From the electrochemical theory of corrosion it follows that corrosion can be stopped by suppressing either the cathodic or the anodic reaction, or by inserting between the cathodic and anodic areas a large resistance, which impedes the movement of ions. Calculations indicate that paint films are so permeable to water and oxygen that they cannot suppress the cathodic reaction. Paints can inhibit corrosion by modifying the anodic reaction; for this to occur the pigment must be either metallic, or basic, or soluble. In general, paint films protect by virtue of their high electrolytic resistance; they readily acquire a charge, consequently they are relatively impermeable to ions. Soap formation is beneficial, since it renders the film less permeable to electrolytes; it also yields soluble inhibitive degradation products, which have been identified.
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