Oxidation temperature is one of impact on the formation of the oxide film in hard anodizing of Al7075

Oxidation temperature is one of impact on the formation of oxide film in hard anodizing of Al7075

  When hard anodizing process of 7075 aluminum alloy, the oxidation temperature and time, current density and concentration of the oxidation bath have the greatest impact on the formation of the oxide film.

A: Analysis From the Forming Principle of Hard Oxide Film

Formation process of aluminum oxide film is an exothermic electrochemical reaction process [2Al+3 [O]=Al2O3+399kcal ]; Generally ,large amout of heat of formation will generate when generating AL2O3. In addition, there is a large amount of electrical energy at the bottom of hole that is converted into Joule heat, causing the temperature of the solution around the workpiece where the anode reaction occurs to be higher. The dissolution effect of sulfuric acid in the solution on the oxide film gradually strengthens with the increase of solution temperature, disrupting the balance between dissolution and film formation, accelerating the chemical dissolution rate of the film layer, causing excessive corrosion, then cause the oxide film to loosen and fall off, resulting in the production of Al2 (SO4) 3 powder:[Al2O3+3H2SO4=Al2 (SO4) 3+3H2O]; From the appearance, it can be seen that the surface of the oxide film appears "white" remark-Like the below picture.

In addition, excessively high temperature can cause heat concentration on the surface of the sample, it generates high impedance and increases the voltage that resulting in "ablation" defect.

  Due to the dissolution of the aluminum oxide film generated by the aluminum element, the silicon phase and other less active metal phases on the surface of the basal body cause the surface of the aluminum parts to become rough after "ablation"

B:Analysis from the aspects of film structure composition and physical properties

    7075 aluminum alloy anodized film consists of two layers: an inner barrier layer (insulation layer) of 20-25nm; an outer porous layer accounts for 99% of the total film thickness

    Due to the insulation of the oxide film, the resistance increases correspondingly when the oxide film is formed. After these resistors are energized, a voltage drop. This will convert a large amount of electrical energy into thermal energy, increasing the temperature of the oxidation solution, and accelerating the dissolution of the film layer. The higher temperature of the oxidation solution, the stronger the dissolution effect. Therefore, as the temperature of the oxidation solution increases, the thickness of the film layer will gradually become thinne

     When the temperature of the oxidation solution is low, the formed oxide film is dense and has a small porosity; As temperature increases, the oxide film gradually becomes loose and the porosity of the film layer gradually increases. Therefore, the temperature of the oxidation solution is an important factor determining the density of the oxide film