What is the main principle of action of protective coatings on metal surfaces?

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What is the main principle of action of protective coatings on metal surfaces? We know that in the environment, metal surfaces, especially carbon steel, require protective measures, and coating protection is one of the common methods used for this purpose. Generally, coatings with anti-corrosion and rust-preventing properties are used. The protection principle of this type of coating is as follows: 1. Barrier effect: On the metal surface, the coating forms a complete layer that separates the environmental agents (oxygen, water, acids, bases, salts, etc.) from the metal surface, thereby eliminating the conditions necessary for the formation of a \"corrosion cell\" and preventing corrosion of the metal surface. Its reliability has certain limits. Mainly, the coating cannot completely isolate the environment; there are always media such as oxygen and water that can penetrate the metal surface. The ability of penetration depends on the material of the coating film, the construction methods, etc. On the other hand, over time the protective effect of the coating diminishes; therefore, physical isolation alone often cannot provide effective protection for the metal surface. 2. Addition of corrosion-resistant components (corrosion inhibitors): This is a unique feature of coatings designed for corrosion and rust prevention. When a certain amount of a substance known as a \"corrosion inhibitor\" is added to the film-forming material, even if the coating cannot completely isolate the metal from its environment and a \"corrosion cell\" has already formed, the corrosion inhibitor can effectively prevent or slow down corrosion on the metal surface, thereby achieving better protection. Adding some anodic metal powders (such as zinc, magnesium, aluminum, etc.) to the coating provides sacrificial protection (cathodic protection). In addition, active pigment systems and additives that have phosphating, passivating, and corrosion-inhibiting effects on steel are used.