Organic coatings polarization resistance

Electrochemical methods are well adapted for characterizing the corrosion behavior of coated metals in solution. Because of the high resistance of organic coatings, ac methods are generally more suited than dc polarization methods. In electrochemical impedance spectroscopy (EIC) one measures the response of the coated electrode to a small amplitude ac perturbation as a function of frequency (Chapter 5). The interpretation of the measured frequency response, in principle, requires a physical model. However, for coated metals useful information is more easily obtained by representing the metal-coating-electrolyte interface by an electrical circuit (equivalent circuit). 

The example suggests that to obtain a uniform current distribution one needs to find conditions where the resistance at the metal-electrolyte interface is much higher than that of the electrolyte in the interelectrode space. In cathodic protection this is achieved by the application of an organic coating of high resistance. In anodic protection, the passive film causes a high polarization resistance and thus favors a uniform current and potential distribution. 

X. Li, J. Gao, Z. Zeng, pH-resistant titania hybrid organic-inorganic sol-gel coating for solid-phase microextraction of polar compounds , Anal3dica Chimica Acta, 590(1), 26-33,(2007). 

The alternating —CH2—and —CF2 groups along the polymer chain provide a unique polarity that influences the polymer s solubility and electric properties. At elevated temperatures PVDF can be dissolved in polar solvents such as organic esters and amines. This selective solubility offers a way to prepare corrosion resistant coatings for chemical process equipment and long-life architectural finishes on building panels. 

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