**2.1 Diffusion coatings obtained by nitriding**

Nitriding is a thermochemical treatment in which nitrogen in atomic or ionic form is introduced by diffusion process into the metallic surface and in the case of steels is based on the solubility of nitrogen in iron (Davis, 2001, 2002; Pye, 2003). The unique of the nitriding process were recognized by the Germans in the early 1920s. It was used in the applications that required:

High torque

296 Corrosion Resistance

attention is directed to the advance methods for surface modification of metals such as laser surface treatment, ion beam surface treatment and electrical discharge machining, which give a modified surface with specific combination of properties in result of nonequilibrium

The obtained by all these methods surface layers can be classified in several ways. Based on

In the overlay coatings, an additional material is placed on the substrate by techniques such as physical vapor deposition (PVD), flame or plasma spraying, etc. The coating in these cases has a mechanical bond with the metallic surface, without much diffusion of the

In diffusion coatings a chemical bond is formed with the metallic surface and is obtained a diffusion layer with modifying chemical composition in the depth of the layer. These coatings are formed generally at high temperatures and include such methods as thermochemical treatment and chemical vapor deposition (CVD). Thermochemical treatment is one of the fundamental methods for surface modification of metals and alloys by forming of diffusion coatings. The plain carbon steels and low-alloy steels are mainly treated by these methods to form on the surface layers with high hardness, wear resistance and corrosion resistance, but these methods are also often used to modify the surface of high-alloy steels, cast irons, nonferrous metals and for obtaining of layers with determinate

The recast layers are obtained after attacking the metallic surface with high energy stream such as laser, ion beam or electrical discharge for a very short time and pulse characteristics that involve local melting of the surface and after that rapidly cooling. The recast layer can be with the same chemical composition as the substrate, but with different microstructure and properties in result of nonequilibrium phase transformations during the rapidly cooling, or with a different chemical composition, microstructure and properties in result of attending diffusion process of surface alloying. In recent years of scientific and practical interests is the electrical discharge machining (EDM) for obtaining of recast layers with different characteristics and properties, mainly high hardness, wear resistance and corrosion

Typical cases of surface modification are diffusion coatings and recast layers, which will be

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

the mechanism of the treating process, they can be categorized as:

microstructural characteristics.

coating constituents into the substrate.

chemical composition, structure and properties.

 Overlay coatings; Diffusion coatings; Recast layers.

resistance.

the objectives of this chapter.

**2. Diffusion coatings** 


Nitrided steels offer improved corrosion and oxidation resistance. The nitrided surface of an alloy steel or tool exhibits increased resistance to saltwater corrosion, moisture and water.

The treatment temperature is usually between 500 and 550 ºC for periods of 1 to 100 h depending of the nitriding method, type of steel and the desired depth of the layer. Since nitriding does not involve heating the steel to austenitic temperatures and quenching to martensite is not required, nitriding can be carried out at comparatively low temperatures and thus produce diffusion coating with high quality without deformations of the workpiece.

This technique is of great industrial interest as it forms structures with hard nitride surface layers, so that the global mechanical performance, hardness, wear resistance and corrosion resistance of steels are greatly improved. In recent years new and innovative surface

Improvement of Corrosion Resistance of Steels by Surface Modification 299

Machlet and was for nitrogenization of iron and steel in an ammonia gas atmosphere diluted by hydrogen. Either a single-stage or a double-stage process can be used when nitriding with anhydrous ammonia. The temperature of the single-stage process is usually between 495 and 525 ºC and it is produced a nitrogen-rich compound zone in a form of white nitride layer on the surface of the nitrided steel. For successful nitriding, it is necessary to control the gas flow so that there is a continuous fresh supply of ammonia at the steel surface. An oversupply of nitrogen may result in the formation of a thick layer of iron nitrides on the steel surface. Independently of some brittleness this nitride layer has a very good corrosion resistance. The principle purpose of double-stage nitriding is to reduce the depth of the white layer on the steel surface, but except for the reduction in the amount of ammonia consuming per hour, there is no advantage in using the double-stage process unless the amount of the white layer produced in the single-stage nitriding cannot be

The gas nitriding for improvement of corrosion resistance of plain carbon steels and lowalloy steels can be carried out for shorter times at elevated temperatures (Minkevich, 1965). The purpose is to obtain on the steel surface non-etched nitrided layer without pores and thickness about 0.015 – 0.030 mm. In Table 1 are given the conditions of this process for

Table 1. Gas nitriding process conditions for improvement of the corrosion resistance for

Plasma nitriding (ion nitriding) is a thermochemical treatment process in which nitrogen ions alone or in combination with other gases react at the workpiece surface to produce hardened and corrosion resistance surface on a variety of steels (Buchkov & Toshkov, 1990; Pye, 2003; Smith, 1993). The process is realized on the creation of gaseous plasma under vacuum conditions. The gases can be selected in whatever ratio to provide required surface metallurgy and the layer can consists of single phase, dual phase, or diffusion zone only. The surface metallurgy can be manipulated to suit both the application and the steel. Ion nitriding has many advantages and is appropriate to many applications that are not possible with the conventional nitrided techniques. The nitrided layer on the steel is of the order of 0.1 mm in depth and is harder than nitrided surface layers produced by gas nitriding. The process requires both hydrogen and nitrogen at the workpiece surface. The hydrogen makes certain that the surface of the steel is oxide-free and the chemical reaction takes place between the steel and nitrogen ions. The oxide-free surface enables the nitrogen to diffuse rapidly into the steel and sustains the nitriding actions. A major advantage of the plasmanitriding process is the enhanced mass transfer of high-energy nitrogen ions to the surface of the steel under the action of an electrical field. The kinetics of the nitrogen ions into the bulk of the steel is controlled by the solid-state diffusion and nitride precipitation. Advantages of plasma nitriding include reduced nitriding cycles, good control of the γ' white iron nitride layer, reduced gas consumption, clean environmental operation, excellent surface quality

Temperature, ºC Time, min Dissociation of

600 60 - 120 35 - 55 600 45 - 90 45 - 65 700 15 - 30 55 - 75

ammonia, %

tolerated on the finished parts.

Type of Steels (GOST)

08, 10, 15, 20, 25, 40,

A12, A15, A20

45, 50,

some plain carbon steels and free-cutting steels.

plain carbon steels and free-cutting steels.

engineering technologies have been developed to meet the rapidly increasing demands from different extreme applications, but gas and plasma (ion) nitriding remain as one of the most widely used techniques for surface engineering.

The case structure of nitrided steel depends on its type, concentration of alloying elements and particular conditions of nitriding treatment. The diffusion zone is the original core microstructure with the addition of nitride precipitates and nitrogen solid solution. The surface compound zone is the region where γ' (Fe4N) and ε (Fe2-3N) intermetallics are formed. The corrosion resistance of steel varies with nitrided layer structure. The surface "white layer" can contain ε nitride, γ' nitride or a two phase mixture ε+γ', below that is the diffusion zone. In acid solutions the iron nitrides corrode more slowly than iron and when the "white zone" is formed on the steel surface the improvement of the corrosion resistance is a fact. In Fig. 1 is shown the typical structure of nitrided plain carbon steel (Minkevich, 1965).

Fig. 1. Microstructure of nitrided GOST 10 steel (x340).

The commonly used steels for nitriding are generally medium-carbon steels that contain strong nitride-forming elements such as aluminium, chromium, vanadium, tungsten and molybdenum. These alloying elements are beneficial in nitriding because they form nitrides that are stable at nitriding temperatures. Other alloying elements such as nickel, silicon and manganese are not so important for the characteristics of the nitrided diffusion coatings. Although these alloy steels are capable to form iron nitrides in the presence of nascent nitrogen, the properties of the nitrided layer are better in those steels that contain one or more of the major nitride-forming alloying elements.

Gas and plasma nitriding are the main methods for obtaining of nitrided diffusion coatings on steels with widely industrial application. The times of gas nitriding can be quite long, that is from 10 to 130 h depending on the application and the depth of the layer is usually less than 0.5 mm. Plasma nitriding allows faster nitriding process and quickly attained surface saturation on the base of the activated nitrogen diffusion. The process provides excellent dimensional control of the white-layer, its composition and properties.

Gas nitriding of steels (Davis, 2001, 2002; Pye, 2003; Smith, 1993) is a thermochemical treatment that takes place in the presence of ammonia gas which dissociates on the steel surface at the operating temperatures. The atomic nitrogen produced is adsorbed at the steel surface, and depending on the temperature and concentration of nitrogen, iron nitrides form at and bellow the steel surface. The patent for gas nitriding was first applied for by Adolph

engineering technologies have been developed to meet the rapidly increasing demands from different extreme applications, but gas and plasma (ion) nitriding remain as one of the most

The case structure of nitrided steel depends on its type, concentration of alloying elements and particular conditions of nitriding treatment. The diffusion zone is the original core microstructure with the addition of nitride precipitates and nitrogen solid solution. The surface compound zone is the region where γ' (Fe4N) and ε (Fe2-3N) intermetallics are formed. The corrosion resistance of steel varies with nitrided layer structure. The surface "white layer" can contain ε nitride, γ' nitride or a two phase mixture ε+γ', below that is the diffusion zone. In acid solutions the iron nitrides corrode more slowly than iron and when the "white zone" is formed on the steel surface the improvement of the corrosion resistance is a fact. In Fig. 1 is

The commonly used steels for nitriding are generally medium-carbon steels that contain strong nitride-forming elements such as aluminium, chromium, vanadium, tungsten and molybdenum. These alloying elements are beneficial in nitriding because they form nitrides that are stable at nitriding temperatures. Other alloying elements such as nickel, silicon and manganese are not so important for the characteristics of the nitrided diffusion coatings. Although these alloy steels are capable to form iron nitrides in the presence of nascent nitrogen, the properties of the nitrided layer are better in those steels that contain one or

Gas and plasma nitriding are the main methods for obtaining of nitrided diffusion coatings on steels with widely industrial application. The times of gas nitriding can be quite long, that is from 10 to 130 h depending on the application and the depth of the layer is usually less than 0.5 mm. Plasma nitriding allows faster nitriding process and quickly attained surface saturation on the base of the activated nitrogen diffusion. The process provides

Gas nitriding of steels (Davis, 2001, 2002; Pye, 2003; Smith, 1993) is a thermochemical treatment that takes place in the presence of ammonia gas which dissociates on the steel surface at the operating temperatures. The atomic nitrogen produced is adsorbed at the steel surface, and depending on the temperature and concentration of nitrogen, iron nitrides form at and bellow the steel surface. The patent for gas nitriding was first applied for by Adolph

excellent dimensional control of the white-layer, its composition and properties.

shown the typical structure of nitrided plain carbon steel (Minkevich, 1965).

widely used techniques for surface engineering.

Fig. 1. Microstructure of nitrided GOST 10 steel (x340).

more of the major nitride-forming alloying elements.

Machlet and was for nitrogenization of iron and steel in an ammonia gas atmosphere diluted by hydrogen. Either a single-stage or a double-stage process can be used when nitriding with anhydrous ammonia. The temperature of the single-stage process is usually between 495 and 525 ºC and it is produced a nitrogen-rich compound zone in a form of white nitride layer on the surface of the nitrided steel. For successful nitriding, it is necessary to control the gas flow so that there is a continuous fresh supply of ammonia at the steel surface. An oversupply of nitrogen may result in the formation of a thick layer of iron nitrides on the steel surface. Independently of some brittleness this nitride layer has a very good corrosion resistance. The principle purpose of double-stage nitriding is to reduce the depth of the white layer on the steel surface, but except for the reduction in the amount of ammonia consuming per hour, there is no advantage in using the double-stage process unless the amount of the white layer produced in the single-stage nitriding cannot be tolerated on the finished parts.

The gas nitriding for improvement of corrosion resistance of plain carbon steels and lowalloy steels can be carried out for shorter times at elevated temperatures (Minkevich, 1965). The purpose is to obtain on the steel surface non-etched nitrided layer without pores and thickness about 0.015 – 0.030 mm. In Table 1 are given the conditions of this process for some plain carbon steels and free-cutting steels.


Table 1. Gas nitriding process conditions for improvement of the corrosion resistance for plain carbon steels and free-cutting steels.

Plasma nitriding (ion nitriding) is a thermochemical treatment process in which nitrogen ions alone or in combination with other gases react at the workpiece surface to produce hardened and corrosion resistance surface on a variety of steels (Buchkov & Toshkov, 1990; Pye, 2003; Smith, 1993). The process is realized on the creation of gaseous plasma under vacuum conditions. The gases can be selected in whatever ratio to provide required surface metallurgy and the layer can consists of single phase, dual phase, or diffusion zone only. The surface metallurgy can be manipulated to suit both the application and the steel. Ion nitriding has many advantages and is appropriate to many applications that are not possible with the conventional nitrided techniques. The nitrided layer on the steel is of the order of 0.1 mm in depth and is harder than nitrided surface layers produced by gas nitriding. The process requires both hydrogen and nitrogen at the workpiece surface. The hydrogen makes certain that the surface of the steel is oxide-free and the chemical reaction takes place between the steel and nitrogen ions. The oxide-free surface enables the nitrogen to diffuse rapidly into the steel and sustains the nitriding actions. A major advantage of the plasmanitriding process is the enhanced mass transfer of high-energy nitrogen ions to the surface of the steel under the action of an electrical field. The kinetics of the nitrogen ions into the bulk of the steel is controlled by the solid-state diffusion and nitride precipitation. Advantages of plasma nitriding include reduced nitriding cycles, good control of the γ' white iron nitride layer, reduced gas consumption, clean environmental operation, excellent surface quality

Improvement of Corrosion Resistance of Steels by Surface Modification 301

By contrast to plain carbon steels, the corrosion resistance of stainless steels can be reduced by nitriding, due to breakdown of the surface chrome oxide barrier to enable nitrogen diffusion into the steel. The stainless steels exhibit generally poor tribological properties, because of that treatments such as nitriding can enhance the surface hardness and improve the wear resistance. Plasma nitriding can be carried out for this purpose at temperatures from 350 to 500 ºC (Castaletti et al., 2008). While giving significant improvement in wear resistance, the higher treatment temperatures tend to adversely affect on the corrosion performance of the stainless steels in result of formation of CrN. Improved corrosion resistance of plasma-nitrided layers on stainless steels are observed when the nitriding process is carried out at a lower temperature (400 ºC) with presence in the layer of "S – phase", which is supersaturated with nitrogen austenite. The expanded austenite layer in nitrided YB 1Cr18Ni9Ti steel has a good pitting corrosion resistance in 1 % NaCl solution and an equivalent homogeneous corrosion resistance in 1 N H2SO4 solution, compared with

Boronizing or boriding is a thermochemical treatment that involves diffusion of boron into the metal surface at high temperatures (Davis, 2002; Liahovich, 1981; Minkevich, 1965; Schatt, 1998). The boriding process is carried out at temperatures between approximately 850 and 1050 ºC by using solid, liquid or gaseous boron-rich atmospheres. Boronizing is an effective method for significant increasing of surface hardness, wear and corrosion resistance of metals. The basic advantage of the boronized steels is that iron boride layers have extremely high hardness values between 1600 and 2000 HV. The typical surface hardness of borided carbon steels is much greater than the produced by any other conventional surface hardening treatments. The combination of a high surface hardness and a low surface coefficient of friction of the borided layer provides also for these diffusion coatings a remarkable wear resistance. Boronizing can considerably enhance the corrosionerosion resistance of ferrous materials in nonoxidizing dilute acids and alkali media and is increasingly used to this advantage in many industrial applications. It is also important that the borided diffusion coatings have a good oxidation resistance up to 850 ºC and are quite

Fig. 3. Microstructure of ion nitrided surface of steel 31CrMoV9 (x600).

the original stainless steel (Lei & Zhang, 1997).

resistant to attack by molten metals.

**2.2 Diffusion coatings obtained by boronizing** 

and reduced distortion of the workpiece. The white layer on the surface of the ion nitrided medium-carbon steel contains mainly from γ' iron nitride with a small amount ε iron nitride. With the control of the process can be obtained layer from single γ' nitride phase. In Table 2 is made a comparison of the white layer structures of gas and ion nitrided GOST 10 steel (Buchkov & Toshkov, 1990).


Table 2. Comparison of the white layer structures of gas and ion nitrided GOST 10 steel

Our investigations on the microstructure of ion nitrided EN 31CrMoV9 steel show that on the surface is formed nitrided white layer consists from ε-nitride and γ'-nitride (Krastev et al., 2010) – Fig. 2. The thickness of the nitride white layer is 10 – 12 μm and in depth follows 150 – 200 μm diffusion zone with nitride precipitates – fig. 3.

Fig. 2. X-ray diffraction patterns of ion nitrided surface layer on steel 31CrMoV9.

The microhardness of the nitride white layer is about 1050 – 1100 HV which together with the improve corrosions resistance provides high wear resistance.

and reduced distortion of the workpiece. The white layer on the surface of the ion nitrided medium-carbon steel contains mainly from γ' iron nitride with a small amount ε iron nitride. With the control of the process can be obtained layer from single γ' nitride phase. In Table 2 is made a comparison of the white layer structures of gas and ion nitrided GOST 10 steel

Type of nitriding Temperature, ºC Time, min ε-nitride, % γ'-nitride, %

Table 2. Comparison of the white layer structures of gas and ion nitrided GOST 10 steel

Fig. 2. X-ray diffraction patterns of ion nitrided surface layer on steel 31CrMoV9.

the improve corrosions resistance provides high wear resistance.

The microhardness of the nitride white layer is about 1050 – 1100 HV which together with

150 – 200 μm diffusion zone with nitride precipitates – fig. 3.

Our investigations on the microstructure of ion nitrided EN 31CrMoV9 steel show that on the surface is formed nitrided white layer consists from ε-nitride and γ'-nitride (Krastev et al., 2010) – Fig. 2. The thickness of the nitride white layer is 10 – 12 μm and in depth follows

540 30 10 90 540 300 10 90 540 720 20 80

540 30 - 100 540 300 - 100 540 720 - 100

(Buchkov & Toshkov, 1990).

Gas

Ion

Fig. 3. Microstructure of ion nitrided surface of steel 31CrMoV9 (x600).

By contrast to plain carbon steels, the corrosion resistance of stainless steels can be reduced by nitriding, due to breakdown of the surface chrome oxide barrier to enable nitrogen diffusion into the steel. The stainless steels exhibit generally poor tribological properties, because of that treatments such as nitriding can enhance the surface hardness and improve the wear resistance. Plasma nitriding can be carried out for this purpose at temperatures from 350 to 500 ºC (Castaletti et al., 2008). While giving significant improvement in wear resistance, the higher treatment temperatures tend to adversely affect on the corrosion performance of the stainless steels in result of formation of CrN. Improved corrosion resistance of plasma-nitrided layers on stainless steels are observed when the nitriding process is carried out at a lower temperature (400 ºC) with presence in the layer of "S – phase", which is supersaturated with nitrogen austenite. The expanded austenite layer in nitrided YB 1Cr18Ni9Ti steel has a good pitting corrosion resistance in 1 % NaCl solution and an equivalent homogeneous corrosion resistance in 1 N H2SO4 solution, compared with the original stainless steel (Lei & Zhang, 1997).

#### **2.2 Diffusion coatings obtained by boronizing**

Boronizing or boriding is a thermochemical treatment that involves diffusion of boron into the metal surface at high temperatures (Davis, 2002; Liahovich, 1981; Minkevich, 1965; Schatt, 1998). The boriding process is carried out at temperatures between approximately 850 and 1050 ºC by using solid, liquid or gaseous boron-rich atmospheres. Boronizing is an effective method for significant increasing of surface hardness, wear and corrosion resistance of metals. The basic advantage of the boronized steels is that iron boride layers have extremely high hardness values between 1600 and 2000 HV. The typical surface hardness of borided carbon steels is much greater than the produced by any other conventional surface hardening treatments. The combination of a high surface hardness and a low surface coefficient of friction of the borided layer provides also for these diffusion coatings a remarkable wear resistance. Boronizing can considerably enhance the corrosionerosion resistance of ferrous materials in nonoxidizing dilute acids and alkali media and is increasingly used to this advantage in many industrial applications. It is also important that the borided diffusion coatings have a good oxidation resistance up to 850 ºC and are quite resistant to attack by molten metals.

Improvement of Corrosion Resistance of Steels by Surface Modification 303

Paste boriding is an attractive technique for producing of boride diffusion coatings on steels surface because of lower cost and less difficulty in comparison with pack boriding. It is carried out usually in a paste from B4C as a boriding agent, Na3AlF4 as an activator, fluxes, and binding agent for the paste formation. The temperature of the process is from 800 to 1000 ºC and heating is mostly inductively or resistively. A layer in excess of 50 μm thickness may be obtained after inductively or resistively heating to 1000 ºC for 20 min. The relationship between the boride layer thickness and time for iron and steel boronized with B4C-Na2B4O7-Na3AlF6 based paste at 1000 ºC is shown in Fig. 5 (ASTM Handbook, 1991).

Fig. 5. Relationship between the boride layer thickness and time for iron and steel boronized

Gas and plasma boriding are carried out in B2C6-H2 or BCl2-H2 mixtures which are high toxic and also there are problems with the explosiveness of the gaseous atmosphere. As a result these techniques have not gained commercial acceptance. Plasma boriding has some advantages, mainly the lower temperature of the thermochemical process of about 650 ºC

Fluidized bed boriding is the recent innovation on the area of boriding technologies. It is carried out in special retort furnace and involves a bed material of coarse silicon carbide particles, a special boride powder and oxygen-free gas atmosphere from nitrogen-hydrogen gas mixture. The process offers several advantages, can be adaptable to continuous production and has low operating costs due to reduced processing time and energy

Our investigations on pack boriding show that it is possible to change the traditional type and amount of activator, and provide a successful diffusion process with high quality of obtained boride layers. The compositions of powder mixtures were from 55 % B4C; 1 to 3 % NaBF4 or Na3AlF6 and diluter Al2O3. The thermochemical treatments are carried out with

with B4C-Na2B4O7-Na3AlF6 based paste at 1000 ºC.

consumption for mass production of boronized parts.

and reduction of the duration.

On the surface of the boronized steels generally a boron compounds layer is formed. It can be a single-phase or double-phase layer of borides with definite composition. The single phase boride layer consists of Fe2B, while the double-phase layer consists of an outer phase of FeB and an inner phase of Fe2B. The FeB phase is brittle and harder, forms a surface under high tensile stress and has a higher coefficient of thermal expansion. The Fe2B phase is preferred because it is less brittle and forms a surface with a high compressive stress, the preferred stress state for a high-hardness, low-ductility case. Although small amounts of FeB are present in most boride layers, they are not detrimental if they are not continuous. Continuous layers of FeB can be minimized by diffusion annealing after boride formation. In Fig. 4 is shown the typical microstructure of borided layer on the surface of plain carbon steel (Schatt, 1998).

Fig. 4. Microstructure of borided layer on the EN C15 steel consisting of FeB (dark) and Fe2B (light) phases.

Boriding can be carried out on most ferrous materials such as plain carbon steels, low-alloy steels, tool steels, stainless steels, cast irons and sintered steels. There are a variety of methods for producing of boride diffusion coatings on steel surface. Thermochemical boronizing techniques include:


Only pack and paste boriding from these methods have reached commercial success. Because of environmental problems gas and liquid boriding have a very limited application. Pack boriding is the most common boriding method with a wide development. The process involves packing the steel parts in a boriding powder mixture from ferroboron, amorphous boron or B4C, fluxes and activators (NaBF4, KBF4, Na2B4O7), and heating in a heat-resistant steel box at 900 to 1050 ºC for one to twelve hours depending on the required layer thickness. The commonly produced case depths are 0.05 to 0.25 mm for carbon steels and low-alloy steels and 0.025 to 0.080 mm for high-alloy steels.

On the surface of the boronized steels generally a boron compounds layer is formed. It can be a single-phase or double-phase layer of borides with definite composition. The single phase boride layer consists of Fe2B, while the double-phase layer consists of an outer phase of FeB and an inner phase of Fe2B. The FeB phase is brittle and harder, forms a surface under high tensile stress and has a higher coefficient of thermal expansion. The Fe2B phase is preferred because it is less brittle and forms a surface with a high compressive stress, the preferred stress state for a high-hardness, low-ductility case. Although small amounts of FeB are present in most boride layers, they are not detrimental if they are not continuous. Continuous layers of FeB can be minimized by diffusion annealing after boride formation. In Fig. 4 is shown the typical microstructure of borided layer on the surface of plain carbon

Fig. 4. Microstructure of borided layer on the EN C15 steel consisting of FeB (dark) and Fe2B

Boriding can be carried out on most ferrous materials such as plain carbon steels, low-alloy steels, tool steels, stainless steels, cast irons and sintered steels. There are a variety of methods for producing of boride diffusion coatings on steel surface. Thermochemical

Only pack and paste boriding from these methods have reached commercial success. Because of environmental problems gas and liquid boriding have a very limited application. Pack boriding is the most common boriding method with a wide development. The process involves packing the steel parts in a boriding powder mixture from ferroboron, amorphous boron or B4C, fluxes and activators (NaBF4, KBF4, Na2B4O7), and heating in a heat-resistant steel box at 900 to 1050 ºC for one to twelve hours depending on the required layer thickness. The commonly produced case depths are 0.05 to 0.25 mm for carbon steels and

low-alloy steels and 0.025 to 0.080 mm for high-alloy steels.

steel (Schatt, 1998).

(light) phases.

 Pack boriding Paste boriding Liquid boriding Gas boriding Plasma boriding Fluidized bed boriding

boronizing techniques include:

Paste boriding is an attractive technique for producing of boride diffusion coatings on steels surface because of lower cost and less difficulty in comparison with pack boriding. It is carried out usually in a paste from B4C as a boriding agent, Na3AlF4 as an activator, fluxes, and binding agent for the paste formation. The temperature of the process is from 800 to 1000 ºC and heating is mostly inductively or resistively. A layer in excess of 50 μm thickness may be obtained after inductively or resistively heating to 1000 ºC for 20 min. The relationship between the boride layer thickness and time for iron and steel boronized with B4C-Na2B4O7-Na3AlF6 based paste at 1000 ºC is shown in Fig. 5 (ASTM Handbook, 1991).

Fig. 5. Relationship between the boride layer thickness and time for iron and steel boronized with B4C-Na2B4O7-Na3AlF6 based paste at 1000 ºC.

Gas and plasma boriding are carried out in B2C6-H2 or BCl2-H2 mixtures which are high toxic and also there are problems with the explosiveness of the gaseous atmosphere. As a result these techniques have not gained commercial acceptance. Plasma boriding has some advantages, mainly the lower temperature of the thermochemical process of about 650 ºC and reduction of the duration.

Fluidized bed boriding is the recent innovation on the area of boriding technologies. It is carried out in special retort furnace and involves a bed material of coarse silicon carbide particles, a special boride powder and oxygen-free gas atmosphere from nitrogen-hydrogen gas mixture. The process offers several advantages, can be adaptable to continuous production and has low operating costs due to reduced processing time and energy consumption for mass production of boronized parts.

Our investigations on pack boriding show that it is possible to change the traditional type and amount of activator, and provide a successful diffusion process with high quality of obtained boride layers. The compositions of powder mixtures were from 55 % B4C; 1 to 3 % NaBF4 or Na3AlF6 and diluter Al2O3. The thermochemical treatments are carried out with

Improvement of Corrosion Resistance of Steels by Surface Modification 305

After boronizing, the corrosion rate of boronized low carbon steel AISI 1018 is about 100 times lower than the corrosion rate of unboronized one based on the electrochemical measurement (Suwattananont, 2005). The boronized high strength alloy steel AISI 4340 and austenitic stainless steel AISI 304 have corrosion rate about several times lower than the corrosion rate of unboronized steels. The comparison of the tafel plots between boronized

Fig. 8. Tafel plots of boronized and unboronized low carbon steel AISI 1018.

**2.3 Diffusion coatings obtained by carbonitriding and nitrocarburizing** 

After boronizing the steel becomes nobler and has lower corrosion rate than the unboronized one which proves that the corrosion resistance of the boronized steels is

Carbonitriding and nitrocarburizing are those thermochemical treatments which involve diffusional addition of both carbon and nitrogen to the surface of steels for production of diffusion coatings with determinate structure and properties (ASTM Handbook, 1991; Chatterjee-Fischer, 1986; Davis, 2002). Carbonitriding is a modified form of gas carburizing rather than a form of nitriding. The modification consists of introducing ammonia into the gas carburizing atmosphere to add nitrogen to the carbonized case as it is being produced. Nascent nitrogen forms at the steel surface by the dissociation of ammonia in the furnace atmosphere and diffuses simultaneously with carbon. Typically, carbonitriding is carried out at a lower temperatures and shorter times than is gas carburizing, producing a shallower case than is usual in production carburizing. The temperature range for the process is normally 750 – 950 ºC and the properties of the obtained diffusion coating are similar with those obtained by carburizing. After the next heat treatment they have high hardness and wear resistance, but the corrosion resistance enhance is unessential. For the improvement of corrosion behaviour of steels the carbonitriding should be carried out at lower temperatures of about 700 ºC for obtaining on the surface a carbonitride compound layer. In this case the thermochemical process transforms into nitrocarburizing. Nitrocarburizing, as definition, is thermochemical treatment that is applied to a ferrous object in order to produce surface enrichment in nitrogen and carbon which form a

and unboronized AISI 1018 steel is shown in Fig. 8.

improved.

EN C60 steel at 950 ºC and time from two to six hours. The results show that it is possible to exchange the traditional for the process NaBF4 with the inexpensive Na3AlF6 as an activator, and amount of 3 % is enough to provide boride layer with the required thickness, structure and hardness. In Fig. 6 are given the structures of borided surface of steel C60 obtained for 2 and 6 h in powder mixture containing 3 % Na3AlF6. The XRD analysis shows that the boride layers consist mainly from Fe2B with a small amount of FeB. The microhardness of the boride diffusion coatings is 1600 – 1800 HV.

Fig. 6. Microstructure of boride layers on C60 steel obtained for 2 (a) and 6 (b) hours pack boriding at 950 ºC in powder mixture containing 3 % Na3AlF6 (x150).

The borided steels have a higher corrosion resistance together with the high hardness and wear resistance. In Fig. 7 is given a comparison in the corrosion resistance of 0.45 % C plain carbon steel before and after boronizing (ASTM Handbook, 1991).

Fig. 7. Corroding effect of mineral acids on boronized and unboronized Ck45 steel.

EN C60 steel at 950 ºC and time from two to six hours. The results show that it is possible to exchange the traditional for the process NaBF4 with the inexpensive Na3AlF6 as an activator, and amount of 3 % is enough to provide boride layer with the required thickness, structure and hardness. In Fig. 6 are given the structures of borided surface of steel C60 obtained for 2 and 6 h in powder mixture containing 3 % Na3AlF6. The XRD analysis shows that the boride layers consist mainly from Fe2B with a small amount of FeB. The microhardness of the

a b Fig. 6. Microstructure of boride layers on C60 steel obtained for 2 (a) and 6 (b) hours pack

The borided steels have a higher corrosion resistance together with the high hardness and wear resistance. In Fig. 7 is given a comparison in the corrosion resistance of 0.45 % C plain

Fig. 7. Corroding effect of mineral acids on boronized and unboronized Ck45 steel.

boriding at 950 ºC in powder mixture containing 3 % Na3AlF6 (x150).

carbon steel before and after boronizing (ASTM Handbook, 1991).

boride diffusion coatings is 1600 – 1800 HV.

After boronizing, the corrosion rate of boronized low carbon steel AISI 1018 is about 100 times lower than the corrosion rate of unboronized one based on the electrochemical measurement (Suwattananont, 2005). The boronized high strength alloy steel AISI 4340 and austenitic stainless steel AISI 304 have corrosion rate about several times lower than the corrosion rate of unboronized steels. The comparison of the tafel plots between boronized and unboronized AISI 1018 steel is shown in Fig. 8.

Fig. 8. Tafel plots of boronized and unboronized low carbon steel AISI 1018.

After boronizing the steel becomes nobler and has lower corrosion rate than the unboronized one which proves that the corrosion resistance of the boronized steels is improved.

### **2.3 Diffusion coatings obtained by carbonitriding and nitrocarburizing**

Carbonitriding and nitrocarburizing are those thermochemical treatments which involve diffusional addition of both carbon and nitrogen to the surface of steels for production of diffusion coatings with determinate structure and properties (ASTM Handbook, 1991; Chatterjee-Fischer, 1986; Davis, 2002). Carbonitriding is a modified form of gas carburizing rather than a form of nitriding. The modification consists of introducing ammonia into the gas carburizing atmosphere to add nitrogen to the carbonized case as it is being produced. Nascent nitrogen forms at the steel surface by the dissociation of ammonia in the furnace atmosphere and diffuses simultaneously with carbon. Typically, carbonitriding is carried out at a lower temperatures and shorter times than is gas carburizing, producing a shallower case than is usual in production carburizing. The temperature range for the process is normally 750 – 950 ºC and the properties of the obtained diffusion coating are similar with those obtained by carburizing. After the next heat treatment they have high hardness and wear resistance, but the corrosion resistance enhance is unessential. For the improvement of corrosion behaviour of steels the carbonitriding should be carried out at lower temperatures of about 700 ºC for obtaining on the surface a carbonitride compound layer. In this case the thermochemical process transforms into nitrocarburizing. Nitrocarburizing, as definition, is thermochemical treatment that is applied to a ferrous object in order to produce surface enrichment in nitrogen and carbon which form a

Improvement of Corrosion Resistance of Steels by Surface Modification 307

resistance. Typical transformed austenite case thicknesses are in the range 50 to 200 μm. However, much deeper cases can be achieved by employing a precarburized treatment prior

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

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

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 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

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

this thermochemical treatment are chromium, aluminium and zinc.

The process is carried out at 900 – 1050 ºC for 6 to 12 hours.

to nitrocarburizing.

wide industrial application.

compound layer. This technique has a wide application in the industry and is carried out as gaseous, plasma and liquid process. There is a tendency for limitation of the liquid process because of its toxicity and environmental problems. Based on the temperature range of the thermochemical treatment, nitrocarburizing can be classified as:


Ferritic nitrocarburizing is this thermochemical treatment which is realized at temperatures completely within the ferrite phase field. The primary object of such treatments is usually to improve the anti-scuffing characteristic of ferrous engineering components by producing a compound layer in the surface which has good tribological characteristics. In addition, the fatigue characteristics can be considerably improved, particularly when nitrogen is retained in solid solution in the diffusion zone beneath the compound layer. This is normally achieved by quenching into oil or water from the treatment temperature, usually 570 ºC. The obtained at these temperatures compound white layer provided the enhancing of the corrosion resistance of the nitrocarburized surface. The compound layer produced by ferritic nitrocarburizing consists mainly from ε carbonitride because of low carbon solubility in γ' nitride. In Fig. 9 is shown the typical microstructure of nitrocarburized surface of low carbon steel (Chatterjee-Fischer, 1986).

Fig. 9. The microstructure of nitrocarbonized EN C15 steel at 570 ºC for 2 hours.

On commercial basis post nitrocarburizing oxidation treatments have been used to enhance the aesthetic properties of gaseous nitrocarburized components. However it is proved that these additional techniques improve the fatigue, wear and corrosion resistance of steel surface and can be successfully combined for this purpose.

When the treatment temperature is such that partial transformation of the matrix to austenite occurs through enrichment with nitrogen, than the treatment is referred to as austenitic nitrocarburizing. With austenitic nitrocarburizing the subsurface is transformed to iron-carbon-nitrogen austenite, which is subsequently transformed to tempered martensite and bainite, with hardness in the range of 750 to 900 HV. The keeping of compound layer from ε carbonitride on the nitrocarburized steel surface together with the transformed subsurface after the nitrocarburizing process at temperatures of about 700 ºC provide enhance of the fatigue resistance of treated parts together with high corrosion and wear

compound layer. This technique has a wide application in the industry and is carried out as gaseous, plasma and liquid process. There is a tendency for limitation of the liquid process because of its toxicity and environmental problems. Based on the temperature range of the

Ferritic nitrocarburizing is this thermochemical treatment which is realized at temperatures completely within the ferrite phase field. The primary object of such treatments is usually to improve the anti-scuffing characteristic of ferrous engineering components by producing a compound layer in the surface which has good tribological characteristics. In addition, the fatigue characteristics can be considerably improved, particularly when nitrogen is retained in solid solution in the diffusion zone beneath the compound layer. This is normally achieved by quenching into oil or water from the treatment temperature, usually 570 ºC. The obtained at these temperatures compound white layer provided the enhancing of the corrosion resistance of the nitrocarburized surface. The compound layer produced by ferritic nitrocarburizing consists mainly from ε carbonitride because of low carbon solubility in γ' nitride. In Fig. 9 is shown the typical microstructure of nitrocarburized surface of low

Fig. 9. The microstructure of nitrocarbonized EN C15 steel at 570 ºC for 2 hours.

surface and can be successfully combined for this purpose.

On commercial basis post nitrocarburizing oxidation treatments have been used to enhance the aesthetic properties of gaseous nitrocarburized components. However it is proved that these additional techniques improve the fatigue, wear and corrosion resistance of steel

When the treatment temperature is such that partial transformation of the matrix to austenite occurs through enrichment with nitrogen, than the treatment is referred to as austenitic nitrocarburizing. With austenitic nitrocarburizing the subsurface is transformed to iron-carbon-nitrogen austenite, which is subsequently transformed to tempered martensite and bainite, with hardness in the range of 750 to 900 HV. The keeping of compound layer from ε carbonitride on the nitrocarburized steel surface together with the transformed subsurface after the nitrocarburizing process at temperatures of about 700 ºC provide enhance of the fatigue resistance of treated parts together with high corrosion and wear

thermochemical treatment, nitrocarburizing can be classified as:

 Ferritic nitrocarburizing Austenitic nitrocarburizing

carbon steel (Chatterjee-Fischer, 1986).

resistance. Typical transformed austenite case thicknesses are in the range 50 to 200 μm. However, much deeper cases can be achieved by employing a precarburized treatment prior to nitrocarburizing.
