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14 Types of Anodizing Coloring Defects: How to Prevent

Updated : Oct. 24, 2023

We are a technologically advanced manufacturer specializing in aluminum alloy anodizing, known for our high-quality products. With extensive experience, we have compiled a list of 14 common defects in aluminum alloy anodizing.Quick Quote

1. Thin film in hanging material causes light coloring, even no coloring

Main Cause: Loose binding of material 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.

2. Partial burn of oxidation film results in blackening

Main Cause: Poor contact between aluminum parts and conductor rod or insufficient contact area, unclean film on conductor rod, or short circuit between anode and cathode.

Solution: Improve contact and eliminate anode-cathode contact.

Partial burn of oxidation film results in blackening

3. Dark film layer

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

Solution: Improve ingot quality if it's a material issue, or strengthen pretreatment if it's a treatment issue. Adjust electrolyte content, sulfuric acid concentration, and appropriately reduce voltage.

Dark film layer

4. Fingerprints appearing

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

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

Fingerprints appearing

5. Poor corrosion resistance

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

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

6. Yellowing

Definition: Impurities mixed in the oxide film cause the film to turn yellow.

Phenomenon: The color tone changes when this film is used for electrolytic coloring.

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.

Phenomenon: Can see the imprint of overlapped profiles, sometimes can see partial rainbows (interference color).

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

Phenomenon: The film is very thin or non-existent at the gaps or corners of the material, and it cannot obtain uniform color during electrolytic coloring.

Causes: Unsuitable hanging angle or material shape affects 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 appear after anodizing due to the local precipitation of β’ intermediate phase.

Causes: The 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 the thermal conductivity of the materials in contact with extruded materials.

Black spots

10. Powdering (powdery film)

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

Phenomenon: After anodizing, the film appears powdery and opaque.

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.

Phenomenon: The material and the electrode short-circuit, and part of the material dissolves due to excessive current.

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.

Phenomenon: Sometimes rainbows (interference color) can be seen on both sides, and normal electrolytic coloring cannot be performed.

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.

Phenomenon: The film lacks transparency and turns 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.

Milky white

14. Film burns (burns)

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

Phenomenon: During anodizing treatment, the local concentration of current increases the temperature, and the film thickness increases, becoming whitened and powdery.

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.Quick Quote

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Anodized aluminum is a truly versatile material that can be used to create a wide variety of products. Its durability, corrosion resistance, and attractive appearance make it a popular choice for many different applications.Quick Quote

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