morphology and composition of anodic films. Concerning the effect of alkaline etching on anodising behaviour of the alloy, the removal of intermetallics increased anodising efficiency; the pre-developed copper-enriched layer advanced the establishment of the steady-state film/alloy interface. Variation of copper-enriched layer structure is revealed by the detection of copper-rich nanoparticles in the copper-enriched layer. The copper-rich nanoparticles have structures consistent with either θ'', θ' or θ phases. Machining damage can be detected not only by chromic acid anodising (CAA) but also by tartaric-sulphuric acid anodising (TSAA). Neither CAA nor TSAA can be used for detecting shallow corrosion pits.
|Date of Award||1 Aug 2011|
- The University of Manchester
|Supervisor||Xiaorong Zhou (Supervisor)|
- AA 2099 aluminium alloy