Metal structures require protective treatment against corrosion, which will extend the operational life of the facility. To isolate the metal from aggressive action, specialized coatings are used to protect it. They include:
Metal coatings: cathodic and anodic – applied by galvanizing, dipping, spraying, diffusion, and cladding.
Non-metallic coatings: polymer films, silicate enamels, paint and varnish compositions, rubbers.
In this article, we will describe the above types in detail.
Protective metal coatings.
In anodic metal coatings, the electrochemical potential is lower than that of the metal structure being treated. If the protective layer is broken, the electrochemical protection functions remain. But cathodic coatings prevent corrosion due to a mechanical barrier, which must be undamaged.
Galvanic coatings. They are formed as a result of galvanization, a method of applying a metal protective layer by electrochemical means. The main types are brassing, gilding, chrome plating, galvanizing, and others. Galvanic coatings not only protect against oxidation and corrosion, but also improve the aesthetic functions and service life of the product.
This method of protection is used in construction, machine-building and aviation industries, electronics and radio engineering.
Gas-thermal spraying is a process where molten material particles are applied to the surface of the workpiece using a gas or plasma method. The features of this method are heat resistance, wear resistance, high electrical conductivity or electrical insulation function.
Special powder compositions made of ceramics, metal, various wires and cords are used for spraying.
Gas thermal spraying can be:
- Gas plasma spraying, which is simple and affordable, is used against corrosion for large areas of metal structures, or to restore the shape of a part.
High-speed gas plasma spraying is used to create metal and metal-ceramic coatings with high density.
-Plasma spraying to create ceramic coatings with high refractoriness.
-Detonation spraying is applied to restore small areas of a structure.
- Sputtering and melting are used when there is a risk of deformation of the part.
Electric arc metallization for applying metal coatings to large areas to protect the object from corrosion.
The method of immersing elements in the melt.
Before the parts are immersed in a melt of zinc, tin or aluminum, they are treated with glycerin and ammonium chloride. This procedure protects the melt from oxidation and removes various salt and oxide films. The disadvantages of this method are uneconomical consumption and unevenness of the applied metal coating. This method cannot be used to protect narrow holes and gaps.
Thermal diffusion coating, which uses zinc, creates a high level of electrochemical protection for both steel and ferrous metals. It perfectly covers parts and structures of complex shapes and does not peel off during operation. Advantages of the coating: resistance to corrosion, deformation, mechanical stress, and high adhesion.
Cladding is a method of applying metal by means of plate deformation and strong compression. It is used to protect decorative and contact coatings on steel, aluminum, and copper parts. Cladding occurs during hot rolling, pressing, stamping, or in the process of explosive welding.
Polymeric coating against corrosion.
Consider the most popular polymers for use: polyethylene, polypropylene, polyisobutylene, epoxy resin.
Polymerization is carried out by gas-thermal spraying, by immersion or by using a brush. When cooled, a protective film is formed.
Antifriction solid lubricants can be classified as polymeric.
The coatings are based on molybdenum, graphite and other solid lubricants. Acrylic components, titanium, and epoxy resin are used as binders.
Oxide films.
Oxidation is a reaction of metals that occurs when they interact with an acid-base composition and oxygen. As a result, a protective film is formed on the metal surface, which increases the service life of the parts.
Rubber coatings.
Rubber and ebonite films protect chemical apparatus and pipelines. They are formed from rubber of different hardness using sulfur. The coating is applied to a degreased surface, and the accumulated air is squeezed out with a roller, followed by vulcanization of the product.
One of the advantages of rubber coating is its resistance to acid and alkali, and the disadvantage is abrasion during use.
Lubricants and pastes.
They are used to protect metals from dirt and dust during transportation. Greases and pastes are based on minerals and wax-like materials.
Among the disadvantages is the easy damage to the protective film, which is why it is better to use solid-lubricating antifriction coatings.