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Modeling of the Current Distribution in Aluminum Anodization

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Modeling of the Current Distribution in Aluminum Anodization. Rohan Akolkar and Uziel Landau Department of Chemical Engineering, CWRU, Cleveland OH 44106. Yar-Ming Wang and Hong-Hsiang (Harry) Kuo General Motors R&D, Warren MI 48090.

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Presentation Transcript
slide1

Modeling of the Current Distribution in Aluminum Anodization

Rohan Akolkar and Uziel Landau

Department of Chemical Engineering,

CWRU, Cleveland OH 44106.

Yar-Ming Wang and Hong-Hsiang (Harry) Kuo

General Motors R&D,

Warren MI 48090.

205th Meeting of The Electrochemical Society, San Antonio, TX.

slide2

Outline

  • Anodic Oxide Films on Aluminum
  • Current distribution –Significance
  • Kinetics of oxide growth
  • Modeling of Current and Potential Distribution
      • Comparison with experiments
      • Effect of operating conditions (t, V, T)
  • Conclusions
slide3

Introduction

  • Aluminum Anodization
  • dc voltage = 12-20 V
  • Alloy 6111
  • 15 wt. % H2SO4
  • time = 15-35 min
  • oxide films ~ 5-25 μm

5-25 μm

Oxide pores

~30 nm

Al2O3 barrier

Al metal

slide4

Important Issues in Al Anodization

  • Anodized parts with complex, non-accessible features experience large oxide thickness variations.
      • What are the current distribution characteristics inside non-accessible cavities ?
      • How are they affected by the operating conditions ?

Objective

  • Analyze and model the current distribution in anodizing systems, and compare with experimental measurements.
slide5

Governing Equations

Net Flux = Diffusion + Migration + Convection

  • Assume :
      • No concentration gradients
      • Steady state

_

+

zj

Potential Distribution

H+

v

Boundary Conditions

  • Insulator (zero current) :
  • Electrode (Resistive Oxide) :

Mott Cabrera Kinetics

slide6

Anodization kinetics

Mott Cabrera Kinetics : i = A exp (B V) A, B: ionic transport parameters within the oxide film

Increasing temperature

VERY HIGH SURFACE RESISTANCE leads to VERY HIGH SURFACE OVER-POTENTIALS

slide7

Oxide Thickness Distribution

_

Current Density :

+

Faraday’s law :

current efficiency

oxide porosity

slide8

Current and Potential Distribution

Methods to compute current distribution

Scaling Analysis

e.g. Wagner number :

Analytical Modeling

e.g. analytical solution of current balance equations

Numerical Modeling

e.g. CELL DESIGN*, FEM, FDM to solve Laplace equation

* CELL DESIGN, L-Chem Inc., Shaker Heights, Ohio 44120.

slide9

Experimental setup

_

_

+

Parallel plate anode assembly

z

y

x

2.5

Anodes

43

Cathode

Cathode

10

30

z

z

0.8

x

y

30

side shields

slide10

Numerical Modeling

Geometry

Potential Map

Electrode Properties e.g. kinetics

Cell Design’sBEM* Solver

Current Distribution

Electrolyte Properties e.g. conductivity

Deposit Profile

Oxide Properties e.g. porosity

* Boundary Element Method

slide11

Simulation Results

Significant potential drop ONLY in the interior of the parallel plates

NON-UNIFORM oxide in the interior

Potential Distribution

Current Distribution

slide12

Measurement of Oxide Distribution

for comparison with modeling results

Anode

86

0

Uniform Oxide

Cathode

Non-Uniform Oxide

43

43

  • Oxide thickness measured along the anode at ~5 cm intervals
slide13

Experimental vs. Modeling

Non-uniform distribution in the interior

Uniform oxide thickness on the exterior

Anodic Oxide Thickness (microns)

Distance Along the Electrode (cm)

slide14

Effect of Anodization Time

35 min

Constant oxide resistance

Anodic Oxide Thickness (microns)

15 min

Distance Along the Electrode (cm)

slide15

Effect of Anodization Time –Distributed resistance

Low growth rates for distributed resistance within entire oxide

Constant oxide resistance

Anodic Oxide Thickness (microns)

35 min

15 min

Distance Along the Electrode (cm)

slide16

Effect of Anodization Voltage

18 V

Uniform oxide

Anodic Oxide Thickness (microns)

Low oxide thickness inside the interior

14 V

Distance Along the Electrode (cm)

slide17

Effect of Anodization Temperature

25 oC

Uniform oxide

Anodic Oxide Thickness (microns)

Low oxide thickness inside the interior

15 oC

Distance Along the Electrode (cm)

slide18

Main Conclusions

  • An electrochemical CAD software used to model the current distribution in anodizing.
  • Excellent agreement between modeling and experiments.
  • The oxide growth rates are independent of time indicating a porous oxide growth – the oxide resistance resides in a compact barrier film at its base.
  • Current distribution was highly non-uniform in high aspect ratio cavities due to dominance of ohmic limitations over surface resistance.
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