**3. Recast layers**

308 Corrosion Resistance

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;

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

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

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

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

Minkevich, 1965; Springer et al., 2011).

(b) x250.

diffusion coatings on steel surface.

zinc (η-phase) or pure zinc.

diffusion coatings despite of their smaller 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 machining.

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 erosion and corrosion resistance of a number of ferrous alloys.

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

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

Improvement of Corrosion Resistance of Steels by Surface Modification 311

The recast white layer as well as the other discussed white layers can not be etched and has very high hardness, corrosion resistance and wear resistance. The phenomenon of surface modification by EDM has been observed for over four decades. Under the high temperature of the discharge column, the white layer can dissolve carbon from the gases formed in the discharge column from the hydrocarbon dielectric and receives higher carbon content than the base material and hence show increased resistance to abrasion and corrosion. Moreover electrode material has been found in the workpiece surface after machining with conventional electrode. Better surface properties have been obtained by machining with powder metallurgy electrodes containing alloying elements which diffuse in the workpiece surface. Fine powders mixed in the dielectric offer another way for achieving desirable surface modification. All this determines the three main directions for surface modification

In the EDM process with conventional electrode has been observed material transfer from the electrode to workpiece surface which is a function of the various electrical parameters of the circuit. The high energy machining results in lower surface deposition, but there is more diffusion in depth. Also it is found that the negative polarity is desirable for increase of material transfer from the tool electrode. The improvement of the surface integrity, wearand corrosion resistance of the workpiece material can be realized by surface alloying during sparking, using sintered powder metallurgy electrode. With the alloying there is a potential to increase workpiece hardness from two to five times and significant enhance the corrosion resistance that of the bulk material. It is possible remarkable to increase the corrosion resistance of carbon steel by using of composite electrodes containing cooper, aluminium, tungsten carbide and titanium. The material from the electrode is transferred to the workpiece and the characteristics of the surface layer can be changed significantly. The same results can be achieved with the addition of metallic and compound powders in the

by electrical discharge machining (Kumar et al., 2009):

 Surface modification by conventional electrode materials Surface modification by powder metallurgy electrodes Surface modification by powder-mixed dielectric

dielectric. In this case are used Ni, Co, Fe, Al, Cr, Cu, Ti, C (graphite), etc.

**3.2 Surface modification by electrical discharge treatment in electrolyte** 

Such a method as EDM is the electrical discharge treatment in electrolyte, where the modification goes by a high energy thermal process in a very small volume on the metallic surface, involving melting, vaporisation, activation and alloying in electrical discharges and after that cooling of this surface with high rate in an electrolyte. The high energy process put together with the nonequilibrium phase transformations in the metallic system causes considerable modifications of the metallic surface and obtaining of layers with finecrystalline and nanocrystalline structure (Krastev at al., 2009; Krastev & Yordanov, 2010). The metallic surface after electrical discharge treatment in electrolyte has a different structure in comparison with the metal matrix which determines different properties. It is observed remarkable increasing of hardness, strength and corrosion resistance related to the nonequilibrium phase transformations and the obtained finecrystalline microstructure. The investigations show that obtained on tools layers have higher hardness, wear resistance, tribocorrosion resistance and corrosion resistance, which give better performance, considerable increasing of working life and wide opportunities for industrial application.

immersed in a dielectric fluid (Asif Iqbal & Khan, 2010). It has been proven to be especially valuable in the machining of super-tough, electrically conductive materials, such as tool steels, hard metals and space-age alloys. These materials would have been difficult to machine by conventional methods, but EDM has made it relatively simple to machine intricate shapes that would be impossible to produce with conventional cutting tools. In EDM process, the shapes of mold cavities are directly copied from that of the tool electrode, so time-consuming preparation work must be done on the fabrication of the corresponding tool electrode.

The basis of EDM can be traced as far back as 1770, when English chemist Joseph Priestly discovered the erosive effect of electrical discharges (Ho & Newman, 2003). In 1943 Russian scientists Boris Lazarenko and Natalya Lazarenko (Satel, 1956) applied the destructive effect of electrical sparks for manufacturing and developed a controlled process of machining difficult-to-machine metals by vaporizing material from the surface. At the recent years the research interests and practice are directed to the novel application of electrical discharge machining in the area of surface modification.
