Corrosion processes and methods to prevent or reduce corrosion of coatings

Corrosion of steel alone is a multitrillion-dollar problem worldwide.  Economic loss is not the only consequence of corrosion;  Structural damage to buildings and Bridges can be fatal. 

Control of corrosion by coating  

When an object is prone to oxidation, one of the most common methods of corrosion control is the use of a protective anticorrosive coating.  Different mechanisms can be used to prevent corrosion, but two methods are used in most protective coatings.  These methods are to block the formation and destruction of electrochemical corrosion cells.  The two mechanisms are usually combined in a single coating to increase corrosion resistance. 

Barrier coating  

One way to disrupt an electrochemical cell is to create a barrier.  Normally, this is a barrier to water and/or oxygen, preventing the formation of a viable electrochemical cell.  This can be achieved by using a coating made of a highly crystallized polymer adhesive that prevents the diffusion of these components to the substrate through the film.  The incorporation of lamellar pigments also forms a barrier to oxygen and water, interrupting the flow of electrons and preventing corrosion from taking place.  

Two well-known types of polymer adhesives used to block coatings are epoxy resins and halogenated copolymers.  These systems form thin films that are highly resistant to the transport of water, water vapor and oxygen.  Prevents water from reaching the surface prevents the formation of a conductive electrolyte path in which electrons flow from the anode to the cathode.  

If oxygen cannot reach the metal surface, there is no acceptor (cathode) for the electrons from the metal and corrosion cannot take place.  

The layers also prevent water and oxygen from moving to the metal surface.  These tabular pigments - such as mica and talc - form various "mazes."  Even though the polymer membrane allows oxygen and water to permeate, the indirect path to the surface is longer.  If not stopped, the corrosion process will slow down and maintain the surface.  

The barrier properties of the coating also depend on other factors, such as film thickness.  In general, thicker barriers perform better against corrosion as long as other properties (i.e., adhesion) are not affected.  

Anticorrosive coatings  

While barrier coatings physically protect metal surfaces, corrosion inhibition coatings protect surfaces through chemical mechanisms.  The metal has higher electrochemical activity and will oxidize rather than the matrix or pigment, which may interrupt the electrochemical process.  Therefore, metals can be used as components of corrosion inhibition coatings.  

When well chosen and correctly applied to a suitably prepared substrate, anticorrosive coatings can provide many years of service in preserving metallic objects.