ã€China Aluminum Industry Network】 1. General principles for the formation of anodized aluminum oxide film An aluminum plate is used as an anode in an electrolyte solution. The process of forming an aluminum oxide film on the surface by electrolytic action is called anodization of an aluminum plate. The cathode of the device is a material with high chemical stability in the electrolytic solution, such as lead, stainless steel, aluminum and the like. The principle of aluminum anodization is essentially the principle of water electrolysis. When the current passes, hydrogen is released on the cathode; on the anode, the oxygen that precipitates is not only molecular oxygen but also atomic oxygen (O) and ionized oxygen, which is usually expressed as molecular oxygen in the reaction. Aluminum, which is the anode, is oxidized by the oxygen deposited on it to form an anhydrous aluminum oxide film. Not all of the generated oxygen reacts with aluminum, and some of it is precipitated in gaseous form. Thermostatic Radiator Valve,Traditional Radiator Valves,Hive Thermostatic Radiator Valve,Chrome Thermostatic Radiator Valve Ningbo Safewell Plumbing Co., Ltd. , https://www.safewellbrass.com
2. Selection of anodic alumina plate oxidation electrolytic solution:
A prerequisite for the growth of the anodized film is that the electrolyte should have a dissolution effect on the oxide film. However, this does not mean that anodic oxidation can produce an oxide film or the resulting oxide film has the same properties in all electrolytic solutions that have a dissolution effect.
3, anodized alumina plate oxidation type:
Anodizing is divided into current form: direct current anodizing, alternating current anodizing, pulsed current anodizing. Divided by the electrolyte: sulfuric acid, oxalic acid, chromic acid, mixed acid and sulfo-organic acid as the main solution of natural coloring anodized. According to the film layer properties, there are anodization such as ordinary film, hard film (thick film), porcelain film, bright modification layer, and barrier layer of semiconductor function. Aluminium and aluminum alloy commonly used anodizing methods and process conditions are shown in Table-5. Among them, the application of direct current sulfuric acid anodizing is more common.
4. Structure and properties of anodized aluminum oxide film
The anodized film consists of two layers. The porous, thick outer layer grows on a dense inner layer with dielectric properties, the latter being called the barrier layer (also called the active layer). Observed by electron microscopy, the vertical and horizontal planes of the film layer almost all show tubular holes perpendicular to the metal surface, and they penetrate the outer layer of the film up to the barrier layer between the oxide film and the metal interface. The pores around the main axis are dense aluminas that form a honeycomb hexagonal body called a unit cell. The entire membrane layer is composed of numerous such cells. The barrier layer is composed of anhydrous alumina, thin and dense, with high hardness and resistance to the passage of current. The barrier layer has a thickness of about 0.03-0.05 μm, which is 0.5%-2.0% after the total film. The porous outer layer of the oxide film is mainly composed of amorphous alumina and a small amount of hydrated alumina, and also contains cations of the electrolyte. When the electrolyte is sulfuric acid, the sulfate content in the film is normally 13%-17%. Most of the excellent properties of the oxide film are determined by the thickness and porosity of the porous outer layer, and they are all closely related to the anodizing conditions.