Aluminum Anodize

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Aluminum Anodize

• By Randall Marks

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Aluminum Oxide

• Amorphous aluminum oxide (alumina, Al2O3) forms
  in a layer 2-3 nm thick when bare aluminum is
  exposed to oxygen
• Protects from further oxidation
• Stress causes fractures in the surface, but not
  peeling
• How can this natural coating be improved upon?

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Anodize Process
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Types of Anodize

• Barrier Anodize
• Near neutral electrolyte solution
• Max 1um thick
• Porous Anodize
• Acidic electrolyte solution
• Can be several hundred um thick

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Barrier Anodize

• Forms oxide on both sides
• Dielectric, which supports up to 1 V/nm
• Too thin for most mechanical applications

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Porous Anodize Formation
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Porous Anodize Sealing

• Hot water forms hydrous oxide crystals, sealing
  the pores.
• Dyes or electroplated metals can be sealed inside
  the pores for various colorations

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Hard Anodize

• Hard Anodize Formation
• Low temperature
• Low acidity
• Applications
• Durable protective layer
• Bearings
• Not sealed to hold lubricants in the pores
• Surface is second hardest material known

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Optics Applications

• Reduction of stray light/spectral reflections
• Bead blasted black anodize
• No outgassing
• Black anodize not good for bearing type surfaces
• Adhesive preparation
• Phosphoric acid preparation
• Left unsealed
• Corrosion resistance

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Important Application Considerations

• AMS governing specs
• AMS 2469-AMS 2474
• Part grows by approximately half the oxide
  thickness
• Surface Youngs Modulus around 300 MPa.
• Does not protect against all chemicals
• Highly susceptible to strong acids and bases

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References

• Case Western Reserve University Electrochemistry
  Encyclopedia
• http//electrochem.cwru.edu/ed/encycl/art-a02-anod
  izing.htm
• AACOA Inc. Anodizing Presentation
• http//www.aacoa.com/presentations/anodizing_prese
  ntation-c.pdf