Introduction to anti-corrosion mechanism of coating

Coating anti-corrosion mechanism The mechanism of coating anti-corrosion is generally believed to be as follows

(1) Anti-penetration mechanism

The coating is like a barrier layer, separating water and oxygen from the surface of the steel to prevent corrosion of the steel. However, by measuring the permeability of the paint film to water and oxygen, it is found that sometimes the coating with a high permeability still has good corrosion resistance, indicating that the protection mechanism may not only be the barrier effect of the coating.

(2) Conductivity control

The mechanism is that the electrochemical reaction conditions require the electrolyte as the medium, therefore, the coating blocks the penetration of ions, which can effectively prevent the occurrence of electrochemical reactions. It can be surmised that coatings with higher conductivity are less protective than coatings with lower conductivity. However, some low-conductivity coatings have little relationship between conductivity and corrosion resistance.

(3) Adhesion anticorrosion mechanism

When water passes through the paint film to reach the steel surface through osmosis, if the "wet adhesion" of the paint film is poor, the water and the dissolved oxygen in it directly contact the anode and cathode regions on the steel surface, while the impurities in the water layer produce less High conductance meets the basic conditions for corrosion. When corrosion progresses, the generated ferrous ions and hydroxide ions generate osmotic pressure under the paint film, allowing water to further pass through the semi-permeable membrane, thereby forming a force that forces the paint film to leave the substrate, resulting in blisters. The osmotic pressure can Up to 2.5~3.0MPa. However, the anti-deformation force of the coating is much lower, only 6~40kPa. As the blisters gradually become larger, the non-protected surface of the steel gradually increases, so more corrosion products are formed, and the corrosion will be faster. It can be seen that the adhesion of the coating to steel is an important factor for the corrosion resistance of the coating.

Cracks in the paint film are unavoidable during the use of the paint. When the paint film is gouged, water and oxygen reach the metal surface and begin to corrode. If the primer does not adhere well to the metal, water will creep under the paint film, the coating will loosen from the metal and spread over wider and wider areas, and the paint film may blister near the gouging point. . Where local adhesion changes, filiform corrosion occurs, which is characterized by fine "filament" corrosion that spreads irregularly under the paint film. These filaments initially start at the edge of a scratch and develop towards weak points of adhesion in the paint film, becoming longer and longer. At the place where the silk begins to form, the infiltrating oxygen oxidizes ferrous ions to ferric ions, and precipitates ferric hydroxide, resulting in a drop in osmotic pressure and the end of the silk; but at the growing end of the silk, most of the oxygen is in the The consumption of the cathode allows the corrosion to continue.

When the paint film is incomplete, it is obvious that having good wet adhesion is the key to corrosion protection. In addition, when corrosion starts, hydroxide ions are generated, so the resistance to alkali-catalyzed hydrolysis is also important. Passivation and cathodic protection, such as primers containing passivating pigments and zinc-rich primers, are also required to minimize the possibility of blister formation near filiform corrosion and gouging damage.