1、 Anodic oxidation concept
Anodic oxidation is the electrochemical oxidation of metals or alloys. Aluminum and its alloys form a layer of oxide film on aluminum products (anode) under the action of external current under corresponding electrolyte and specific process conditions. Anodizing, if not specified, usually refers to sulfuric acid anodizing.
In order to overcome the defects of surface hardness and wear resistance of aluminum alloy, expand the scope of application and prolong the service life, surface treatment technology has become an indispensable part in the use of aluminum alloy, and anodic oxidation technology is the most widely used and successful one at present.
2、 Production process
1. Mechanical polishing;
2. Chemical treatment to remove the copper component on the surface of some alloys;
3. Clean and degrease (for anodized parts, if it is necessary to re anodize, remove the original anodized surface layer with alkali or special agent);
4. Put it into dilute sulfuric acid as an anode for power on to generate a surface oxide layer (porous, white translucent film);
5. Dyeing;
6. Fixing (heating or sealing the holes of the surface oxide layer with chromate solution).
3、 Classification of anodic oxidation
1. Difference between anodic oxidation and conductive oxidation
1) Anodic oxidation is carried out under the condition of high voltage. It is an electrochemical reaction process; Conductive oxidation (also known as chemical oxidation) does not need to be powered on. It only needs to be soaked in liquid medicine. It is a pure chemical reaction.
2) Anodic oxidation takes a long time, often tens of minutes, while conductive oxidation only takes a few tens of seconds.
3) The film formed by anodic oxidation has several microns to tens of microns, and is hard and wear-resistant; The film produced by conductive oxidation is only 0.01-0.15 microns, and the wear resistance is not very good, but it can be conductive and resistant to atmospheric corrosion, which is its advantage.
4) The oxide film is not conductive, but because the film generated by conductive oxidation is very thin, it is conductive.
2. Hard anodizing and ordinary anodizing
Film thickness: the thickness of ordinary oxidation film is relatively thin, with a thickness of 5 ~ 30 μ m; Generally, the thickness of hard oxide film can reach 25 ~ 150 μ m. Ordinary oxidation is generally hv250 -- 350, and hard oxidation is generally Hv350 -- 550.
Surface state: ordinary oxidized surface is smooth, while hard oxidized surface is rough (microscopic, related to substrate surface roughness).
Strong adhesion: according to the oxidation characteristics generated by hard oxidation: 50% of the generated oxide film penetrates into the aluminum alloy and 50% adheres to the aluminum alloy surface (bidirectional growth).
Insulation: the breakdown voltage of hard anodizing can reach 2000V (perfect hole sealing).
Wear resistance: hard anodizing for aluminum alloy with copper content less than 2%, the maximum wear index is 3.5mg/1000 rpm. The wear index of all other alloys shall not exceed 1.5mg/1000 rpm.
Different application occasions: ordinary oxidation is mainly suitable for decoration; Hard oxidation is mainly functional and is generally used in wear-resistant and electric resistant occasions.
4、 Anodizing is better than electroplating in surface treatment of aluminum products
Electroplating on aluminum and aluminum alloy is much more difficult and complex than electroplating on steel, copper and other metal materials. The main reasons are as follows.
1. Aluminum and aluminum alloys have high affinity for oxygen and are easy to form oxide film. Once this oxide film is removed, a new oxide film will be produced in a very short time, which will seriously affect the adhesion of the coating.
2. The electrode potential of aluminum is very negative. When immersed in electroplating solution, it is easy to replace with metal ions with positive potential, which affects the adhesion of coating.
3. The expansion coefficient of aluminum and aluminum alloy is larger than that of other metals, so it is not suitable for electroplating in the range of large temperature change. The difference of expansion coefficient between aluminum and aluminum alloy coating and other metal coatings will cause large stress, so that the bonding force between coating and aluminum and aluminum alloy is not strong.
4. Aluminum is an amphoteric metal, soluble in acid and alkali, and unstable in both acidic and alkaline electroplating solutions.
5. Aluminum alloy die castings have sand holes and pores, which will leave plating solution and hydrogen, easy to bubble, and reduce the adhesion between the coating and the base metal.