Aluminum and aluminum alloys show excellent corrosion resistance in atmospheric and fresh water environments, but black oxidation and surface corrosion usually occur in actual use,local corrosion such as pitting corrosion, intergranular corrosion and stress corrosion may also occur in some environmental media.
7075 alloy elements contain strengthening elements such as Cu and Zn. As the top grade of the 7 series aluminum alloy sheet, although the strength is high, the corrosion resistance is poor. During use, special attention should be paid to protection.
Main reasons:
1. Aluminum will form a passive protective film in an oxidizing acid solution, which is not easy to corrode, but in a non-acidic acid (such as hydrochloric acid) solution, the passive oxide film will be dissolved to accelerate corrosion. In alkaline solutions with pH > 8.5, the corrosion of aluminum will also be accelerated.
2. In water containing Cl-, the passivation protective film will cause defects and pitting corrosion, especially when Cu2+ and Fe3+ coexist, it is more harmful, so it cannot be used in water containing trace heavy metal ions and industrial water.
3. Reasons for the black change on the aluminum surface: the precipitation of dissimilar metal elements in a fine state adheres to the aluminum surface and the exposure of aluminum itself reduces the reflectivity of the aluminum surface; the primary color of the precipitated metal attached to the aluminum surface and the exposed metal elements Changes to colored oxides, hydroxides. Aluminum surfaces are prone to blackening in wet and oily environments.
4. Aluminum and aluminum alloys are in contact with dissimilar metal materials for a long time, and it is easy to cause contact corrosion: compared with high-potential copper (-0.20V) and iron (-0.58), low-potential aluminum (-0.85V) and high-potential metal materials When placed in an electrolytic solution in an electrically bonded manner, cell-action corrosion occurs.
Precaution:
1. Use environment: aluminum parts should be stored and used in a dry environment without metal ion pollution as much as possible
2. Processing medium: aluminum parts should be processed with special cutting fluid to avoid contact with overly acidic and overly alkaline liquids
3. Daily contact: avoid long-term contact with metal materials such as copper and steel in oily and humid environments
4. Surface treatment: such as painting, sandblasting, anodizing, etc., to form a protective layer on the surface
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