**1. Introduction**

238 Corrosion Resistance

[94] H. Z. Zheng, S. Q. Lu, X. J. Dong, D. L. Quyang, Materials Science and Engineering A

[99] Y. Shida, N. Ohtsuka, J. Muriama, N. Fujino and H. Fujikawa, Proc. JIMS-3, High Temp.

[100] H. J. Grabke, E. M. Muller-Lorenz, S. Strauss, E. Pippel, J. Woltersdorf, Oxidation of

[107] X. Y. Zhang, M.H. Shi, C. Li, N. F. Liu, Y.M. Wei, Mater Sci and Engineering A 448

[108] O. E. Kedim, S. Paris, C. Phigini, F. Bernard, E. Gaffet, Z. A. Munir, Mater Sci and

[113] X. Peng, F. Wang, Oxidation – resistant nanocrystalline coatings, Development in high-

[114] I. Kaur, W. Gust, L. Kozma, Handbook of grain and interphase boundary diffusion

[116] R. Gupta, R. K. Singh Raman, C. C. Koch, Materials Science and Engineering A 494

[119] F.J. Humphreys, M. Hatherly, Recrystallization and related annealing phenomena,

[120] R. K. Gupta, R. K. Singh Raman, C. C. Koch, TMS 2008 Annual Meeting 1 (2008) 151-157. [121] K.S. Darling, R.N. Chan, P.Z. Wong, J. E. Semones, R.O. Scattergood, C.C. Koch,

[122] R. K. Gupta, K. S. Darling, R. K. Singh Raman, K. R. Ravi, C. C. Koch, B. S. Murty, R. O. Scattergood, Journal of Materials Science (2011) DOI 10.1007/s10853-011-5986-6.

temperature corrosion and protection of materials, W. Gao, Z. Li (Eds.), CRC Press,

[101] M. Martin, N. Lakshmi, U. Koops, H.-I. Yoo, Z. Phys. Chem. 221 (2007) 1449.

[95] J. E. May, S. E. Kuri, P. A. P Nascente, Mater Sci and Engineering A 428 (2006) 290. [96] J. E. May, G. Galerie, T. P. Busquim, S. E. Kuri, Materials and Corrosion 58 (2008) 87 [97] Y. Niu, Z. Q. Cao, F. G, Farne, G. Randi, C. L. Wang, Corrosion Science 45 (2003) 1125.

[92] F. Wang, X. Tian, Q. Li, L. Li, X. Peng, Thin and Solid Films 516 (2008) 5740.

[93] G. Cao, L. Geng, Z. Zheng, M. Naka, Intermetallics 15 (2007) 1672.

[98] S. Leistikow, I. Wolf and H.J. Grabke, Werkst. Korros., 38 (1987) 556.

Corros., Trans. Jap. Inst. Met., 1983, 631.

[106] H. Hindam, D. P. Whittle, Oxidaiton of Metals, 18 (1982) 245

[109] W. W. Smeltzer, D. P. Whittle, J. Electrochem. Soc. 125 (1978) 1116.

[89] G. Chen, H. Lou, Scripta materialia 43 (2000) 119. [90] G. F. Chen, H. Y. Lou, Materials Letters 45 (2000) 286. [91] L. Liu, F. Wang, Materials Letters 62 ( 2008) 4081.

496 (2008) 524.

Metals, 50 (1998) 314.

(2007) 259.

2008.

(2008) 253.

Pergamon, 1996.

[102] G. C. Wood, Oxidation of Metals, 2 (1970) [103] V. R. Howes, Corrosion Science, 7 (1967) 735 [104] R. Lobb, H. Evans, Metal Science 26 ( 1981). [105] M. Schutz, Oxidation of Metals, 44, (1995), 29

Engineering A 369 (2004) 49.

[110] C. J. Wagner, J. Electrochem. Soc. 99 (1952) 369. [111] C. J. Wagner, J. Electrochem. Soc. 103 (1956) 571. [112] C. J. Wagner, J. Electrochem. Soc. 63 (1959) 772.

data: Stuttgart: Zigler Press, (1989), 523. [115] A. W. Bowen, G .M. Leak, Metall. Trans. 1 (1970), 1695.

[118] F. Wang, Oxidation of Metals 47 (1997) 247.

Scripta Materlialia 59 (2008) 530.

[117] J. G. Goedjen, D. A. Shores, Oxidation of Metals 37 (1992) 125

The modifying of the surface, which involves altering only the surface layers of a material, is becoming increasingly important with the aim to enhance the corrosion resistance of many kinds of materials. The advantage of this approach lies in the fact that the natural physical and mechanical properties of the material are retained, while at the same time the corrosion resistance is increased. It is well known that electroplated zinc coating is employed as active galvanic protection for low and middle-content alloyed steels (Almeida et al.,1998; Hagans & Hass, 1994; Kudryavtsev, 1979; Lainer, 1984; Zaki, 1988). However, zinc is a highly reactive element, and therefore high corrosion rates of this coating are observed in cases of indoor and outdoor exposures. For this reason a post-treatment is needed to increase the lifetime of zinc coatings. This kind of treatment is applied in the current industrial practice to prolong the lifetime of zinc coatings and the steel substrates, respectively. This treatment consists of immersion in a chemical bath, which forms a conversion layer over the plated zinc. The so formed layer is a dielectric passive film with high corrosion resistance and it is also a better surface for paint adherence (Zaki, 1988). The main problem with the traditionally applied post-treatment procedures is the presence of Cr6+ salts that are considered to be carcinogenic substances, which are known to be very harmful to human health and environment (Schafer & Stock, 2005) and whose use is forbidden by European regulations (Hagans & Hass, 1994).

Molybdates, tungstates, permanganates and vanadates, including chromium-like components, were the first chemical elements to be tested as hexavalent chromium substitutes (Almeida et al., 1998a, 1998b; Korobov et al, 1998; Schafer & Stock, 2005; Wilcox & Gabe, 1987; Wilcox et al., 1988). Recently many alternative coatings have been developed, based on zirconium and titanium salts (Barbucci et al., 1998; Hinton, 1991), cobalt salts (Barbucci et al., 1998; Gonzalez et al., 2001) and organic conductive polymers (Gonzalez et al., 2001; Hosseini et al., 2006). The use of salts of rare-earth metals as the main component in the electrolytes, developed for the formation of cerium, lanthanum and other oxide protective films is also a very promising alternative to the chromate films and it is one of the advanced contemporary methods for corrosion protection of metals and alloys (Bethencourt et al., 1998; Crossland et al., 1998; Davenport et al., 1991; Fahrenholtz et al., 2002; Forsyth et al., 2002; Hinton, 1983, 1992; Hosseini et al., 2007; Liu & Li , 2000; Montemor et al., 2002; Montemor & Ferreira, 2008; Pardo et al., 2006; Wang et al., 1997). However, some aspects of

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 241

of cerium oxide, deposited on aluminum alloys and they ascertained that the trivalent cerium is oxidized to tetravalent by the oxygen dissolved in the electrolyte, which leads to precipitation/formation of non-soluble CeO2 on the cathodic sections of the electrode surface. Montemor and coworkers (Montemor et al., 2001, 2002) studied the effect of the composition of the electrolytes (based on Ce(NO3)3 and the regime of preparing conversion layers on galvanized (zinc coated) steel. The increase in the thickness of the cerium conversion films in the process of formation leads to their enrichment in Ce4+. According to the same authors the conversion films, formed in La(NO3)3, are more efficient in view of anticorrosion protection, compared to those formed in electrolytes, containing Ce(NO3)3 and Y(NO3)3 (Montemor et al, 2002). The mechanism, involved in such a process of reducing the corrosion rate of the substrate, may be related to precipitation of cerium oxides and hydroxides in the vicinity of the anodic areas. These precipitates reduce the cathodic activity

The mechanism of zinc corrosion inhibition, when zinc is treated in solutions of Ce(NO)3, has been studied by Aramaki (Aramaki, 2001a, 2001b, 2002a; 2002b). He established the formation of hydrated or hydroxylated Ce-rich layer. This process, in its turn, leads to the formation of Ce2O3 on the electrode/the protected surface, respectively to inhibition of the cathodic reactions of the corrosion process in solutions of NaCl. Тhe work by Lu and Ives (Lu & Ives, 1995) has been extended further to study the effect of cerium salt solution treatment. Rotating disk assemblies were employed to monitor the cathodic electrode process and its inhibition by cerium salt treatment on austenitic stainless steels in a solution simulating sea water. The reduction of oxygen and hydrogen cations on both kinds of non-treated steels has been shown to be controlled by mass transfer processes in the solution. Cerium treatment effectively inhibits the cathodic reduction of oxygen, which is controlled primarily by charge transfer on the electrode. The over-potential for cathodic reduction of hydrogen cations is increased after the cerium treatment and the electrode reaction is controlled both by the mass transfer process in solution and by the charge transfer on the electrode. As a result of inhibition of the electrode processes, cerium improves the localized corrosion resistance, and in particular the crevice corrosion

The electrochemical behavior of stainless steels - SS304 and 316L, following various cerium and cerium/molybdenum prereatment steps, was studied aiming at gaining more information on the process, by which cerium and molybdenum can modify the properties of passive film formed on stainless steels (Breslin et al, 1997). The coatings were analyzed by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy in order to identify the cerium species, which play the main role in the promotion of the passivation behavior. The pre-treatment step, denoted as Ce(CH3CO2)3 and CeCl3, involved immersion treatment of the electrodes in the Ce(CH3CO2)3 solution for 1 h and then in the CeCl3 solution for one additional hour at approximately 92○C. Regardless of the nature of the cerium salt, no changes in the rate of the cathodic reduction reaction could be observed. The increase in the corrosion potential (Ecorr). is mainly due to the decrease in the passive current density, suggesting that treatment in cerium solutions does not affect the rate of the cathodic reaction, but rather reduces the rate of the passive film dissolution. However a significant lowering of the oxygen reduction current could be observed, following the electrodeposition of small amount of cerium onto the electrode surface. Thus, it appears that efficient

and hinder the transfer of electrons from the anodic to the cathodic spots.

resistance, of stainless steels.

the preparation and of the corrosion behavior of these coatings are not quite clear yet and their practical utilization is still uncertain. In order to find an attractive alternative to Cr6+ conversion coating, several treatment procedures that should manifest both efficient anticorrosive behavior as well as an optimal benefit/cost ratio, and mainly insignificant environmental impact, have yet to be developed. It has been found out that cerium species can be successfully applied to protect zinc from corrosion (Aramaki, 2001, 2002; Arenas et al., 2003, 2004; Ferreira et al., 2004; Otero et al., 1996, 1998; Wang et al., 2004; Virtanen et al., 1997), aluminum and aluminum containing alloys (Aldykiewicz et al., 1995; Amelinckx et al., 2006; Arnott et al., 1989; Davenport et al., 1991; Mansfeld et al., 1989,1991, 1995; Pardo et al., 2006; Zheludkevich et al., 2006; Di Maggio et al., 1997; Lukanova et al.. 2008), stainless steels (Breslin et al., 1997; Lu & Ives, 1993, 1995), magnesium containing alloys (Arenas et al., 2002; Liu et al., 2001) even SiC/Al metal matrix composites. All these can be used to reduce the rate of general corrosion, pitting and crevice corrosion as well as stress corrosion (Breslin et al., 1997; Lu & Ives, 1993, 1995). The oxide films of rare-earth elements and refractory compounds can be formed mostly by means of chemical or electrochemical methods (Amelinckx et al., 2006a, 2006b; Avramova et al., 2005; Balasubramanian et al., 1999; Di Maggio et al., 1997; Marinova et al., 2006; Montemor et al., 2001, 2002; Schmidt et al., 1997; Stefanov et al., 2000a, 2000b, 2004a, 2004b; Stoychev et al., 2000, 2003, 2004; Tsanev et al., 2008; Tyuliev et al., 2002; Valov et al., 2002; Zheludkevich et al., 2005). It is supposed that cerium oxide/ hydroxide formation is the main reason for the corrosion protection property of cerium compounds. In spite of the growing number of investigations during the last years, focused on the mechanisms via which the oxides of rare-earth metals (mainly cerium oxides) lead to improvement of the corrosion stability of the systems "oxide(s)/protected metal", still a series of issues remain problematic. The first hypotheses in this respect have been put forward by Hinton (Hinton & Wilson, 1989; Hinton, 1992). He supposed in his early works that the cathodic reactions (reduction of oxygen and evolution of hydrogen) lead to alkalization of the near-to-the-electrode layer, which in its turn results in precipitation of the oxide of the rare-earth element, respectively in formation of protective film on the electrode surface.

The modern technologies for surface treatment, aimed at modifying the surface composition and structure of metals and alloys, including stainless steels, are becoming more and more important instruments for improving their stability to corrosion and for attributing the desired outside appearance and/or functional properties. Wang and coworkers (Wang et al., 2004) have studied the corrosion resistance of stainless steel SS304 after immersion treatment in electrolytes, containing Ce3+ ions, КМnO4 and sulfuric acid. The obtained experimental results prove the considerable increase in the corrosion stability of steel in 3.5% NaCl solution. The corrosion potential of the steel, treated by immersion, has more positive values than that of the non-treated steel, while the potential of pitting formation is also shifted in the positive direction, which is the criterion for weakening the tendency of the studied steel to undergo pitting formation. It has also been observed that the values of the current of complete passivation are decreased by one order of magnitude. As far as the cathodic reaction is concerned, the cerium conversion coatings blocked the matrix steel, which caused the reduction of the oxygen and protons to take place at a higher overpotential and the cathodic reaction was inhibited. The analysis of the chemical state of cerium in the conversion film indicated that the prevailing amount of cerium is in trivalent state. Aldykiewicz and coworkers (Aldykiewicz et al, 1996) have investigated the influence

the preparation and of the corrosion behavior of these coatings are not quite clear yet and their practical utilization is still uncertain. In order to find an attractive alternative to Cr6+ conversion coating, several treatment procedures that should manifest both efficient anticorrosive behavior as well as an optimal benefit/cost ratio, and mainly insignificant environmental impact, have yet to be developed. It has been found out that cerium species can be successfully applied to protect zinc from corrosion (Aramaki, 2001, 2002; Arenas et al., 2003, 2004; Ferreira et al., 2004; Otero et al., 1996, 1998; Wang et al., 2004; Virtanen et al., 1997), aluminum and aluminum containing alloys (Aldykiewicz et al., 1995; Amelinckx et al., 2006; Arnott et al., 1989; Davenport et al., 1991; Mansfeld et al., 1989,1991, 1995; Pardo et al., 2006; Zheludkevich et al., 2006; Di Maggio et al., 1997; Lukanova et al.. 2008), stainless steels (Breslin et al., 1997; Lu & Ives, 1993, 1995), magnesium containing alloys (Arenas et al., 2002; Liu et al., 2001) even SiC/Al metal matrix composites. All these can be used to reduce the rate of general corrosion, pitting and crevice corrosion as well as stress corrosion (Breslin et al., 1997; Lu & Ives, 1993, 1995). The oxide films of rare-earth elements and refractory compounds can be formed mostly by means of chemical or electrochemical methods (Amelinckx et al., 2006a, 2006b; Avramova et al., 2005; Balasubramanian et al., 1999; Di Maggio et al., 1997; Marinova et al., 2006; Montemor et al., 2001, 2002; Schmidt et al., 1997; Stefanov et al., 2000a, 2000b, 2004a, 2004b; Stoychev et al., 2000, 2003, 2004; Tsanev et al., 2008; Tyuliev et al., 2002; Valov et al., 2002; Zheludkevich et al., 2005). It is supposed that cerium oxide/ hydroxide formation is the main reason for the corrosion protection property of cerium compounds. In spite of the growing number of investigations during the last years, focused on the mechanisms via which the oxides of rare-earth metals (mainly cerium oxides) lead to improvement of the corrosion stability of the systems "oxide(s)/protected metal", still a series of issues remain problematic. The first hypotheses in this respect have been put forward by Hinton (Hinton & Wilson, 1989; Hinton, 1992). He supposed in his early works that the cathodic reactions (reduction of oxygen and evolution of hydrogen) lead to alkalization of the near-to-the-electrode layer, which in its turn results in precipitation of the oxide of the rare-earth element, respectively in formation of protective

The modern technologies for surface treatment, aimed at modifying the surface composition and structure of metals and alloys, including stainless steels, are becoming more and more important instruments for improving their stability to corrosion and for attributing the desired outside appearance and/or functional properties. Wang and coworkers (Wang et al., 2004) have studied the corrosion resistance of stainless steel SS304 after immersion treatment in electrolytes, containing Ce3+ ions, КМnO4 and sulfuric acid. The obtained experimental results prove the considerable increase in the corrosion stability of steel in 3.5% NaCl solution. The corrosion potential of the steel, treated by immersion, has more positive values than that of the non-treated steel, while the potential of pitting formation is also shifted in the positive direction, which is the criterion for weakening the tendency of the studied steel to undergo pitting formation. It has also been observed that the values of the current of complete passivation are decreased by one order of magnitude. As far as the cathodic reaction is concerned, the cerium conversion coatings blocked the matrix steel, which caused the reduction of the oxygen and protons to take place at a higher overpotential and the cathodic reaction was inhibited. The analysis of the chemical state of cerium in the conversion film indicated that the prevailing amount of cerium is in trivalent state. Aldykiewicz and coworkers (Aldykiewicz et al, 1996) have investigated the influence

film on the electrode surface.

of cerium oxide, deposited on aluminum alloys and they ascertained that the trivalent cerium is oxidized to tetravalent by the oxygen dissolved in the electrolyte, which leads to precipitation/formation of non-soluble CeO2 on the cathodic sections of the electrode surface. Montemor and coworkers (Montemor et al., 2001, 2002) studied the effect of the composition of the electrolytes (based on Ce(NO3)3 and the regime of preparing conversion layers on galvanized (zinc coated) steel. The increase in the thickness of the cerium conversion films in the process of formation leads to their enrichment in Ce4+. According to the same authors the conversion films, formed in La(NO3)3, are more efficient in view of anticorrosion protection, compared to those formed in electrolytes, containing Ce(NO3)3 and Y(NO3)3 (Montemor et al, 2002). The mechanism, involved in such a process of reducing the corrosion rate of the substrate, may be related to precipitation of cerium oxides and hydroxides in the vicinity of the anodic areas. These precipitates reduce the cathodic activity and hinder the transfer of electrons from the anodic to the cathodic spots.

The mechanism of zinc corrosion inhibition, when zinc is treated in solutions of Ce(NO)3, has been studied by Aramaki (Aramaki, 2001a, 2001b, 2002a; 2002b). He established the formation of hydrated or hydroxylated Ce-rich layer. This process, in its turn, leads to the formation of Ce2O3 on the electrode/the protected surface, respectively to inhibition of the cathodic reactions of the corrosion process in solutions of NaCl. Тhe work by Lu and Ives (Lu & Ives, 1995) has been extended further to study the effect of cerium salt solution treatment. Rotating disk assemblies were employed to monitor the cathodic electrode process and its inhibition by cerium salt treatment on austenitic stainless steels in a solution simulating sea water. The reduction of oxygen and hydrogen cations on both kinds of non-treated steels has been shown to be controlled by mass transfer processes in the solution. Cerium treatment effectively inhibits the cathodic reduction of oxygen, which is controlled primarily by charge transfer on the electrode. The over-potential for cathodic reduction of hydrogen cations is increased after the cerium treatment and the electrode reaction is controlled both by the mass transfer process in solution and by the charge transfer on the electrode. As a result of inhibition of the electrode processes, cerium improves the localized corrosion resistance, and in particular the crevice corrosion resistance, of stainless steels.

The electrochemical behavior of stainless steels - SS304 and 316L, following various cerium and cerium/molybdenum prereatment steps, was studied aiming at gaining more information on the process, by which cerium and molybdenum can modify the properties of passive film formed on stainless steels (Breslin et al, 1997). The coatings were analyzed by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy in order to identify the cerium species, which play the main role in the promotion of the passivation behavior. The pre-treatment step, denoted as Ce(CH3CO2)3 and CeCl3, involved immersion treatment of the electrodes in the Ce(CH3CO2)3 solution for 1 h and then in the CeCl3 solution for one additional hour at approximately 92○C. Regardless of the nature of the cerium salt, no changes in the rate of the cathodic reduction reaction could be observed. The increase in the corrosion potential (Ecorr). is mainly due to the decrease in the passive current density, suggesting that treatment in cerium solutions does not affect the rate of the cathodic reaction, but rather reduces the rate of the passive film dissolution. However a significant lowering of the oxygen reduction current could be observed, following the electrodeposition of small amount of cerium onto the electrode surface. Thus, it appears that efficient

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 243

Ce in the oxide films has been studied in regard to the corrosion potential of the steel in the same corrosion medium. Thereupon it was found out that the increase in the surface concentration of Ce in the oxide films results in a gradual shift of the corrosion potential of steel in the positive direction - from the zone characteristic of anodic dissolution to the zone of deep passivity – defined by the anodic potentiodynamic curve. Moreover it has been proved that there occurs a cathodic reaction of reduction of the electrochemically active

The present work discusses a hypothesis, aimed at elucidation of the question: how the change in the surface concentration of Ce in the mixed Ce2O3-CeO2 oxide film electrodeposited on ОС 404 steel influences the processes of anodic passivity of the studied steel, respectively the values of the potentials of complete passivation and the potentials of pitting formation, as well as the current density in passive state, determining the corrosion behavior of the steel under consideration. As far as the oxide of Се3+ (i.e. Ce2O3) is chemically unstable and it is dissolved in sulfuric acid medium (Achmetov, 1988) the investigations carried out in ref.(Guergova et al, 2011) established an effective inhibitory action of the cerium ions (Ce3+, Ce4+), passing over from the system Ce2O3-CeO2/SS into the corrosion medium. A possible inhibitory interaction has been supposed to occur on the

The stainless steel (SS) samples (SS type OC 404 containing 20% Cr, 5.0% Al, 0.02% C, the rest being Fe) were 10x10 mm plates of steel foil, 50 m thick. The deposition of the films was carried out in a working electrolyte consisting of absolute ethanol saturated with 2.3 M LiCl and 0.3 M CeCl3x7H2O salts. The cathodic deposition was performed in a galvanostatic regime at current density of 0.1mA.cm-2. The deposition time interval was 60 min. Platinum coated titanium mesh was used as counter electrode (anode). It was situated symmetrically around the working electrode and its surface was chosen specially to ensure a low anode polarization, which hindered Cl⎯ oxidation. Because of the relatively low equivalent conductance of the working electrolyte ( - 1.10-2 -1 cm-1), it becomes warmed up during the electrolysis. For this reason, the electrochemical measurements were carried out in a specially constructed electrochemical cell. The cell was kept at a constant temperature of 5–7oC by circulation of cooling water. The obtained СеО2-Ce2O3 coatings had a thickness of 1m (Avramova et al., 2005; Stefanov et al., 2004). The system СеО2-Ce2O3/SS was investigated prior to and after thermal treatment (t.t.) at 450C for 2 h in air. The model aggressive solution (0.1N H2SO4) was prepared by dilution of analytical grade 98% H2SO4 ("Merck") with distilled water. In order to evaluate the inhibitory effect of lanthanide salt, variable concentrations of Ce(SO4)2.4H2O from 0.1 to

The morphology and structure of the samples was examined by scanning electron microscopy using a JEOL JSM 6390 electron microscope (Japan) equipped with ultrahigh resolution scanning system (ASID-3D) in regimes of secondary electron image (SEI) and

back scattered electrons (BEC) image. The pressure was of the order of 10-4 Pa.

CeO2 – one of the components of the electrodeposited mixed Ce2O3-CeO2 film.

**2.1 Specimen preparation and structure characterization** 

surface of the steel.

**2. Experimental** 

1500 ppm were added to 0.1N H2SO4.

formation of cerium hydroxide/oxide does not occur upon immersion of electrodes at elevated temperatures in cerium solutions. It was possible to observe a yellow colored film indicative of cerium in the 4+ oxidation state on the surface of the stainless steel, following a 24-h immersion time interval in the Ce(NO)3 solution at room temperature. Breslin and coworkers (Breslin et al, 1997) proved that the treatment of SS304 in cerium-salt solutions gave rise to an increase in the value of the pitting potential Epit, with the greatest increase resulting from immersion in CeCl3 at 90-95oC for 30 min, followed by immersion in Ce(NO)3 solution at 90-95oC for additional 60 min time interval. The enhanced resistance to the onset of pitting, according to these authors, could be due to the dissolution of surface MnS inclusions during the immersion in the chloride-containing solution and possibly chromium enrichment of the passive film during treatment in the sodium nitrate solution, which is highly oxidizing. The presence of cerium in the solution seemed to have only a minor effect on Epit. The survey of the various mechanisms, proposed in the current literature, indicates that the role of rare-earth elements as inhibitors of corrosion and as protective coatings is not completely elucidated. It is accepted that their presence leads to improvement of the corrosion stability of metals and alloys and therefore they are a promising alternative, in conformity with the requirements for protection of the environment prohibiting the conventional Cr6+ conversion treatment.

At the same time it is known that thin films of Ce2O3-CeO2, have also an important functional designation for the manufacture of catalytic converters, where ceria is widely used in such kind of catalytic processes as a reducible oxide support material in emission control catalysis for the purification of exhaust gases from various combustion systems (Trovarelli, 1996). In the so called "three-way automotive catalysis", for example, the reducibility of ceria contributes to oxygen storage/release capability, which plays an important role in the oxidation of CO and hydrocarbons catalyzed on the surface of precious metal particles (Bunluesin et al, 1997). It is because of their specific interactions with oxygen that the cerium oxides are included in the support layers (Al2O3, ZrO2, etc.) of the proper catalytically active components of the converters (noble metals like Pt, Ro, Pd and others) and they participate directly in the decontamination of exhaust gases (reduction of NOx, oxidation of СО and hydrocarbons, etc.) originating from internal combustion engines (Mcnamara, 2000). In this connection it is important to point out that during the process of operation the main construction elements of the catalytic converters, which are made of stainless steel (Lox et al, 1995; Nonnenmann, 1989) (for example steel ОС 404), are subjected simultaneously to over-heating and at the same time to the aggressive action of the nitrogen oxides, being liberated in the course of the processes of combustion, of sulfur oxides, of water vapor and incompletely oxidized hydrocarbons etc. (respectively resulting in formation of HNO3, H2SO4 etc). In this respect and in the light of the data available in the literature about the protective action of the cerium oxides and hydroxides, it is essential to know what is the intimate mechanism of their anti-corrosion action and to what extent they could contribute, in particular, to the prolongation of the exploitation life-time of the catalytic converters, made of stainless steel.

Our studies on the protective effect of mixed Ce2O3-CeO2 films electrochemically deposited on stainless steel ОС 404 (SS) in model media of 0.1N HNO3 and 0.1N H2SO4 (Nikolova et al., 2006a, 2006b, 2008; Stoyanova et al., 2006a, 2006b, 2010), have shown that these films in their nature are in fact cathodic coatings. The influence of the change in the concentration of Ce in the oxide films has been studied in regard to the corrosion potential of the steel in the same corrosion medium. Thereupon it was found out that the increase in the surface concentration of Ce in the oxide films results in a gradual shift of the corrosion potential of steel in the positive direction - from the zone characteristic of anodic dissolution to the zone of deep passivity – defined by the anodic potentiodynamic curve. Moreover it has been proved that there occurs a cathodic reaction of reduction of the electrochemically active CeO2 – one of the components of the electrodeposited mixed Ce2O3-CeO2 film.

The present work discusses a hypothesis, aimed at elucidation of the question: how the change in the surface concentration of Ce in the mixed Ce2O3-CeO2 oxide film electrodeposited on ОС 404 steel influences the processes of anodic passivity of the studied steel, respectively the values of the potentials of complete passivation and the potentials of pitting formation, as well as the current density in passive state, determining the corrosion behavior of the steel under consideration. As far as the oxide of Се3+ (i.e. Ce2O3) is chemically unstable and it is dissolved in sulfuric acid medium (Achmetov, 1988) the investigations carried out in ref.(Guergova et al, 2011) established an effective inhibitory action of the cerium ions (Ce3+, Ce4+), passing over from the system Ce2O3-CeO2/SS into the corrosion medium. A possible inhibitory interaction has been supposed to occur on the surface of the steel.
