ABSTRACT
This chapter focuses on three mechanisms by which organic coatings protect against corrosion: stabilizing the passivating surface oxide, cathodic protection, and passivating pigments. Organic coatings are barriers against oxygen and water, but they are not impermeable to these species. Barrier properties against water and oxygen cannot explain the protective effect of the coating. The chapter presents a short overview of passivity; that is, protective surface oxides stop the electrochemical reaction between oxygen and the meta. Cathodic protection by painting is achieved with zinc-rich paints, whose zinc pigment acts as a sacrificial anode, corroding preferentially to the steel substrate. In order for the zinc to provide cathodic protection, the zinc must be in electric contact with the steel substrate, which means that the zinc-rich paint must be the first coat applied. The passivation mechanism can be enhanced by pigmenting the paint with reactants or species that precipitate on the metal surface, strengthening or adding to the protection of the original oxide.
