In our aluminum alloy etching process,we summarize and analyze the following factors that affect the hardness of the oxide film and the growth rate of the oxide film.
(1)H2SO is used for the effect of electrolyte concentration;when the electrolyte is hard anodized,its H2S concentration range is between 10-30%.When the concentration is low,the obtained film has high hardness,especially for pure aluminum.However,for alloy materials with higher Cu content,because there are CuA1:metal compounds in alloys with higher Cu content,the dissolution rate is faster during the oxidation process,and it is easy to become the current concentration center and be broken down.Therefore,for alloy materials with higher Cu content,a higher concentration of H2SO4 electrolyte should be used for hard oxidation.
(2)Electrolyte temperature Electrolyte temperature has a great influence on the hardness and wear resistance of the oxide film.Generally speaking,the lower the temperature,the greater the wear resistance of the film.The main reason is that the dissolving effect of the electrolyte on the membrane is significantly weakened at low temperature.In order to obtain a wear-resistant and high-hardness membrane,low-temperature oxidation should be used,and the temperature change should not exceed±2°C.If pure aluminum is used for hard anodizing,the hardness decreases when the temperature is close to 0°C.Therefore,in order to obtain a high hardness oxide film layer,it is advisable to carry out hard anodizing under the condition of 4-8℃for workpieces covered with aluminum.
(3)The current density increases the current density,the growth rate of the oxide film is accelerated,the oxidation time is shortened,the time for the film to be dissolved by H2SO4 is reduced,the dissolved amount of the film is reduced,and the hardness and wear resistance of the film are improved.However,when the current density exceeds the limiting current,the temperature of the anode workpiece interface is too high due to the large increase in the calorific value during oxidation,the dissolution rate of the film is accelerated,and the hardness of the film is reduced instead.If the current density is too low and the voltage rises too slowly,although the calorific value is reduced,the oxidation time must be prolonged to obtain a thicker oxide film,resulting in a longer time for the film to be dissolved by H2SO,so the hardness of the oxide film decreases.The relationship between current density and oxide film hardness,wear resistance,and oxide film growth rate is complicated.In order to obtain an ideal oxide film layer,the appropriate current density should be selected according to different alloy materials,usually 2–2./dm2.
(4)Influence of alloy composition The composition and impurities of the aluminum alloy have a great influence on the quality of the hard oxide film.It is mainly manifested in the effect on the uniformity and integrity of the film layer.For Al-Cu,Al-Si,Al-Mn alloys,it is more difficult to use DC hard anodizing.When the Cu content in the alloy exceeds 5 wefts and the Si content exceeds 7.5%,DC hard anodizing is not suitable.If the hard anodic oxidation is carried out by AC and DC superposition or pulse current method,the content range of alloying elements and impurities can be expanded.
(5)The growth rate of the oxide film increases when the current density is increased and the temperature of the electrolyte is lowered.When the electrolyte concentration is low,the dissolved amount of the film layer is reduced,which can promote the growth of the oxide film layer.In order to obtain a dense and corrosion-resistant oxide film,the growth rate of the film must be reduced.
