**2. Experimental**

242 Corrosion Resistance

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

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

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

conventional Cr6+ conversion treatment.

catalytic converters, made of stainless steel.

### **2.1 Specimen preparation and structure characterization**

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 1500 ppm were added to 0.1N H2SO4.

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.

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 245

region of potentials somewhere between the passivation potential (Еp) and the potential of transpassivity (Еt) (or the potential of pitting formation (Еpit)), in which the increase of the effectiveness of the cathodic process is leading not to enhancement but rather to abatement of the corrosion rate (for example in the course of the transition from one cathodic process К2 to another one К3) one can observe a system more stable to corrosion (easily passivating system). Obviously in this case of minimal corrosion currents there will appear a cross-point between the anodic and the cathodic curves of the corrosion diagram within the zone of stable passive state. Under these conditions it is quite possible that a smaller corrosion current is corresponding to a more efficient cathodic process in comparison to the system displaying a lower cathodic efficiency (if we exclude the conditions where the potential of the system is reaching the value of the potential of transpassivation and the potential of pitting formation - К4). Therefore one can conclude that during the occurring of an efficient cathodic process the system will pass over spontaneously to a stable passive state and it will be corroding at a much lower rate, corresponding to the current of complete passivation. The stationary corrosion potential of such a system will be more positive than the potential of complete passivation (Ecp) and at the same time more negative than the potential of break through the passive film and the potential of transpassivation. In this way the rate of corrosion can be reduced to a considerable extent by the correct use of the phenomenon

"passivation".

A

C

K1

i <sup>t</sup> i

<sup>K</sup> <sup>E</sup> <sup>4</sup> pit

D

Ecorr Et

Ecp

Ep

Eo a

Ecorr


cp i

complete passivation, pitting formation and corrosion in transpassive state.

pit

i k**3**

i p

B

K3

E

lg i

So, in order to create a system stable to corrosion and to decrease the rate of corrosion, it is necessary to find a way to promote the cathodic effectiveness (for example as it is in this specific case of investigations, carried out by us, to modify the steel surface with СеО2-Ce2O3

Fig. 1. Schematic polarization diagram explaining the action of the effective cathodic coatings on the steel corrosion: ip, icp., ipit, it- respectively currents of initial passivation,

K2
