Troubleshooting for sulfuric acid anodizing of aluminum parts

Sulfuric acid anodizing can obtain porous, colorless, and transparent anodic oxide films. This film not only has good corrosion resistance and wear resistance, but also has the ability to adsorb pigments and coatings. The quality of the anodized film mainly depends on the composition of the aluminum component, film thickness, electrolyte temperature, current density, water quality, and the filling and sealing after anodizing.
Below, Jiechang Electroplating Factory will introduce the causes and solutions of common quality problems in sulfuric acid anodizing films.
1. Loose or powdery anodic oxide film
Cause of occurrence
(1) The electrolyte temperature is too high.
(2) Anodizing treatment takes too long.
(3) The current density is too high.
resolvent
(1) Adjust the electrolyte temperature. The temperature of the electrolyte is generally controlled at 18-20 ℃. If the temperature increases, the dissolution rate of the oxide film accelerates, resulting in more pores, looseness, pulverization, and poor adsorption capacity in the film layer.
(2) Reduce processing time. The usual anodizing treatment time is 30-60 minutes. If the processing time is extended, the film layer becomes thicker, and it is easy to crack or loosen.
(3) Adjust the current density. At the beginning of oxidation, a smaller current density (about 2A/dm2) should be used for oxidation for 30 seconds, and then gradually (in a stepwise manner) it should rise to normal process parameters. If the current density is too high, the thermal effect inside the membrane pores will increase, accelerating the dissolution of the membrane. During the oxidation process, the current density should be controlled within a fluctuation range of (± 0.05) A/dm2.
2. The oxide film appears rainbow colored
Cause of occurrence
(1) The anodizing treatment time is too short.
(2) The current density is too low.
(3) Poor conductivity during workpiece oxidation.
resolvent
(1) Extend processing time. Properly extending the processing time, generally controlled within 30-60 minutes, can meet the requirements.
(2) Increase current density. Increase the current density appropriately to accelerate the growth rate of the membrane. When the current density reaches a certain value, the film forming rate actually decreases.
(3) Check the fixture. Universal fixtures should have a certain degree of elasticity, and there should be no oxide film at the contact point of the fixture. The film should be removed once every use to ensure good contact between the workpiece and the fixture. It is best to use copper sheets at the conductive contact between the fixture hook and the oxidation tank.
3. Roughness or localized corrosion of the oxide film
Cause of occurrence
(1) Voltage and current density too high
(2) Excessive chloride ions in the electrolyte.
resolvent
(1) Reduce voltage. In the early stage of anodic oxidation, the voltage has a significant impact on the formation of the oxide film. If the voltage is too high at the beginning, it will cause the edges and corners of the workpiece to break down, and the current density will increase, causing the film layer to dissolve. During operation, the voltage should be gradually increased from 0V, and it is strictly prohibited to reach the desired position in one step. At the same time, the voltage fluctuation during the oxidation process should be kept within 1-2 V.
(2) Analyze chloride ions. The chloride ion in the electrolyte should be controlled below 0.05g/L. When its mass concentration exceeds the allowable value, it will cause membrane perforation and cause local corrosion. Therefore, the water used for configuring electrolyte and cleaning workpieces must be deionized water or distilled water.
4. Dark or gray oxide film
Cause of occurrence
(1) The workpiece is not energized for a long time in the oxidation tank.
(2) There is a phenomenon of power outage and energization during the oxidation process.
resolvent
(1) Live down slot. Minimize the residence time of the workpiece in the solution as much as possible (referring to situations where there is no electricity). The surface of the workpiece is darkened due to non conductivity and no oxide film is formed. Ash is a silicon compound left behind by acid etching.
(2) Check the motor. Daily attention should be paid to whether the power motor (DC rectifier) used is in good condition, and maintenance and upkeep should be strengthened to maintain normal operation and prevent power outages during operation.
5. Local parts of the workpiece are not easy to generate oxide film
Cause of occurrence
(1) The surface of rolled sheet aluminum parts has dense burnt paste that cannot be removed.
(2) The workpiece is not thoroughly rinsed in nitric acid after alkaline washing.
(3) Excessive accumulation of aluminum impurities in the alkaline washing solution.
resolvent
(1) Grind the surface. Before oxidation, the workpiece is polished off with fine sandpaper to remove the dense burnt paste on the surface, and then enters oxidation.
(2) Neutralization with nitric acid. After alkali washing, the workpiece should be thoroughly rinsed in a nitric acid (1:1) solution to neutralize the alkali and make the surface of the workpiece smooth.
(3) Replace the alkaline solution. Due to the large amount of AL3+in the old alkaline solution, the viscosity of the solution increases. This state of alkaline solution is difficult to wash off the oil stains on the surface of the workpiece, and some grease blocks the formation of the oxide film.
6. It is difficult to generate oxide film around the workpiece eyelet and its surroundings
Cause of occurrence
(1) The workpiece is not cleaned thoroughly after oil removal, and there is residual alkaline solution in the hole.
(2) There is butter around the eyelets of the workpiece.
resolvent
(1) Wash with hot water. After the workpiece is washed with alkali, the thermal expansion cleaning method can effectively remove the alkali in the workpiece hole.
(2) Gasoline washing. The butter mainly comes from the lubricating oil used for mechanical tapping of screw holes. For this, it is necessary to brush the workpiece with organic solvent or gasoline before alkaline washing, and then perform alkaline washing treatment. In addition, an appropriate amount of emulsifier should be added to the alkaline washing solution to help remove butter.