What Are the Main Aluminum Anodizing Processes?

When an aluminum anode is oxidized in an acidic solution by an external current, oxide-film growth and dissolution occur simultaneously. An anodic oxide film exists only when its formation rate exceeds its dissolution rate. Common methods include sulfuric-acid, chromic-acid, and oxalic-acid anodizing. Other methods include hard and porcelain-like anodizing.

Sulfuric-Acid Anodizing

Sulfuric-acid anodizing applies direct or alternating current in an electrolyte containing 10% to 20% H₂SO₄ by mass. It produces a colorless, transparent, highly adsorptive film 5 to 20 μm thick. The process is simple, the solution is stable, and operation is convenient.

Effect of Sulfuric-Acid Concentration

A high sulfuric-acid concentration accelerates chemical dissolution and produces a thin, soft, highly porous film with strong adsorption and good dyeability. Lower concentration increases film-growth rate and produces lower porosity, higher hardness, good wear resistance, and good reflectivity.

Effect of Temperature

Electrolyte temperature strongly affects film quality. At 10 to 20°C, the film is porous, adsorptive, elastic, and suitable for dyeing, but has lower hardness and wear resistance. Above 26°C, it becomes loose and soft. Below 10°C, it becomes thicker, harder, and more wear resistant but less porous. Temperature must therefore be controlled strictly.

Effect of Current Density

Increasing current density accelerates growth, shortens oxidation time, reduces chemical dissolution, and produces a harder, more wear-resistant film. Excessive current density increases dissolution through Joule heating and can reduce the growth rate. Very low current density requires a long process and produces a loose, softer film.

Effect of Agitation

Agitation promotes convection and uniform temperature, preventing local metal heating from degrading film quality. Compressed-air systems and pumps are used for agitation.

Effect of Alloy Composition

Alloy composition strongly affects film quality, thickness, and color. Other elements generally reduce quality. If an Al-Mg alloy contains more than 5% magnesium by mass and has a nonuniform structure, suitable homogenizing heat treatment is required to preserve transparency. As silicon increases in Al-Mg-Si alloys, the film changes from colorless and transparent through gray and purple to black, making uniform color difficult. Copper increases porosity and looseness and reduces quality in Al-Cu-Mg-Mn alloys. Under identical conditions, pure aluminum produces the thickest and hardest film with the best corrosion resistance.

Effect of Oxidation Time

Required time depends on electrolyte concentration, temperature, current density, and target thickness. Under the same conditions, increasing thickness increases porosity, dyeability, and corrosion resistance, but growth eventually slows and stops. A film of specified thickness and hardness requires 30 to 40 minutes; a decorative film with greater porosity for dyeing requires 60 to 100 minutes.

Effect of Alternating Current

With AC, each workpiece is anodic for only half of each cycle, so sulfuric-acid concentration should be lower and current density can be higher. The film has high transparency and porosity but lower hardness and wear resistance. Workpieces may be mounted on both electrodes if their areas are equal. Producing the same thickness as DC anodizing requires twice the oxidation time.

Chromic-Acid Anodizing

Chromic-acid films are much thinner than sulfuric- or oxalic-acid films, at 2 to 5 μm. They have low porosity, are soft and elastic, and have lower corrosion and wear resistance than sulfuric-acid films. Because little aluminum dissolves, parts retain their original accuracy and surface roughness, making the process suitable for precision components. The film adheres well to paint and is widely used as a paint base and on rubber-bonded parts.

Chromic Anhydride Concentration

Oxidizing power decreases when chromic anhydride content is either too high or too low, although a slight excess within the permitted range is acceptable. An electrolyte with too little chromic anhydride is unstable and reduces film quality.

Effect of Impurities

Chloride, sulfate, and trivalent chromium ions are harmful impurities. Chloride etches parts. Increasing sulfate changes the film from transparent to opaque and shortens solution life. Excess trivalent chromium makes the film dark and dull.

Effect of Voltage

During the first 15 minutes, voltage is raised gradually from 0 to 40 V in increments no greater than 5 V to keep current within the specified range. Once 40 V is reached, it is maintained until oxidation ends. Voltage must rise progressively during anodizing to maintain the specified current density and achieve the required film thickness.

Oxalic-Acid Anodizing

Oxalic acid is weak and attacks aluminum only slightly. Oxalic-acid anodizing therefore produces a relatively hard, thick film up to 60 μm with good corrosion resistance and electrical insulation. Costs are high because the electrolyte has greater resistance than sulfuric or chromic acid, consumes more electrical energy, and heats readily, requiring effective cooling. Changing process conditions can directly produce decorative films in different colors without subsequent dyeing, except on copper-containing aluminum.

The oxalic-acid electrolyte is highly sensitive to chloride ions. Above 0.04 g/L, corrosion spots appear on the film. Trivalent aluminum ions must not exceed 3 g/L.