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 AnodizingGrow 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, ~12VThe part is the anode (+) (thus the name)Electrolysis and chemical reaction occursPorous aluminum oxide layer grows on the aluminumUp to 3000 times thicker than naturally occuring Al2O3 layerDye goes into pores, results in bright colorsPlace 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 (H2SO4) our procedure

Electrolysis: Extracts constituent elements from solutionAnode Evolution of oxygen 2Al + 3H20 Al203 + 6H+ + 6e-

Cathode Evolution of hydrogen 6H20 + 6e- 3H2 (g) + 6OH-

Pore growth

Acid electrolyte acts as solvent for oxideDissolves portions of barrier oxide layerOxide grows at metal/oxide interfaceRate of growth dependent on current, concentration, temperature, voltageHexagonal shape

Photo from Artists Anodizing Aluminum, D. LaPlantz, 1988, p. 17

Anodizing Setup

Materials

Aluminum item (anode)Aluminum wireAluminum sheet (cathode)Sulfuric Acid 15%Non-metal containerPower supplyDistilled/de-ionized waterDye (RIT clothes dye)Baking soda

Photo by Kris Magri

Process

Diagram by Kris Magri

Anodizing Tank

Photo by Kris MagriPhoto by Ed Troxell

Cathode design

Cathode at least 1/3 the area of the part

Experimental 1st cathode very large and very far away, poor results

Radial cathode worked best

Sealing

Aluminum oxide converted to hydrated form

Al2O33H2O has more volume than Al2O3

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/ft2) desired rate

4A for 20 min, part is 4.42 in2 43.4 ASF

1A for 20 min, part is 4.42 in2 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, 1984Materials Science and Metallurgy, Herman W. Pollack, 3rd Edition, Reston Publishing, 1981Artists anodizing aluminum: The sulfuric acid process, David LaPlantz, Press de LaPlantz, 1988Wikipediahttp://en.wikipedia.org/wiki/ElectrolyteMr. Titaniumhttp://mrtitanium.com/interference.htmlAnodizing Aluminum, by Ron Newmanhttp://www.focuser.com/anodize.html