Application of Pickling in the Surface Treatment of Aluminium Alloys

 

Removal of Oxide Film and Impurities: A natural oxide film will form on the surface of aluminium alloys during processing or storage, and impurities may also be attached. Pickling can effectively remove these substances. For example, when using dilute sulfuric acid for pickling, sulfuric acid (₂₄) will react with aluminium oxide (₂₃) on the surface of the aluminium alloy. The chemical equation is ₂₃₂₄₂₄₃₂. Through this reaction, the oxide film on the surface is removed, leaving the surface of the aluminium alloy in a relatively pure state, providing a good foundation for subsequent treatments. Moreover, pickling can also remove some intermetallic compound impurities on the surface of the aluminium alloy, and these impurities may affect the quality and performance of the subsequent oxide film.

 

Surface Activation: The surface of the aluminium alloy after pickling has higher reactivity. This is because after the oxide film is removed, fresh aluminium atoms are exposed on the surface and are more likely to react with other substances. For example, pickling before anodizing can improve the reaction efficiency of the aluminium alloy during the anodizing process, making the generated oxide film more uniform and dense. At the same time, the activated surface is also beneficial to subsequent chemical conversion treatment and other processes, enhancing the reactivity between the aluminium alloy and the treatment solution.

 

Application of Alkali Cleaning in the Surface Treatment of Aluminium Alloys

 

Removal of Oil Stains: The surface of aluminium alloys will be contaminated with oil stains during processing (such as cutting, stamping, etc.). Alkali cleaning is an effective method for removing oil stains. Usually, sodium hydroxide () solution or sodium carbonate (₂₃) solution and other alkaline cleaning agents are used. Taking sodium hydroxide as an example, it has a saponification reaction with grease. The glyceride of higher fatty acids in the grease reacts with sodium hydroxide to produce sodium higher fatty acid (the main component of soap) and glycerol. This reaction can decompose the oil stains into water - soluble substances, so they can be easily removed from the surface of the aluminium alloy. For example, during the manufacturing process of aluminium alloy doors and windows, after alkali cleaning, the oil stains on the surface are removed, ensuring the quality of subsequent surface treatments.

 

Synergistic Effect of Surface Cleaning and Pretreatment: Alkali cleaning can not only remove oil stains but also play a certain cleaning role on the surface of the aluminium alloy. During the alkali - cleaning process, some slight stains and dust will also be removed together. Moreover, the surface of the aluminium alloy after alkali cleaning is alkaline, which has a certain promoting effect on some subsequent chemical pretreatment processes (such as chemical conversion treatment). For example, before chromating treatment, the alkaline surface after alkali cleaning is beneficial to the better reaction of the chromate solution with the surface of the aluminium alloy, forming a chromate conversion film with better quality.

 

 

 

 

 

 

 

 

 

 

Grade

(wt%)Chemical Composition(wt%)

C

Si

Mi

P

S

Cr

Ni

Mo

Cu

As

Sb

Sn

12Cr2Mo1R(H)

≤0.15

≤0.15

0.30-0.60

≤0.010

≤0.010

2.0-2.5

≤0.20

0.90-1.10

≤0.20

≤0.016

≤0.003

≤0.015

 

 

Chromating Treatment

 

Formation of Corrosion - Resistant Conversion Film: Chromating treatment is to form a chromate conversion film on the surface of aluminium alloys. The chromate ions (₄²⁻) in the chromate solution will react with the aluminium atoms on the surface of the aluminium alloy to form a chromate conversion film with good corrosion resistance. For example, the reaction formula may be ₄²⁻₂₂₃⁻. This chromate conversion film can form a barrier on the surface of the aluminium alloy, effectively preventing the corrosive medium from contacting the aluminium alloy substrate.

 

Auxiliary Role in Oxide Film Formation: The chromating film plays a role in protecting the substrate in the initial stage of oxide film formation and is conducive to the continuous growth of the subsequent oxide film. In the process of surface treatment of aluminium alloys, chromating treatment is carried out first, and then the oxide film formation process (such as anodizing or thermal oxidation) is carried out, which can make the oxide film grow more uniformly and completely. For example, in the surface treatment of some aluminium alloy products with high requirements for corrosion resistance (such as aerospace components, automotive parts, etc.), the aluminium alloy after chromating treatment can generate an oxide film with better quality in the subsequent oxidation process, thereby improving the service life and reliability of the product.

 

 

 

 

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