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Analysis and Prevention of Anodic Oxidation Failure of Aluminum Alloy

1.Common Faults and Analysis
  (1) After the aluminum alloy products were anodized by sulfuric acid, the local oxidation free elusive occurred, showing macroscopic black spots or streaks, and the oxide film had bubbling or holes.Such failures, although rare, also occur
  The above failure causes, generally related to the composition, organization and phase homogeneity of aluminum and aluminum alloys, or some metal ions or suspended impurities dissolved in the electrolyte, etc.Aluminum and aluminum alloys the chemical composition, the organization, and the homogeneity of the metallic phase can affect the oxide film generation and properties.Oxide films of pure aluminum or aluminum magnesium alloys are easily produced and the quality of the films is also better.However, aluminum silicon alloy or aluminum alloy with higher copper content, oxide film is harder to produce, and the produced film is dark, graying, and not glossy.If the surface produces inhomogeneity of the metallic phase, tissue segregation, segregation of micro impurities or inhomogeneity of various parts of the tissue caused by improper heat treatment, etc., it is easy to produce selective oxidation or selective dissolution.Such as partial segregation of silicon content in aluminum alloys, which tends to create localized unoxidized films or black spotty streaks or localized selective dissolution producing holes, etc.In addition, if the amount of suspended impurities, dust or metallic impurity ions such as copper and iron in the electrolyte is too high, it will often make the oxide film black spots or black stripes, affecting the corrosion protection performance of the oxide film
  (2) Anodized parts treated in the same tank, some with oxide free film or film layers that are either slightly thin or incomplete, some with ablative corrosion at the contacts of clamps and parts.Such failures tend to occur more frequently in stream acid anodization process practices and seriously affect the anodization quality of aluminum alloys
  Since aluminum oxide film is better insulating, aluminum alloy manufactures must be securely mounted on universal or specialized clamps before anodization treatment to guarantee good electrical conductivity.The conductive rods should be of copper or copper alloy material of choice and adequate contact area should be guaranteed.The clamps are in contact with the parts, both to guarantee free passage of current and to minimize clamps and part to part contact impressions.Too small a contact area and too high a current density can produce excessive heat easily burned parts and clamps.No phenomena such as oxide film or incomplete membrane layer were observed mainly due to poor clamp and piece contact, poor electrical conduction or due to incomplete removal of oxide film layer on the clamp
  (3) Aluminum alloy after sulfuric acid anodization treatment, the oxide film is loose and powdered even hand elusive, especially after filling closure, the surface of the fabrication appears a severe powder layer, and corrosion resistance is low.This type of failure mostly occurs in summer, especially for sulfuric acid anodization troughs without cooling devices, and the phenomenon of loose powdering can occur after the components of troughs 1-2 are often processed, obviously affecting the quality of the oxide film
  Due to the large anodic oxide film resistance of aluminum alloy, a large amount of Joule heat will be generated during the anodization process, and the higher the tank voltage generates more heat, which leads to the continuous rise of electrolyte temperature.So in the anodization process, a stirring or cooling device must be employed to keep the electrolyte temperature within a certain range.Generally, the temperature should be controlled at 13-26 ◦ C, and the quality of the oxide film is optimal.If the electrolyte temperature exceeds 30 ℃, the oxide film will produce loose powdering, the quality of the membrane layer is low, and in severe cases, the"" burnt out"" phenomenon occurs.In addition, when the electrolyte temperature is constant, the anode current density must also be limited because the anode current density is too high and the temperature rises dramatically, and the oxide film is also prone to loose and powdery or sand grain shape, which is quite unfavorable to the quality of the oxide film
  (4) Occasionally, the oxide film after sulfuric acid anodization of aluminum alloy is dull and sometimes produces pitting corrosion, and in severe cases, the black pitting corrosion is significant, resulting in end-of-life of the part and causing large loss
  Such failures tend to occur by chance and have special causes.In the process of aluminum alloy anodization, half-way power loss and re electricity supply, often make the oxide film dim and light-free, while half-way power loss parts stay too long in the cleaning tank, the high acidity of the washing sink, the net water quality, containing suspensions, mud and so on, tend to make aluminum alloy manufacture electrochemical corrosion, the occurrence of punctate corrosion black spots and so on.Sometimes the addition of tap water to the electrolyte, water treated with bleach powder with exceedingly high Cl-content or sometimes the container filled with HCl without thorough washing and contained sulfuric acid, can make the mixture in the anodizing electrolyte with exceedingly high Cl-, and thus cause the anodization of aluminum alloy parts to make the product end-of-life and so on

2.Precautions against failure
  The quality of aluminum alloy sulfuric acid anodized oxide film will be good or bad, and the superiority of the corrosion protection performance will mainly depend on the composition of aluminum alloy, the thickness of membrane layer as well as the anodization treatment process conditions, such as temperature, current density, water quality used and the filling and closing process after anodization.To reduce or avoid anodization failure to improve product quality it is important to start at the microscale and take effective measures
(1) For different aluminum alloys, such as casting, casting or machining molding or heat treatment welding and other procedures, the appropriate pretreatment method should be chosen according to the actual situation.For example, the surface of aluminum alloy cast and its non machined surface should generally be sandblasted or blasted to clean its original oxide film, sticky sand, etc.Aluminum alloys with high content of silicon (especially cast aluminum) should be dip activated by a mixed acid solution of nitric acid containing about 5% hydrofluoric acid to effectively maintain a good activated surface and ensure the quality of the oxide film.Aluminum alloys of different materials, bare aluminum and pure parts or parts of aluminum and aluminum alloys with different size specifications, generally should not be treated by oxidation in the same trough
  For lap, spot welding or swaged aluminum alloy combinations, for aluminum alloy preparations which can not easily be excluded by easily forming gas bags during anodization, sulfuric acid anodization process is generally not allowed from the quality consideration
  (2) Mounting jigs materials must ensure that the conduction is good, generally hard aluminum rods are selected, and plates are guaranteed to be of some elasticity and strength.The barbs are appropriately chosen of copper or copper alloy materials.Re use when already used specialized or general purpose jigs such as anodization treatments, their surface oxide film must be thoroughly removed to ensure good contact.Working jigs should both guarantee adequate conductive contact area and minimize jig impressions.If the contact surface is too small, it will cause ablation to melt out the anodized part
  (3) The temperature of the sulfuric acid anodization solution must be tightly controlled, and the optimal temperature range is 15-25 ◦ C.Compressed air stirring is required during the sulfuric acid anodization process and refrigeration should be provided.The addition of 1.5%-2.0% of propionic or carboxylic acids such as oxalic acid and lactic acid to the sulfuric acid electrolyte without refrigeration device can avoid or reduce the loosening or powdering of the oxide film by anodizing the solution temperature range beyond 35 ° C. Some process tests and production practices have confirmed that the addition of an appropriate amount of carboxylic acid or propriol into the sulfuric acid anodization electrolyte can effectively reduce the adverse effects of the thermal effects of the reaction, can improve the temperature of the anodization electrolyte without reducing the thickness and hardness of the oxide film, and can improve the production efficiency, under the premise of quality assurance.In addition, with the control of constant temperature, care is also taken to effectively control the anode current density in order to better guarantee the oxide film quality
  (4) The water quality used by the sulfuric acid anodizing electrolyte and the harmful impurities in the electrolyte must be strictly controlled.Formulating sulfuric acid anodization solutions tap water should not be used, especially turbid tap water containing high levels of Ca2+, Mg2+, SiO32-, and Cl-.Generally, the Cl- concentration in water up to 25 mg/l can have a harmful effect on the anodization treatment of aluminum alloys.Cl-(including other halo elements) can disrupt oxidative film generation or even form no oxide film at all.Sulfuric acid anodization should be performed with demineralized, deionized or distilled water containing ≤15 mg/L of Cl- in the electrolyte and ≤50 mg/L of total minerals
  Sulfuric acid solution in the anodization process, will produce a smeary foam and suspended impurities, which should be regularly excluded.Other harmful impurities commonly found in the oxidation solution of sulfuric acid anode plates are Cu2+, Fe3+, Al3+ and so on.Harmful effects can occur if the impurity content exceeds the permitted content, and the sulfuric acid solution can be partially or fully replaced to effectively guarantee the quality of sulfuric acid anodization of aluminum alloys
  Aluminum alloy sulfuric acid anodization treatment is a widely used and well-established treatment process for anti-corrosion protection decoration, as long as the process conditions are strictly enforced and the operation is serious, the quality of sulfuric acid anodization film is completely guaranteed

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