Q311 : Modelling coated plates in MALZ / HIFREQ

Question
I have to model a well coated metallic plate to obtain the resistance between the plate and an anode. Is it possible to model the plate in MALZ, using a wiremesh approximation?

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Answer
A wire-mesh model would work for a bare plate, but not a coated one. When there is no coating, the resistance is that of the material between the plate and the anode. The precise configuration (and density) of the conductors used to model the plate is not very important as long as they cover the surface of the plate quite well.

The problem when a coating is present is that the impedance will tend to be dominated by the resistance of the coating. This should be inversely proportional to the total surface area of the coating, and so will depend very strongly on the conductor density: the more conductors there are, the larger the surface area of the coating and the smaller the impedance. There is no real way to obtain the correct impedance, short of putting conductors every few centimeters or so (essentially touching each other).

A possible way to compute the impedance is to assume that only the coating contributes (assuming also that the current distribution is uniform over the plate, admittedly a rather rough approximation). The following formula can be used:

R = Rho * L / A

where Rho is the resistivity of the coating, L the thickness of the coating and A the total surface area of the plate, i.e.

A = PI * r^2

where r is the radius of the plate.

This should be a lower bound on the impedance. If the current distribution in the plate is not uniform, the effective area of the coating seen by the current should be smaller, yielding a larger value for the resistance.

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