Advances in Treatment Technology of Surface Chemical Conversion Coatings on Aluminum and Aluminum Alloys

[China Aluminum Industry Network] 1 Chemical Oxidation

At a certain temperature, a thin oxide film, called the chemical oxidation of aluminum, is formed on the surface of the aluminum part by a chemical reaction. This method does not require current flow, and is simpler and less costly than electrochemical oxidation. The resulting oxide film is very thin. Generally, the film thickness is about 0.5 to 4 μm. The film is soft and has low abrasion resistance. Therefore, it cannot be used alone. The film has good physical adsorption ability and is a good bottom layer for painting. After oxidation, the resulting protective layer can greatly increase the protection of the parts.

2 Anodic oxidation

The anodization of the aluminum alloy uses aluminum or an aluminum alloy as an anode, and lead, carbon, or stainless steel as a cathode, and is electrolyzed in an aqueous solution of oxalic acid, sulfuric acid, chromic acid, or the like. Anodic oxidation by electric power can provide a film formation rate that is difficult to obtain by the natural oxidation method. In the oxide film formation process, two processes occur simultaneously: one is the process of forming an Al2O3 oxide film on the surface of the aluminum plate, and the other is that the oxide film formation process is accompanied by the dissolution process of the oxide film. Only when the oxide film formation rate exceeds the dissolution rate, can get a certain thickness of the oxide film, the film thickness is generally 5 ~ 15μm. The film is a two-layer structure composed of a dense barrier layer and a porous layer of a columnar structure. Anodized films can be divided into two major categories: porous type - films formed in acid electrolytic baths such as sulfuric acid, oxalic acid and the like and forming very fine pores perpendicular to the surface; barrier type - in neutral salt electrolysis such as ammonium borate The non-porous extremely thin film produced in the bath is generally used for electrolytic capacitors and the like.

3 micro-arc oxidation

Micro-arc oxidation, also known as plasma oxidation, is a surface treatment technique for growing a ceramic layer in-situ on the surface of a metal substrate on the basis of anodization. Its basic performance is similar to ceramic (corundum), with high hardness and wear resistance, high temperature performance, but also has a high degree of insulation and acid and alkali corrosion resistance.

4 rare earth conversion film

At present, great progress has been made in this field at home and abroad. The rare earth passivation technology developed has attracted much attention due to its non-toxic, non-polluting and good anti-corrosion effect. At present, the rare earth conversion film process can be roughly divided into three categories: 1 chemical methods with strong oxidizing agents and other film-forming accelerants; 2 chemical and electrochemical processes; 3 rare earth bohmite layer processes. The addition of strong oxidants such as H2O2, KMnO4, (NH4)2S2O4, etc. can greatly reduce the processing time, the solution treatment temperature is not high, and the chemical process containing strong oxidants formed at a low temperature for a short time is more potential for development; The combination of electrochemical processing steps is cumbersome, and the solution processing temperature is also boiling; the rare earth bohmite layer process also has the disadvantage of higher processing temperature.

5 organosilation treatment

Silanization has traditionally used a dip-coating process in which aluminum alloys are immersed in this dilute silanization solution for a certain period of time and then cured at a certain temperature to form a few hundred nanometers thick coating on the aluminum alloy surface (compared with The traditional conversion coating is much thinner and the coating can effectively prevent various forms of corrosion of the aluminum alloy. Hu Jiming et al. used electrochemical techniques to deposit dodecyltrimethoxysilane (DTMS) films on the surface of LY12 aluminum alloys. Reflective absorption infrared spectroscopy shows that the DTMS silane reagent has a chemical bond interaction with the surface of the aluminum alloy substrate, forming -SiOAl bond to achieve film formation, electrochemical impedance spectroscopy (EIS) test results show that, compared with the open circuit potential, using the cathode potential deposition method The corrosion resistance of the silane film was significantly improved.

6 phosphate primer treatment

Phosphating primer base material, the main component of polyvinyl butyral resin, plus chromate and other rust-proof pigments and auxiliaries, component two for phosphoric acid, the use of the two components before the required proportion It is evenly mixed and sprayed on the surface of the aluminum plate to combine part of the phosphoric acid with the metal aluminum, so that the metal surface and the coating film are integrated, the coating film has a certain anti-corrosion ability, and can have good binding ability with the primer in the coating system. However, the phosphating primer requires high construction conditions. If it is not careful, it will cause the film to become brittle and cause serious consequences of large-scale paint loss.

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