Anodizing

Part of Electrochemistry

The general principles and practice of anodic oxidation — growing protective or decorative oxide films on metals through electrolysis.

Why This Matters

Anodizing is an electrochemical surface treatment that replaces the thin native oxide on a metal surface with a much thicker, harder, more controlled oxide layer. The process converts the outer layer of the metal itself into its own protective coating — unlike plating, which deposits a foreign metal on top.

For aluminum, the most important anodizing material, the result is a surface that resists corrosion in marine environments, withstands abrasion from daily use, electrically insulates, and accepts permanent dye in any color. For titanium, anodizing produces brilliant interference colors used in jewelry and implant marking. For magnesium, it provides critical corrosion protection for a naturally reactive metal.

In any civilization that works with light metals, anodizing extends service life and enables marking and identification systems that survive harsh conditions.

The Anodizing Principle

Anodizing is the reverse of plating. In plating, metal deposits on the part (cathode). In anodizing, oxide grows at the part (anode):

At the anode (metal part): Metal → Metal^(n+) + n e⁻

The metal ions react with oxygen provided by electrolysis of water: Metal^(n+) + (n/2) O²⁻ → Metal oxide

For aluminum: 2 Al + 3 H₂O → Al₂O₃ + 6 H⁺ + 6 e⁻

At the cathode (counter electrode): 2 H⁺ + 2 e⁻ → H₂ (hydrogen gas)

The net result is conversion of the metal surface to its oxide, with the oxide growing simultaneously inward into the metal and outward from the original surface. Typically half the oxide thickness grows into the metal, half grows out.

Metals that Can Be Anodized

Metal	Anodizing Electrolyte	Key Benefit
Aluminum	Sulfuric acid (15–20%)	Corrosion resistance, hardness, dyeability
Titanium	Phosphoric or sulfuric acid	Decorative interference colors; biocompatibility
Magnesium	Proprietary (fluoride-based)	Essential corrosion protection
Zinc	Alkaline electrolytes	Limited use
Copper/brass	Not typically anodized	—

Anodizing Process Steps (General)

1. Surface Preparation

Surface quality before anodizing determines surface quality after. The oxide faithfully replicates the metal surface underneath.

• Mechanical finishing: Machine, grind, or polish to desired finish.
• Alkaline cleaning: Remove oils and grease. For aluminum: 5–10% NaOH solution, 50–60°C, 1–3 minutes.
• Etching (optional): Produces a matte, satin finish by lightly dissolving the surface. Same NaOH bath, longer time.
• Brightening (optional): Produces a mirror finish. Phosphoric/nitric acid mixture (Alupol, Alox) or electropolishing.
• Desmutting: After alkaline etch, aluminum surfaces develop a dark smut of alloying element compounds. Nitric acid (15–30%) rinse removes this.

2. Racking

Parts must be electrically connected to the anode bus bar. The connection point will not anodize — it remains bare metal. Place rack contact points in hidden locations (inside holes, on screw threads that will be covered).

Materials for racks: titanium (ideal — durable, does not anodize thickly), aluminum alloy, stainless steel.

3. Anodizing Bath

Sulfuric acid for aluminum (standard type II):

• Concentration: 180–200 g/L H₂SO₄ (≈15–18%)
• Temperature: 18–22°C (held constant — use cooling if necessary)
• Current density: 1.0–2.0 A/dm²
• Time: 10–60 minutes depending on desired thickness
• Agitation: essential for temperature uniformity and electrolyte circulation

Type III (hard anodizing):

• Lower temperature: 0–5°C
• Higher current density: 2.5–3.5 A/dm²
• Produces 25–100 μm thick oxide
• Very hard (Vickers hardness 300–500 HV vs. 60 HV for aluminum)
• Used for wear-resistant parts: hydraulic components, gears, tooling

4. Dyeing (Optional)

Immediately after anodizing, the oxide is porous and ready to absorb dye:

• Anodizing dyes (proprietary, wide color range)
• Fabric dyes (Rit, Dylon) — work adequately for most colors
• Tea/coffee/plant extracts — limited color range, UV-stable varies
• Temperature: 50–60°C
• Time: 10–20 minutes (deeper color with longer immersion)
• Rinse lightly before sealing

5. Sealing

Converts porous oxide to closed, stable form. Two methods:

Hot water seal:

• Boiling distilled water, 20–30 minutes
• Converts Al₂O₃ to Al₂O₃·H₂O (boehmite) — increases volume, closes pores
• Simple, no chemicals required

Nickel acetate seal:

• 5–10 g/L nickel acetate, 80–85°C, 20 minutes
• Better corrosion resistance than hot water seal
• Requires nickel compound availability

Quality Checks

Test	Method	Pass Criterion
Coating thickness	Eddy current gauge	≥ 10 μm (Type II), ≥ 25 μm (Type III)
Sealing quality	Acid dip test: 10% H₂SO₄, 10 min	No visible corrosion or dye bleed
Adhesion	Tape test	No oxide delamination
Corrosion resistance	Salt spray test (5% NaCl, 35°C, 100+ hours)	No base metal corrosion

Common Problems

Problem	Likely Cause	Solution
Burning/pitting	Current too high; bath too warm	Reduce current density; cool bath
Powdery white deposit	Excessive etching time	Reduce pre-etch
Streaky appearance	Non-uniform current distribution	Improve rack contacts; add agitation
Dye blotchy	Oil contamination remained; non-uniform oxide	More thorough degreasing
Poor corrosion resistance	Inadequate sealing	Extend seal time; check seal bath temperature