14 Types of Anodizing Coloring Defects and Strategies for Prevention

1. Thin Film Causing Light or No Coloring

Issue: Thin film in hanging material resulting in light coloring or no coloring.

Main Cause: Loose binding or loosening after alkali etching, leading to poor contact between material and conductor rod.

Solution: Tighten the binding wire further with pliers after alkali etching to ensure secure contact.

2. Partial Burn of Oxidation Film Leading to Blackening

Issue: Partial burn of oxidation film causing blackening.

Main Causes:

• Poor contact between aluminum parts and conductor rod.
• Insufficient contact area.
• Unclean film on conductor rod.
• Short circuit between anode and cathode.

Solutions: Improve contact between aluminum parts and conductor rod, ensuring a clean conductor rod and eliminating anode-cathode contact.

3. Dark Film Layer

Issue: Presence of a dark film layer.

Main Causes: Issues with alloy composition, interruption of current during oxidation, low electrolyte concentration, high oxidation voltage, poor pretreatment.

Solutions: Improve ingot quality for material issues, strengthen pretreatment for treatment issues, adjust electrolyte content, sulfuric acid concentration, and appropriately reduce voltage.

4. Fingerprints Appearing

Issue: Fingerprints visible on the anodized film.

Main Cause: Touching the anodized film with fingers during operation.

Solution: Wear clean gloves and avoid touching with fingers as much as possible.

5. Poor Corrosion Resistance

Issue: Poor corrosion resistance of the anodized material.

Main Cause: High concentration of sulfuric acid, aluminum ion content exceeds 20g/L.

Solution: Maintain sulfuric acid concentration between 150-200 g/L and consider replacing 1/2-3/4 bath fluid if aluminum ion exceeds 20 g/L.

6. Yellowing

Definition: Impurities mixed in the oxide film causing a yellow tint.

Causes: Iron, silicon, etc., in the electrolyte or alloy material mix into the film.

Countermeasures: Reduce the concentration of iron, silicon, etc., in the alloy and electrolyte. Optimize anodizing conditions.

7. Overlapping

Definition: Abnormal film formation due to material overlapping during oxidation.

Causes: Possible abnormal contact due to dense arrangement of aluminum materials during electrolysis.

Countermeasures: Maintain appropriate binding spacing, tighten fixtures, remove deformed fixtures, and do not hang deformed, bent profiles.

8. Accumulated Gas (Air Pockets)

Definition: Gas produced during electrolysis or air used for stirring remains in the gaps or corners of the material.

Causes: Unsuitable hanging angle or material shape affecting gas and air retention.

Countermeasures: Use hanging angles and loading methods that allow gas to escape easily.

9. Black Spots

Definition: Black or white spots after anodizing due to local precipitation of β’ intermediate phase.

Causes: Belly button material undergoes rapid cooling and reheating, leading to the precipitation of the intermediate phase (Mg2Si).

Countermeasures: Suppress reheating with cooling fans and reduce thermal conductivity of materials in contact with extruded materials.

10. Powdering (Powdery Film)

Definition: White powder forming on the surface of the film after anodizing.

Causes: Long-time electrolysis in high temperature, high concentration electrolyte, or long-time immersion after electrolysis.

Countermeasures: Lower electrolyte concentration and temperature, reduce aluminum ion content, and shorten immersion time.

11. Short Circuit (Electric Corrosion, Melting Film, Sparking)

Definition: Material dissolves partially due to contact with the electrode during electrification.

Causes: Material and electrode contact, or a short circuit occurs through fallen profiles.

Countermeasures: Improve arrangement, prevent material sway, remove fallen materials, and adjust electrode spacing.

12. Poor Electrolysis (Poor Electrification)

Definition: Poor conductive contact during anodizing, different from the set current value, almost no film is generated.

Causes: Interruption of electrolysis due to power outage, power supply failure, deteriorated, contaminated fixtures.

Countermeasures: Strengthen management of fixture joints, increase contact area, and confirm set current values.

13. Milky White

Definition: Impurities mixed in the anodized film, causing the film structure to differ and appear milky white.

Causes: High-temperature electrolytic treatment, short hot water wash time, poor extrusion conditions, fluctuations in the content of silicon, iron, manganese, etc.

Countermeasures: Normalize anodizing treatment conditions, normalize water wash conditions, confirm set current values, and adjust alloy components.

14. Film Burns (Burns)

Definition: During anodizing treatment, the local current density is too high, forming a burn-like appearance.

Causes: Insufficient contact area, too close proximity between electrode and material, etc., leading to locally high current density.

Countermeasures: Ensure appropriate contact point area, improve electrode configuration, increase bath fluid circulation, ensure uniformity, set appropriate current density, and optimize process conditions, especially determining the content of aluminum ions.