**2.4 Diffusion coatings with high corrosion resistance on metals basis**

The diffusion coatings on metals basis for improvement the corrosion resistance of steels have a wide industrial application. The main aim of the thermochemical treatment in this case is to form on the steel surface a layer from metals with high corrosion resistance, their solid solution in the metal matrix, or their compounds. The metals that are usually used for this thermochemical treatment are chromium, aluminium and zinc.

Chromizing is a surface treatment process of developing a chromized layer on metals and alloys for heat-, corrosion-, and wear resistance (Davis, 2001; Liahovich, 1981; Minkevich, 1965). The technique is applied principally for different types of steels and cast irons, but it is also of interest for surface modification of nickel, molybdenum, tungsten, cobalt and their alloys. Chromized steels offer considerably improved corrosion and oxidation resistance of the surface and can work successfully in complicated conditions combining wear, high temperature, corrosion, erosion and cavitation. If the plain carbon or alloy steel for chromizing contains carbon more than 0.4 %, a corrosion and wear resistant compound layer from Cr23C6 and Cr7C3 with thickness 0.01 – 0.03 mm will be formed on the surface. On steels with low carbon content compact chromium carbides layer cannot be formed, but because of high solubility of chromium in iron it will be formed on the steel surface a solid solution with chromium content up to 60 % which provide the high corrosion resistance of the diffusion layer. There are a variety of methods for producing of chromium diffusion coatings on steel surface, such as gaseous, liquid, pack and vacuum chromizing, but only vacuum and pack processes are developed as thermochemical treatment technologies with a wide industrial application.

The pack chromizing is often preferred because of its easily process conditions and low cost. The components to be chromized are packed with fine chromium powder and additives. A typical chromizing mixture consists of 60 percent chromium or ferrochromium powder, up to 2 percent halide salt as an activator and about 38 percent aluminium oxide as inert filler. The process is carried out at 900 – 1050 ºC for 6 to 12 hours.

The aluminizing pack-cementation thermochemical treatment has also the most widely industrial application for production of aluminium diffusion coatings. The process is commercially practiced for a wide range of metals and alloys, including plain carbon steels, low-alloy steels and high-alloy steels, cast irons, nickel- and cobalt-base superalloys. Sample aluminide coatings have high corrosion resistance and resist high-temperature oxidation by the formation of an aluminium oxide protective layer and can be used up to about 1000 ºC. The powder mixture for pack aluminizing usually consists from about 50 % aluminium or frroaluminium powder, 1 to 2 % NH4Cl as an activator and about 48 % aluminium oxide as inert filler. As the other processes of pack-cementation, the aluminizing technique consists of packing the steel parts in the powder mixture and heating in a heat-resistant steel box at 800 to 1100 ºC for three to fifteen hours, depending on the alloy type and required layer thickness. The aluminized diffusion coatings on plain carbon steels and low-alloy steels are usually about 0.05 – 0.8 mm thick and represent a white layer with a complicated

Improvement of Corrosion Resistance of Steels by Surface Modification 309

Sherardizing is a diffusion process in which the steel parts are heated in the presence of zinc dust or powder in inert medium. Aluminium oxide or sand in amount of 20 % is added to the zinc powder as inert filler and 1 to 2 % halide salts are used as activator. The thermochemical treatment can be carried out in retort, rotated drum or as a packcementation process at 350 to 500 ºC for three to twelve hours. The structure of the obtained layer is the same as the structure on steel surface after hot-dip galvanizing with a thickness

The recast layers on metals and alloys are created by treating the surface with high energy stream such as laser, ion beam or electrical discharge for a very short time and pulse characteristics. The high energy attack on the surface involves local melting and in many cases vaporizing of metal microvolumes. After the cooling, on the treated metal surface a recast layer with different structure and properties from the substrate is formed. This recast layer can be with the same chemical composition as the substrate or with different one if in the thermal process suitable conditions for surface alloying are created. When the recast process is not controlled there are on the surface microcracks and pores which have negative influence on the surface properties and the recast layer must be removed. In the controlled recast processes it is possible to produce surface layer with determinate chemical composition, thickness, structural characteristics and properties, which are unique for the material with the very high hardness, corrosion- and wear resistance. The basic techniques that give opportunities in this direction are laser surface treatment and electrical discharge

Laser surface treatment is widely used to recast and modify localized areas of metallic components. The heat generated by the adsorption of the laser light provides a local melting and after controlled cooling is obtained a recast layer on the metal surface with high hardness, wear resistance and corrosion resistance. The laser surface melting is based on rapid scanning of the surface with a beam focused to a power density scale of 104 W/cm2 to 107 W/cm2. Quench rates up to 108 - 1010 K/sec provide the formation of fine structures, the homogenization of microstructures, the extension of solid solubility limits, formation of nonequilibrium phases and amorphous phases or metallic glasses, with corrosion resistance 10–100 time higher compared to crystalline (Bommi et al., 2004). Laser surface melting is a simple technique as no additional materials are introduced, and it is especially effective for processing ferrous alloys with grain refinement and increase of the alloying elements content in solid solution. In fact the process has been employed for improving the cavitation

The laser surface melting can be combined with a simultaneous controlled addition of alloying elements. These alloying elements diffuse rapidly into the melt pool, and the desired depth of alloying can be obtained in a short period of time. By this means, a desired alloy chemistry and microstructure can be generated on the sample surface and the degree of microstructural refinement will depend on the solidification rate. The surface of a lowcost alloy, such as low carbon steels, can be selectively alloyed to enhance properties, such

Electrical discharge machining is a thermoelectric process that erodes workpiece material by series of discrete but controlled electrical sparks between the workpiece and electrode

erosion and corrosion resistance of a number of ferrous alloys.

as resistance to wear and corrosion (Davis, 2001).

about 50 – 400 μm.

**3. Recast layers** 

machining.

composition from iron aluminides (Fe3Al, FeAl, FeAl2 and Fe2Al5) with high corrosion and oxidation resistance and solid solution of aluminium in α-iron (Davis, 2001; Liahovich, 1981; Minkevich, 1965; Springer et al., 2011).

Our research group has been carried out investigations on high-temperature corrosion and abrasive resistance of chromium and aluminium diffusion coatings on EN C45 steel as parts of sintering machine in an agglomeration process of iron ores. The specimens are produced by pack-cementation process at the optimal characteristics for the both heat treatment techniques. The structure of the obtained diffusion coatings is given in Fig. 10. The comparison shows that better behaviour in these work conditions have the chromium diffusion coatings despite of their smaller thickness.

Fig. 10. Diffusion coatings on C45 steel produced by chromizing (a) x150, and aluminizing (b) x250.

a b

Zinc diffusion coatings are traditional method for corrosion protection of steels with a great practical importance and a wide industrial application (Proskurin et al., 1988). Zinc has a number of characteristics that make it a well-suited corrosion protective coating for iron and steel products in most environments. The excellent field performance of zinc coatings results from its ability to form dense, adherent corrosion product films and a rate of corrosion considerably below that of ferrous materials. Many different types of zinc coatings are available and each has unique characteristics, which affect not only on the applicability, but also on the relative economics and expected service live. Hot-dip galvanizing and sherardizing are the main thermochemical treatment techniques for producing of zinc diffusion coatings on steel surface.

The hot-dip galvanizing process, also known as general galvanizing, produces a zinc coating on iron and steel products by immersion of the material in a bath of liquid zinc. Before the coating is applied, the steel surface is cleaned to remove all oils, greases, soils, mill scale, and rust. Galvanized coatings are used on a multitude of materials ranging in size from small parts such as nuts, bolts, and nails to very large structural shapes. The process is usually carried out at 440 to 470 ºC for 1 to 10 minutes and in result is obtained zinc diffusion coating which consists of a series of zinc-iron compound layers from FeZn13 (ξ–phase), FeZn7 –FeZn10 (δ-phase), Fe5Zn21 (Γ1-phase) with a surface layer of solid solution of iron in zinc (η-phase) or pure zinc.

Sherardizing is a diffusion process in which the steel parts are heated in the presence of zinc dust or powder in inert medium. Aluminium oxide or sand in amount of 20 % is added to the zinc powder as inert filler and 1 to 2 % halide salts are used as activator. The thermochemical treatment can be carried out in retort, rotated drum or as a packcementation process at 350 to 500 ºC for three to twelve hours. The structure of the obtained layer is the same as the structure on steel surface after hot-dip galvanizing with a thickness about 50 – 400 μm.
