How much of EMF change after aluminum is anodized (chromated) depends on 
how good/thick the coating is.  It is fairly easy to measure the relative 
potential delta of two materials.  Check the book "Corrosion Engineering" 
by Fontana/Greene or any other electrochemistry book and you'll know how to 
do it.  Remember, (1) surface area is an important factor, (2) Redox 
(reduction/oxidation) potential is only a thermodynamic indicator, i.e., 
the tendency to corrode.  It does not provide kinetic information, i.e., 
how fast it corrodes.  

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