**2. Diffusion coatings**

The diffusion coating process is one of the most effectively and with a great practical application method for improvement of corrosion resistance together with wear resistance, hardness and working live of metals and alloys. This is very important for the carbon steels as the most widely used engineering material accounts more than 80% of the annual world steel production. Despite its relatively limited corrosion resistance, carbon steel has a wide application in whole nowadays industry and the cost of metallic corrosion to the total economy is remarkable high. Because of that the carbon steels and in many cases low-alloy steels are the most used constructional metallic materials for surface engineering on the base of diffusion coatings.

The diffusion coatings are products of thermally activated high temperature processes, that form on the metallic surface chemically bonded layer with determinate chemical composition, structure and properties. For decades a variety of diffusion coatings have been developed and used to improve the properties of metallic surface. There are several kinds of coating methods among which the most commonly used and with the most widely industrial application is thermochemical treatment.

Thermochemical treatment technologies for surface modification of steels have been very well investigated and developed on research and industrial level. These are methods by which nonmetals or metals are penetrated into the metallic surface by thermodiffusion after chemical reaction and adsorption. By thermochemical treatment the surface layers change their chemical composition, structure and properties and in many cases this modified surface can work in conditions which are impossible for the bulk material. Carburizing, nitriding, carbonitriding, nitrocarburizing, boronizing, chromizing, aluminizing and zinc coating are the most popular methods for industrial application. Only carburizing from all these methods could not perform the requirements to form coatings on the surface with high corrosion resistance in the most cases of steel treatment. The other techniques often are used for improvement of the corrosion behaviour of steels by surface modification and more of them increase the wear resistance and hardness of the treated materials.
