How does arc anodizing differ from traditional anodizing?

Fundamental Differences Between Arc Anodizing and Traditional Anodizing

Arc Anodizing (often used interchangeably with Plasma Electrolytic Oxidation or Micro-arc Oxidation) and traditional anodizing are both electrochemical surface treatments, but they differ radically in process mechanics, coating characteristics, and final performance. The core distinction lies in the use of high-voltage plasma discharges to create a fundamentally different type of protective layer.

Process Mechanics and Coating Formation

Traditional Anodizing (Type II & III): This is a low-voltage process that operates below the dielectric breakdown voltage of the oxide. It grows a porous, amorphous aluminum oxide layer through straightforward electrolysis. The coating grows outward from the substrate, and its porous structure requires a secondary "sealing" step in hot water or steam to improve corrosion resistance.

Arc Anodizing (PEO/MAO): This process employs voltages high enough to cause controlled dielectric breakdown, generating numerous microscopic plasma discharges across the component's surface. These instantaneous, high-temperature plasma events (>2,000°C) sinter and fuse the oxide, transforming it from an amorphous state into a dense, crystalline ceramic layer rich in hard alpha-alumina (α-Al₂O₃). The coating grows both inward and outward from the original substrate surface.

Coating Properties and Performance

The different formation mechanisms result in coatings with vastly different properties:

• Thickness and Hardness:

  • Traditional: Thin (5-25 µm). Hard anodizing (Type III) reaches ~400-500 HK.

  • Arc Anodizing: Very thick (25-100+ µm). Extreme surface hardness (1000-2000 HV), rivaling tool steel.

• Structure and Corrosion Resistance:

  • Traditional: Porous structure. Good corrosion resistance only after effective sealing.

  • Arc Anodizing: Dense, low-porosity, monolithic ceramic. Provides a superior barrier, often achieving 500 to 1000+ hours in ASTM B117 salt spray testing without a separate sealing step.

• Wear Resistance and Adhesion:

  • Traditional: Good wear resistance but can crack under high stress. The coating is adhered but can be chipped.

  • Arc Anodizing: Exceptional abrasion and wear resistance due to the alpha-alumina phase. The coating is metallurgically bonded, forming a gradient interface that is highly resistant to delamination and spalling.

• Appearance and Dyeing:

  • Traditional: Can be dyed in a wide range of vibrant, uniform colors. Offers a smooth, glossy finish.

  • Arc Anodizing: Typically produces matte finishes in shades of gray, dark gray, or bronze. The inherent micro-roughness and process physics make consistent, bright coloring impossible. Its appearance is technical and functional.

Summary: When to Choose Which Process

Choose Traditional Anodizing for: Decorative applications, corrosion protection in mild environments, and when a wide color palette or glossy finish is required.

Choose Arc Anodizing for: Engineering components subjected to severe abrasive wear, cavitation, or high loads, applications in highly corrosive environments, and where electrical insulation or thermal stability is critical. It is the definitive choice for maximizing the service life of critical components in aerospace, automotive, and heavy industry.