Technology And Mechanism Of Electrolytic Etched Graining Of Al-Mg-Si Alloy

The process of electrolytic etching and graining on the surface of Al-Mg-Si alloy has been studied systematically by means of electrochemical measurement. The morphology, component and structure of electrolytically etched surface are characterized and the mechanism of electrolytic etching and graining is also discussed based on the theoretical analysis.The components that can form barrier layer or dissolve the resisting layer on the surface of Al-Mg-Si alloy specimens are investigated through voltamperometric and chronoamperometric analysis in several solutions. Four systems of NaBO2-H3BO3, C4H6O6-Na2CO3, Na3PO4-C4H8O2 and Na3C6H5O7-H2O-C6H8O7-H2O are selected as etching solutions. It has been shown that the pattern obtained from Na3C6H5O7-H2O-C6H8O7-H2O is relatively shallow due to no ideal etchant, but after optimizing the solution and technique conditions, picturesque pattern can be obtained. In the system of NaBO2-H3BO3, Na2CO3 is used as etchant, with Na3PO4 as enhancing bond-agent, and as a result, the pattern is picturesque and the depth of mark is moderate. After anodic oxidation and electrolytic coloring, the color of the pattern is distinctive and natural. The pattern is very similar to wood graining. The effect of NaBO2 on electrolytic etching is in accordance to that of H3BO3. H3BO3 is an auxiliary agent during the formation of resisting layer and also a buffer. The optimal combination of current density and time makes electrolytic etching under ambient temperature possible. In this system, operation is simple and convenient, electrolyte is stable and it is easy to maintain and adjust. The pattern obtained by electrolytic etching can be preserved for a long time and its corrosion resistance, wear resistance and climate resistance are the same as that of the film colored by anodic oxidation.SEM observation shows that the components of aluminum alloy affect greatly the result of electrolytic etching and patterning, low hydrogen over voltage elements such as Si and Mg should be used as additives in aluminum alloy in order to obtain good pattern. The results of SEM, XRD and XPS analysis show that electrolytically etched surface consists of Al and A12O3. A12O3 concentrates in pattern area and exists in forms of a- A12O3 and y- A12O3. Anodic oxidation does not changeobviously the structure and composition of oxide film; but gives rise to the existence of SO42" in oxide film. This seems to be resulted from the diffusion and electromigration of anion towards the inner of porous layer. Anodic oxidation also increases the micro structure difference between the pattern area and the pattern-free area. So do the pore density and size in pattern area. Electrolytic coloring metal interlinks and piles up in the micropores of pattern area and makes color deep. The oxide film of pattern area is thicker than that of pattern-free area. The interface between oxide film and substrate is distinctive; but there is no obvious interface between electrolytic coloring film and oxide film; which indicates that coloring metal is deposited in micropores.The theoretical analysis results show that the electrolytic etching and patterning on the surface of aluminum alloy is naturally an metal partial corrosion process associated with hydrogen evolution; spot and line corrosion. Spot corrosion happens at the beginning of electrolytic etching and patterning and followed by line corrosion. The process of electrolytic etching and patterning is a complex electrochemistry process; together with the charge exchange; partial current density distribution and electromigration. On the basis of the above results;the mathematical relation between the structure parameters of porous layers and its growth conditions is proposed.