Field-based Prediction Of Localized Anodic
Dissolution Events Taking Place On ZnAl Alloy
Coatings In The Presence Of 5% NaCl Solution
Author(s)
S. G. R. Brown & N. C. Barnard
Abstract
An extension to a previously-developed numerical model is presented that
predicts the sites of localized anodic dissolution on exposed surfaces of ZnAl
alloy coating; often employed in the protection of steel-strip substrate material.
Long-established thermodynamic concepts and galvanic-coupling mechanisms
are used to identify sites susceptible to local metallic dissolution. Estimation of
metal loss over time is linked to the electrode potentials predicted at the exposed
alloy surface. Both alloy composition and the concentration of multiple species
in the NaCl solution are considered in determining these electrode potentials.
The temporal evolution of the Zn-Al-Fe system – represented using a cellular
automaton framework – is predicted via field-based calculations on steady-state
voltage and non-steady state diffusion/migration fields. Concentration
perturbations in the electrolyte are captured and resultant potential fields are
generated using a straight-forward finite difference technique on an irregularstructured
computational grid. The influence of the microstructural features in
the ZnAl alloy coatings is assessed in terms of metal loss, current density fields
and pitted-depth. Simulated results are validated at the meso-scale in
comparisons made to experimental observations in accelerated testing of these
alloys in concentrated aqueous NaCl solutions. Simulations have been performed
that quantitatively assess the localized corrosion at the surface of ZnAl alloy
coatings and cut-edge scenarios where Zn, Al and Fe are exposed. The model is
extended to predict the effect of altering the processing conditions, i.e. cooling
rate and coating thickness.
Keywords: Zn-Al coatings, galvanic corrosion, de-alloying, finite difference.
Keywords
Zn-Al coatings, galvanic corrosion, de-alloying, finite difference
Related Book
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