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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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.


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


  • 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


    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.


    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


    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


    Oxide Thickness Distribution

    _

    Current Density :

    +

    Faraday’s law :

    current efficiency

    oxide porosity


    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.


    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


    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


    Simulation Results

    Significant potential drop ONLY in the interior of the parallel plates

    NON-UNIFORM oxide in the interior

    Potential Distribution

    Current Distribution


    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


    Experimental vs. Modeling

    Non-uniform distribution in the interior

    Uniform oxide thickness on the exterior

    Anodic Oxide Thickness (microns)

    Distance Along the Electrode (cm)


    Effect of Anodization Time

    35 min

    Constant oxide resistance

    Anodic Oxide Thickness (microns)

    15 min

    Distance Along the Electrode (cm)


    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)


    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)


    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)


    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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