Introduction of intergranular corrosion in aluminum alloy etching process:
Intergranular corrosion is also a common corrosion phenomenon in localized corrosion of aluminum alloys,like pitting corrosion.This corrosion occurs along or near the grain boundary,so it is called intergranular corrosion.Regarding the cause of intergranular corrosion,it is generally believed that it is caused by the potential difference between grain boundaries and adjacent grains.Taking the AI-Cu-Mg alloy as an example,after aging and desolubilization,there is a non-precipitation zone at the grain boundary,which contains less Cu and has a negative potential,which constitutes the anode phase.The higher Cu content in the grains,the more positive potential,constitutes the cathode phase.
During the corrosion process,the Cu-poor area with negative potential will be preferentially corroded,and with the dissolution of aluminum,the isolated Cu in the grains will also enter the solution,especially the dissolution of aluminum in the cathode phase makes Cu directly enter the solution solution to generate Cu2+.Since the redox potential of Cue+is very positive(relative to the potential of aluminum),it will be reduced and precipitated in the cathode area ofthe aluminum surface.This precipitation process is called secondary precipitation.
Due to this secondary precipitation behavior,aluminum forms a new Cu-rich cathode phase in the corrosion medium,so that the previous corrosion process is repeated.This phenomenon should be avoided as far as possible for the protection of aluminum.However,in the surface texture etching of aluminum alloys,intergranular corrosion is also another cause of texture,especially materials with acicular grain structure are more important for the formation of filamentous texture.In order to achieve this purpose,an appropriate amount of metal ions or oxidizing anions with positive redox potential can be added to the corrosive medium to achieve the purpose of etching and strengthening the filamentous texture on the surface of the aluminum alloy.
But this only has a significant effect on those alloy materials that have a tendency to corrode with a filiform texture.There are also information on the filamentous texture etching of the aluminum alloy surface by the action of an external electric field in an electrolyte solution with an inhibitor.
Through limited experiments,it is found that the effect is not obvious for those alloy materials that do not have the tendency of filiform texture corrosion.Further research is needed to be widely used in mass production.
During the etching process,due to the potential difference,it is possible that the boundary of the crystal grain is etched first,and then the entire crystal grain is etched and dissolved.Therefore,it is easier to obtain a textured effect with high roughness by texture etching on a material with a larger grain size.However,the excessively large grain size will affect the strength of the material after etching,especially after a longer period of etching,the impact will be greater.Therefore,it is very important to control the etching time(ie,control the etching amount of the material)when performing texture etching.
