Effect of Copper Content on etching Response of Aluminum in Alkaline and Acid Solutions
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Copper are used as an alloying element in aluminum alloys to increase the strength of the material. By mixing copper and aluminum the good corrosion resistance of the pure aluminum decreases giving the alloy a lower corrosion resistance. After years of investigation on corrosion of aluminum alloys several results have shown increasing corrosion rates of aluminum that have been alloyed with both copper and zinc, giving a grainy appearance on the surface of the alloy. By adding copper to the aluminum increased intergranular corrosion and preferential etching of surface grains has been found after heat treatment and etching of the alloy.Before etching the aluminum alloys in alkaline and acidic environments, Glow Discharge Mass Spectroscopy (GDMS) measurements was done for all the alloys to determine the correct amount of alloying elements and impurities in each alloy. Preliminary etching trials have been performed on pure aluminum (Al 5N), aluminum containing 10 ppm copper (AlCu10), aluminum containing 100 ppm copper (AlCu100), and aluminum containing 1000 ppm copper (AlCu1000), as well as corrosion potential tests. Further, the surface of all the etched test specimens has been investigated in light microscope, Scanning Electron Microscope (SEM), Electron Backscattered Diffraction (EBSD), Confocal Microscope (IFM), Energy Dispersive Spectroscopy (EDS),and Glow Discharge Optical Electron Spectroscopy (GD-OES). These methods have been used to determine height differences between etched neighboring grains, the crystallographic orientation of grains, surface roughness, detecting elements occurring on the surface of etched specimens, and giving a depth profile of the etched specimens.For the alkaline etching trials, an increasing etching rate was found by increasing the amount of copper in the alloys, however the AlCu10-alloy showed little or no difference from the pure aluminum both in the etching trials and the corrosion potential tests. By increasing the copper amount in the aluminum the corrosion potential increased drastically for AlCu100-alloy and the AlCu1000-alloy. Further an increasing etching rate of grains having close to  crystallographic orientation could be seen in alkaline environment, a difference in the surface roughness between grains with different crystallographic orientations could also be seen for the alkaline etched test specimens. The GD-OES investigations done on test specimens that had been alkaline etched revealed differences in their depth profiles dependent on the etching temperature and how the specimens was treated after being etched. AlCu1000-alloys etched in acidic environment showed a different type of etching and surface after etching than the other alloys etched in the same environment. A clear difference could also be seen between the alkaline and the acidic etched AlCu1000 specimens.