Intergranular corrosion

Intergranular corrosion

Intergranular corrosion is a common corrosion phenomenon in aluminum alloys as well as pitting corrosion.This corrosion occurs along or near grain boundaries, so it is called intergranular corrosion.As for the cause of intergranular corrosion, it is generally believed that it is caused by the potential difference between grain boundaries and adjacent grains.Taking ai-cu-mg alloy as an example, there is no precipitation zone in the grain boundary after the aging desolubility, with less Cu and negative potential, forming the anode phase.The Cu contained in the grain is higher and the potential is positive, forming the cathode phase.In the corrosion process, the negative cu-poor area will be corroded preferentially, and along with the dissolution of aluminum, the isolated Cu in the grain will also enter the solution, especially the dissolution of aluminum in the cathode phase leads Cu directly into the solution to form 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 of the aluminum surface. This precipitation process is called secondary precipitation.

As a result of this secondary precipitation behavior, a new cu-rich cathode phase is formed in the corrosion medium and the previous corrosion process is repeated.This phenomenon should be avoided for the protection of aluminum.But in aluminum alloy surface texture etching, another cause is the same texture intergranular corrosion, especially with acicular crystal structure of materials for silky texture formation is more important. To achieve this goal, you can add people to the corrosive medium amount of oxidation reduction potential is metal ions or oxidizing anion, silky texture etching on aluminum alloy surface in order to achieve the purpose of strengthening.

But this is only apparent for alloys with a tendency to corrode with filament-like texture.It is also reported that in electrolyte solution with inhibitor, the filamentous texture is etched on the surface of aluminum alloy by applied electric field.Through limited experiments, it is found that the effect is not obvious for those alloys that do not have the corrosion tendency of filamidal texture.

In the etching process, due to the potential difference, it is possible to etch the boundary of the grain first, and then the whole grain is etched and dissolved.Therefore, it is easier to obtain the texture effect with high roughness by texture etching on the material with larger grain size.However, excessive grain size will affect the strength of the material after etching, especially after etching for a long time.Therefore, it is important to control the etching time (that is, to control the amount of etching to the material) when etching the texture.

In a word, the chemical etching of the aluminum alloy is actually the corrosion process on the surface of the aluminum alloy.From the above aluminum corrosion behavior, the basic corrosion types of aluminum mainly include: uniform corrosion, pitting corrosion, intercrystalline corrosion, layered corrosion and corrosion cracking.If there is potential difference between one part of aluminum alloy and other parts, pitting corrosion will occur.If there is potential difference between grain boundary and grain grain boundary of aluminum alloy, there is tendency of intergranular corrosion.In the presence of stress, this intercrystalline corrosion accelerates the corrosion cracking of the metal.Controlled pitting and intergranular corrosion are needed for texture etching in these corrosion types, and the rest should be avoided.To achieve this goal, the key is how to control the different corrosion conditions and corrosion environment, so as to cause different corrosion tendency, get different texture and appearance of the texture effect.There are many types of aluminum alloys with many influencing factors, and the relationship is complicated. For example, the alloy materials of different grades have different composition of alloy elements, grain composition, alloy composition or the distribution uniformity of impurities on the alloy surface.Even if the same type of alloy material, there may be different internal metallographic structure due to different batches, different processing methods or other factors.In addition, the surface state of the alloy material before corrosion, the processing and forming method, and the change of composition content of the corrosive medium during the corrosion process all affect the formation and texture of the texture.Therefore, it is difficult to develop a chemical texture etching solution that can be widely used in various aluminum alloy materials and can control the texture roughness arbitrarily.In practical application, only by selecting the appropriate alloy material and the corresponding texture etching fluid;Or by heat treatment of the material (such as those aluminum alloy materials with severe stripe corrosion and non-uniform texture), dislocation defects and control of appropriate grain size, and then choose the appropriate texture etching solution, to etch the texture with adjustable roughness and uniform texture.

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