Aluminum Anodize By: Randall Marks

Aluminum Oxide Amorphous aluminum oxide (alumina, Al 2 O 3 ) 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?

Anodize Process

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

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

Porous Anodize Formation

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

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

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

Important Application Considerations AMS governing specs: –AMS 2469-AMS 2474 Part grows by approximately half the oxide thickness Surface Young’s Modulus around 300 MPa. Does not protect against all chemicals –Highly susceptible to strong acids and bases

References Case Western Reserve University Electrochemistry Encyclopedia – a02-anodizing.htmhttp://electrochem.cwru.edu/ed/encycl/art- a02-anodizing.htm AACOA Inc. Anodizing Presentation – g_presentation-c.pdfhttp:// g_presentation-c.pdf