Corrosion products ("rust") forming on steel, where the protective
coating system has been damaged mechanically (scratch).

There are several mechanisms by which paint coating systems provide corrosion protection, as described by Hare (see reference below):

1. Barrier: limiting the access of chemical species involved in electrochemical corrosion reactions, notably oxygen in the cathodic reaction.
   
   
2. Barrier: maintaining a high electrical resistance at the substrate interface, restricting the access of ionic species.
   
   
3. Inhibition: corrosion inhibitors in the primer, passivating the underlying metal surface.
   
   
4. Cathodic protection: Sacrificial protection of the underlying substrate, a galvanic effect (for example, steel protection by zinc).

References/Literature:

C.H. Hare: "Barrier Coatings", JPCL-PMC, April 1998, pp.17-37.

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