The mastery of the anodic oxidation process conditions is closely related to the quality of the oxide film, this is because the process conditions are based on different process formulations, after a series of experiments, before the anodic oxidation, the specified process conditions should be known, the operation should be done in strict accordance with the process requirements. Among them, the most basic most sensitive factors are: solution temperature, voltage current density control range, anodic oxidation time, solution stirring method, current density of tank liquid the ratio of tank liquid to anodic oxidation area. If there is a deviation in the process of implementing these process conditions, the quality of the anodic oxide film will be significantly affected. When the deviation is too large, it may cause the scrap of the parts, resulting in economic losses.
When the process conditions exceed the control range, the influence on the quality of the anodic oxide film, different symptoms correction methods are discussed in the following questions.
How is the voltage controlled during anodizing?
The voltage is regulated according to the solution temperature. When the solution temperature is low, the voltage of the upper limit should be used. This is because the oxide film obtained when the solution temperature is low is dense, the oxide film resistance is large, to thicken the oxide film must take a higher voltage, otherwise it is difficult to obtain the normal quality of the oxide film. On the contrary, when the solution temperature is high, the voltage should be reduced, otherwise, the film solution will be too fast due to the generated loose oxide film, it is difficult to obtain the ideal oxide film thickness.
For example, in units without cooling devices, the solution temperature will be close to the limit temperature in summer. If the operation still needs to continue, the voltage should exceed 12V. In winter, when the solution temperature is below the limit temperature limit, the voltage has to rise to a high value, such as 18V.
Anodic oxidation is an exothermic reaction, when the workload is full, the temperature of the solution will gradually rise, so it should be tested at any time, as a basis for voltage regulation. If the temperature continues to rise, then the voltage will be below the specification is difficult to guarantee the quality. Production should be stopped at this time. Take corresponding measures to cool down, to meet the process requirements when processing.
How to control the current density during anodic oxidation?
Under normal temperature conditions (around 20℃), except for special process formulations, the current density of anodized aluminum its alloys is generally controlled between 1 -- 1.5A/dm2.
According to the solution temperature, solution concentration, product shape other relevant process conditions.
Under the possible conditions, appropriately increasing the current density is conducive to accelerating the rate of film formation, shortening the time of anodic oxidation, increasing the porosity of the film improving the coloring effect. However, when the current density continues to increase, the influence of joule heat will be increased in the process of anodic oxidation, the thermal effect in the film hole will be increased, the local temperature will rise significantly, thus accelerating the dissolution rate of the oxide film, the film forming speed will decline, the complex parts will also cause uneven current distribution, affecting the coloring effect. In the surface of the parts may also be easy to wipe off the loose oxide film, film layer brittle, cracking, white traces, serious may also cause ablative parts.
Choosing the appropriate current density in a certain range can accelerate the growth rate of the film, but when it exceeds a certain value, the film formation rate decreases.
According to the above rules, in order to ensure product quality improve production efficiency, the following methods can be adopted.
When the cooling condition is good the solution can meet the requirement of strong stirring, the upper limit of current density can be used to improve the working efficiency.
Under the condition of neither cooling device nor strong stirring, although the temperature of the solution was moderate at that time, the current density should be properly controlled to prevent the quality problems caused by too fast temperature rise in the process of anodic oxidation, may cause the ablation of parts in serious cases. The most effective method is to reduce the volume current density.
The correct estimation of surface area of anodized parts is also an important condition for the reasonable control of current density.
The surface of the deep concave part of the anodized part shall have the same current density as other surfaces.