Corrosion protection refers to measures to prevent damage to surfaces caused by external influences. Corrosion is a chemical or electrochemical reaction that leads to a change and destruction of materials made of metal, glass, ceramics and polymers. Oxygen, hydrogen, sulphur dioxide, hypochlorous acids, peroxide compounds and the like are among the reactants. A generally known type of corrosion is the rusting of iron.
In the case of metallic materials, metal atoms change into their originally non-metallic chemical compound in this process. This happens when the metal forms a compound together with e.g. oxygen, thus forming metal oxide. Oxidation is a chemical process in which one substance gives up electrons to another substance. An example of the process of oxidation is the reaction of iron with oxygen in the air. When iron is exposed to air, it reacts with oxygen and forms rust (iron oxide). In the process, iron gives up electrons to the oxygen.
Iron oxide has a different chemical structure than pure iron and is less stable, which means that it oxidises further and becomes rust. A thin invisible layer of oxidation first covers the metal. The oxide can prevent corrosion if the layer adheres firmly and without gaps to the surface, thus shielding the metal from the oxygen. This protection is called passivity. Whether it forms on a metal or whether the oxidation leads to corrosion depends on the purity of the metal. The more impure a metal is, the more impervious and thus oxygen-permeable the oxide layer forms, which ultimately leads to corrosion rather than passivation.
There are different types of corrosion protection by coating, including:
- Barrier protection: A coating is applied to the metal surface to protect it from direct contact with the environment. Here, the coating forms a barrier that prevents the penetration of oxygen, moisture and other corrosive substances.
- Sacrificial layer protection: Here, a coating is applied that consists of a material that corrodes more easily than the metal underneath. The material of the coating therefore “sacrifices” itself to protect the metal underneath. An example of this is zinc coating on steel (also known as galvanising).
- Cathodic protection: In this corrosion protection mechanism, a material is applied to the metal surface. It acts as a cathode (negative pole or also negatively charged electrode) and thus reduces or mitigates the corrosion process. An example of this is the application of zinc to steel or aluminium.
- Inhibitor protection: Here, a special chemical is applied to the metal surface that inhibits the corrosion process. Inhibitors can, for example, reduce or stop the electrochemical reactions that lead to corrosion.
There are also hybrid coatings that combine several of these protective mechanisms to achieve higher corrosion resistance. The choice of the appropriate corrosion protection process depends on several factors, such as the type of metal, the environment and the requirements for durability and resistance of the coating.
There are a variety of coatings that can be used for corrosion protection. Some commonly used coatings are:
- Epoxy coatings: Characterised by their high chemical resistance and good adhesion to many surfaces.
- Polyurethane coatings: To be applied as a topcoat over epoxy coatings to provide additional protection. They are known for their abrasion resistance and flexibility.
- Zinc coatings: Used as sacrificial coating protection. Galvanised steel is used, for example, in construction and the automotive industry.
- Phosphating: Used to pre-treat steel for painting or coating to improve coating adhesion.
- Powder coatings: Possess good adhesion, are durable and offer excellent protection against corrosion. They are often used in the automotive industry and in mechanical engineering.
- Chrome coatings: Offer excellent protection against corrosion and are often used in the aerospace and electronics industries.
The choice of the appropriate coating depends on various factors, such as the type of material to be protected, the environmental conditions and the requirements for durability and resistance of the coating.
Various coatings are offered at ROSE Systemtechnik. Special requests are also possible after consultation with the in-house surface technology department (effort, costs, minimum order quantities for desired coating, etc.). To test the corrosion behaviour of the coatings and materials, salt spray is used in accordance with DIN EN ISO 9227. The resulting corrosion provides information about the quality. Consistent quality is necessary to comply with the ISO 9001 standard.