Anodizing Aluminum and Microstucture of a Steel Sprocket

Ian Buchanan & Kris Magri Santa Rosa Junior College ENGR 45, Younes Ataiiyan Dec 07, 2009

Authors

Ian Buchanan is a Civil Engineering major, hoping to work in construction Kris Magri is a Mechanical Engineering major who plans to work in robotics

Purpose of Anodizing

Grow an aluminum oxide layer on the aluminum so it can be dyed Corrosion and wear resistance Hardening (Type III) Color – cosmetic Photo by Ron Newman, http://www.focuser.com/anodize.html

Overview

Aluminum part immersed in acid electrolyte Apply electrical current, DC, ~12V The part is the anode (+) (thus the name) Electrolysis and chemical reaction occurs Porous aluminum oxide layer grows on the aluminum Up to 3000 times thicker than naturally occuring Al 2 O 3 layer Dye goes into pores, results in bright colors Place in boiling water to seal pores

Electrochemistry

Electrolyte in Solution: Free ions ,conductive  Sulfuric, oxalic, or phosphoric acid typically used  15% solution of sulfuric acid (H 2 SO 4 ) our procedure Electrolysis: Extracts constituent elements from solution Anode   Evolution of oxygen 2Al + 3H 2 0  Al 2 0 3 + 6H + + 6e Cathode   Evolution of hydrogen 6H 2 0 + 6e  3H 2 (g) + 6OH -

Pore growth

Acid electrolyte acts as solvent for oxide Dissolves portions of barrier oxide layer Oxide grows at metal/oxide interface Rate of growth dependent on current, concentration, temperature, voltage Hexagonal shape Photo from Artists Anodizing Aluminum, D. LaPlantz, 1988, p. 17

Anodizing Setup

Materials

Aluminum item (anode) Aluminum wire Aluminum sheet (cathode) Sulfuric Acid 15% Non-metal container Power supply Distilled/de-ionized water Dye (RIT clothes dye) Baking soda Photo by Kris Magri

Process

Diagram by Kris Magri

Anodizing Tank

Photo by Ed Troxell Photo by Kris Magri

Cathode design

Cathode at least 1/3 the area of the part Experimental 1 st cathode very large and very far away, poor results Radial cathode worked best

Sealing

Aluminum oxide converted to hydrated form Al 2 O 3  3H 2 O has more volume than Al 2 O 3 Clogs the pores Hot water seal: Boiling de-ionized water or steam Other sealing: nickel acetate, cobalt acetate (cold), sodium or potassium dichromate (hot)

Results

Photo by Kris Magri

Current Density

10-15 ASF (Amp•hr/ft 2 ) desired rate 4A for 20 min, part is 4.42 in 2  43.4 ASF 1A for 20 min, part is 4.42 in 2  10.8 ASF Poor results at 4A but good results at 1A Rate of oxide thickening proportional to current density up to some point Dissolution point – barrier layer being removed faster than new oxide layer being formed

More about Anodizing

Type II  This is the procedure we used  .00007" to .001" oxide layer thickness Type III  Low temp (50 degree) at higher current (24 ASF)  Over .001"

Anodizing in General

Other metals that can be anodized   Titanium, magnesium, niobium, tantalum, tungsten, zirconium Ti utilizes interference property of oxide film instead of dye for color History  Anodizing developed around 1917 with first US patent in 1925 (*AAA)

Resources

Aluminum: Properties and Physical Metallurgy, Edited by John E. Hatch, American Society for Metals, 1984 Materials Science and Metallurgy, Herman W. Pollack, 3rd Edition, Reston Publishing, 1981 Artists anodizing aluminum: The sulfuric acid process, David LaPlantz, Press de LaPlantz, 1988 Wikipedia http://en.wikipedia.org/wiki/Electrolyte Mr. Titanium http://mrtitanium.com/interference.html

Anodizing Aluminum, by Ron Newman http://www.focuser.com/anodize.html