THE PREVENTIVE MEASURES OF UNEVEN FAULTS FOR OXIDE FILM DYEING

　　In fact,the uneven film layer colors of aluminum alloy oxide film will present a lot of phenomenons(such as some faults introduced above).How to guarantee the uniformity of  oxide film layer dyeing because of that the color uniformity of aluminum alloy sulfuric acid anodized film is mainly depended on the aluminum alloys’ components and anodized treatment technical conditions,such as temperature,current density,fixture,using water quality and inter-process protection.Only start from the fine place of actual process can effectively take measures to obtain uniform color and excellent performance of the oxide layer.

1.Select Appropriate Pre-Treatment Methods

　　For the different aluminum alloys,such as casting,rolling or mechanical processing or heating treatment of welding and other aluminum alloy parts of processes processing,it should select appropriate pre-treatment methods(including degreasing, bright dip)according to actual situation.For example,the aluminum alloy parts surface shaped by castings,its non-machined processing surface usually adopts sand blast or shot blasting to remove the original oxide films and burnt-on sands.For the aluminum alloy(cast aluminium)with high silicon content,it should be etched and excited in mixed solutions of 5% of hydrofluoric acids and nitric acids to keep a good excitation surface and ensure the qualities of anodized film layers.Different materials of aluminum alloys,bare aluminum and pure aluminum parts or different specifications of aluminum and aluminum alloy parts,generally not in the same tank for oxidation.

2.Adopt Appropriate Craft Equipment

　　The hanging equipment materials should ensure good electric conduction,general adopt the pure aluminum wire or aluminium rib with high specifications and ensure a certain degree of elasticity and strengths.Determine whether heat treatment is required as needed. Used special or general 2.craft equipment such as anodic oxidation when used again,must be removed from the surface oxide film to ensure good contact.It is necessary to ensure that there is enough conductive contact area,but also to minimize the equipment moulage and ensure the smooth discharge of gas in oxidation,to avoid some oxide parts forming gasbags,resulting in oxide film layer is too thin or no oxide film.

3.Seriously Control The Solution Temperature Of Anodizing Treatment

　　From the film formation processes of anodic oxidation,it is known that the thickness of the film gradually becomes darker as the anodizing temperature increases,and the thickness of the film is gradually thinned.The main reason is that the anodized film is insulating,and when the oxide films are formed,the resistances will be increased accordingly.These resistances once power on will produce voltage drop, which will make a lot of electric energy into heat energy,so that the temperature of oxide solutions increased,accelerating the dissolution of the film layer.The temperature of oxide solution is higher,the dissolution is stronger,so that the thickness of the film gradually becomes thinner as the temperature of oxide solution increases.When the temperature of oxide solution is low,the creating oxide films are dense,with small porosity and not easy to color.When the temperature of oxide solution is high,the oxide films are sparse gradually,with large porosity and the colors of the films will darker as the temperature increases.Therefore the temperature of oxide solution is a important element which decides on the dense degree and color uniformity of oxide film.The temperature of sulfuric acid anodized solution must be controlled strictly,and selecting different temperature range according to different aluminum alloy.For duralumin,ultralumin and pure aluminum,the best solution temperature range is 15℃~25℃.For antirust aluminum alloy parts,the best solution temperature range is 10℃~18℃.In the sulfuric acid anodized technology,it needs adopt compressed air to stir and equip refrigeration equipment.In a situation without cooling installation,it can add 1.5%~2% of glycerol or organic carboxylic acids such as oxalic acid and lactic acid in sulfuric acid solution,which make the temperature of anodized solution more than 30℃to avoid or reduce the loosening and pulverization of oxide film.Some oxidation processes and production experiments have shown that adding moderate organic carboxylic acid or glycerol to the sulfuric acid anodized solution can effectively reduce the influence of the heat effect and increase the temperature upper limit of anodized solution without reducing the thickness and hardness of oxide film.In addition,under a condition of controlling the constant oxide solution temperature,the anode current density should be effectively controlled.

４.Seriously Control The Anode Current Density

　　As the anode current density increases, the color of the oxide film becomes deeper and the thickness of the film becomes thicker firstly and then thinner. The reason is that with the increase of the anode current density, the pore of the oxide film increases and is easy to color,while with the decrease of the anode current density,the growth rate of the film decreases and the film layer becomes dense.In the other conditions remain unchanged,improving the anode current density,the generation of the oxide film will be faster, which can shorten the anodizing time and reduce the chemical dissolution of the film layer, resulting in getting a harder and excellent wear-resistance film layer.But the anode current density cannot rise too high,otherwise, due to the impact of Joule heat, the internal heat effect will increase,the local temperature rises significantly to accelerate the dissolution of the oxide film,and the film-forming speed decreases that is easy to cause the ablation of the parts.In other words,when the anode current density increases,the generation rate of oxidation film-forming will accelerate within a certain range.But to a certain value,the film-forming speed is reduced.The size of the anode current density directly affects the uniformity of the film color,according to different aluminum alloys choose different anode current density,for all kinds of pure aluminum and aluminum parts,the best current density is 1．0A／dm²～1．5A／dm².For cast aluminium and wrought aluminium parts,the best current density is 1．5A／dm²～2．5A／dm².For duralumin and superhard lead parts,the best current density is l．3A／ dm²～1．5A／dm².For rust-proof aluminum alloy parts,the best current density is 1．0A／dm²～1．3A／dm².

5.Ensure The Water Quality During The Processing Of Anodic Oxidation

　　Must strictly control the water quality used by sulfuric acid anodized solutions and the harmful impurities in the solution.Preparation of sulfuric acid anodized solution must be used softened water, deionized water or distilled water, and not use the tap water which contains Ca2 +, Mg2 +, SiO32- and Cl-.In general,when the CI-concentration in water is at 25mg/L that will be harmful to the anodizing processing of aluminum alloys.CI-ion(include other halogen elements)can destroy the forming of oxide films,or cannot even form an oxide film.The water used by sulfuric acid anodizing cleaning and sealing should use deionized water or distilled water and be required for CI≤15mg/L,total minerals≤50mg/L.

６.Enhance The Protection Of Inter-Process

　　Generally believed that aluminum is not rust, it is not true.Due to the aluminum alloys contain a variety of alloying elements,such as CI and other active anions in interface.In addition, due to large production volume and long production cycle, during this period, if no effective protection measures are taken, the corrosion points on the local surface will appear and develop rapidly to the depths. After the alkali corrosion, it is obvious that the oxide film surface appears small "pitting" or small "pits".In order to avoid the occurrence of keyhole corrosion, firstly, the parts surface must be coated with anti-rust oil during the inter-process, especially after finishing finishing.Secondly, parts in the process of turnover, it is strictly forbidden to take with "naked hands" and pay attention to "anti-rust" and "moisture-proof".

7.Secondary Heat Treatment For Special Aluminum Alloy Parts

　　For some special parts that are not uniform in heat treatment,according to practical production,after the completion of rough machining, the process of secondary heat treatment (quenching + artificial aging) is added, and the quenching time and quenching cooling speed are strictly controlled during quenching.These two factors have great influence on the mechanical properties and corrosion resistance of quenching materials.It is generally required that the transfer time of super hard aluminum alloy is not more than 15s, otherwise, it will make a large number of Tηand other strengthening phase separating from the crystal and reduce the strength and corrosion resistance after quenching due to the lower material temperature.The influence of quenching cooling rate is as giant as that of transfer time. The cooling rate during quenching must ensure that the supersaturated solid solution is fixed and it plays a decisive role in the performance of the alloy.In order to ensure that the mechanical properties and corrosion resistance should be obtained after quenching, a high quenching and cooling rate must be adopted.For different aluminum alloy with different quenching cooling rate, the general requirements of hard aluminum alloy quenching cooling rate is not less than 170 ℃ / s, otherwise can not get ideal quenching effect.

８.Regularly clean the impurities in the electrolyte

　　The impurities in the anodized solutions are Al3+,Cu2+,Fe2+,Cl-,F-,NO3-.When C1-,F-,NO3- and anionic impurity content is high, the porosity of anodized films increases greatly, and the oxidation surface becomes rough and loose.The allowable content of these impurities in the oxidation solution is [Cl] .059 / L, [F] <0.019 / L and so on.Al3 + in the oxidation solution mainly comes from the dissolution of the work-piece during the oxidation process. When the solution [A13 +]> 209 / L, the surface of the oxide film appear white or massive white spot,and the adsorption performance of the oxide film is lowered, resulting in difficulty in dyeing.When [Cu2 +] content reaches 0. At 029 / L, dark streaks and black spots appear on the oxide film.To exclude Cu2 +, can be through the direct current, anode current density is controlled at 0. 1A/dm² ~ 0. 2A/dm², the copper in the solution is separated out of the cathode.When the content of Cu2 + is high, the lead plate can be used directly in the electrolyte, and the lead plate will soon be replaced with a layer of metal copper.This method can remove the high content of copper ion, and then it can be treated with electricity, which can completely remove Cu2 +. If the content of impurities exceeds the allowable value, the anodic oxidation solution may be partially or completely replaced.

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