L NaCl aqueous solution

JOURNAL OF RARE EARTHS, Vol. 33, No. 11, Nov. 2015, P. 1212

Self-healing effect of ceria electrodeposited thin films on stainless steel in aggressive 0.5 mol/L NaCl aqueous solution Desislava Guergova1, Emilia Stoyanova1, Dimitar Stoychev1,*, Ivalina Avramova2, Plamen Stefanov2 (1. “Rostislaw Kaischew” Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria; 2. Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria) Received 20 November 2014; revised 19 August 2015

Abstract: The self-healing effect of electrochemically deposited CeO2-Ce2O3 films on stainless steel OC404 (SS) in 0.5 mol/L NaCl solution was studied. It was established that the corrosion potential of the steel, after covering it with CeO2-Ce2O3 layer and thermal treatment (i.e. potential of the system CeO2-Ce2O3/SSt.t.), was shifted sharply to a considerably more positive value, while the corrosion current was reduced noticeably. The X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM) analyses on the observed scratched surface area of the system CeO2-Ce2O3/SSt.t., after exposure of investigated specimens to 0.5 mol/L NaCl corrosion media, showed partial accumulation of ceria, as well as remarkable increase in the concentrations of oxides of Al, Cr and Fe, on the mechanically revealed steel surface. On the basis of the obtained results one could conclude that the secondary passive oxide/hydroxide films, formed after a definite time interval of exposure to corrosion media, acted as barriers, hindering the corrosion processes in active zones. A hypothesis was put forward about the mechanism of self-repairing oxide layers on the steel surface and their corrosion protection effect respectively. Keywords: stainless steel; ceria films; corrosion; self-healing effect; rare earths

The corrosion protection of metal substrates depends on the existence of naturally formed surface layers or on the presence of thin films, obtained artificially by various methods. The main function of the anticorrosion coating is to protect the metal surface by forming an effective barrier against corrosive reagents present in different kinds of environments. The corrosion of metallic substrate is taking place when oxygen is being transported through the pores and cracks to the metal-coating interface. The effect of the blocking of active sites by “self-healing” process provides an attractive option to prevent the corrosion of metals. It is known that thin films of oxides of rare-earth elements possess good self-healing properties[1,2]. In this connection electrochemical methods have been developed to obtain cerium-containing oxide films from aqueous and non-aqueous solutions, such as cathodic electrodeposition and conversion coatings in particular and these have attracted considerable attention during past years. The appearance of defects/cracks in the coating is promoting the diffusion rate of aggressive electrolyte components towards the substrate thus decreasing the protective ability of the coatings[3–7]. Živković et al.[8] concluded that the corrosion stability and the formation the crack-free coatings depend on the time interval and on the applied potential. Therefore it becomes important

to develop some new protective systems and procedures to prevent the corrosion in active zones, and to reinforce the protective performance of rare earth element oxide or hydroxide films. At the same time the elucidation of the mechanism of protective influence of the cerium containing oxide films is very important. There are studies, in which the object to be protected is Al or its alloys, and it is reported that the observed corrosion protection is owing to the active protector defense, analogous to those of the chromate films, which are replaced in this case by the cerium oxides[4,5,9–19]. The authors are claiming that the protective effect of the formed oxide/hydroxide films of rare-earth elements is in direct connection with the cathodic processes, occurring on the electrode surface in media containing chloride ions. The cathodic reaction of oxygen reduction in the course of the corrosion process appears to be the dominating reaction in a series of cases, as its rate is determining the rate of corrosion. As reported by Klapper and Goellner the electrochemical reduction of oxygen in neutral and alkaline solutions is the process determining the formation of the ОН– groups[20]. This is leading to alkalization of the space around the cathode, whereupon the process of precipitation of oxide film is facilitated on the active sections on the surface in the form of Ce(OH)3/Cе2О3, resulting in retardation of the corrosion process[21]. A num-

Foundation item: Project supported by the National Science Fund, Bulgaria (Т02-22/12.12.2014) * Corresponding author: D. Stoychev (E-mail: [email protected]; Tel.: +3592 9792529) DOI: 10.1016/S1002-0721(14)60548-2

Desislava Guergova et al., Self-healing effect of ceria electrodeposited thin films on stainless steel in …

ber of authors are ascertaining that in case of prolonged exposure of the cerium oxide film to corrosive medium, its composition is changing, whereupon beside the oxides/hydroxides of Ce3+ it is being enriched also by oxides/hydroxides of Ce4+. Aldykiewicz and co-authors[11] proposed a mechanism of oxidation of Ce(III) into Ce(OH)22+ in the vicinity of the protected metal surface and consecutive precipitation of insoluble oxides of CeO2 on it. This effect is associated with the oxidation process, due to the oxygen dissolved in the medium[21–24]. These precipitates are creating a barrier either to supply oxygen (H+ in case of acidic medium) and/or to lower the cathodic activity and hinder the transfer of electrons from the anodic to the cathodic sites[11,25–27]. On the basis of the above mentioned film-forming properties of the lanthanides and in view of their protective performance some alternative methods have been proposed aimed at the formation of anti-corrosion coatings also on bronze[28,29], Mg[30–32], Ni[33], Zn[34–38] and SS[36,39–41]. Taking into consideration the results of numerous foreign studies and our own investigations, the authors draw the conclusion that in the case of stainless steels the mechanism and the rate of reduction of oxygen depend strongly on the chemical composition of the steel and on the state of the surface (respectively on the possibility for forming a passive film), the nature of the corrosion medium (pH factor, composition) and of course also on the corrosion conditions-stirring, temperature, polarization, etc[42]. Virtanen and co-authors[43] have established that the formation of protective cerium oxides film on the surface of stainless steels leads to increase in the content of chromium in the modified surface oxide film. This effect determines the enhancement of their corrosion stability and decreases the inclination of the studied systems to undergo local corrosion. Trabelsi et al.[1] investigated the electrochemical behaviour of galvanised steel pretreated with bis-[triethoxysilylpropyl] tetrasulfide silane, doped with cerium nitrate after immersion in 0.005 mol/L NaCl solution. The presence of cerium nitrate in the silane coating reduces the corrosion rate of the substrate. Kong and co-authors[44] investigated a complex film (Ce(NO3)3 and silane on hot-dip steel. The authors observed that this film, having a thickness of about 100 nm, reduced the rate of both the anodic and the cathodic reactions of the corrosion process and it had a better corrosion protection ability. The incorporation of CeO2 nanoparticles in silane coatings[45] is leading to the improvement in the barrier properties and stabilization of the passive film on electro-galvanized steel substrates via an anodic inhibition mechanism. The synergistic effect between UV-cure polyester acrylate resin with conducting polymer (PAni-PA) and ceria nanoparticle enhances the protective properties of carbon steel[46].

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The self-healing mechanism of cerium nitrate, mixed with silica in a silane polymer, was discussed by Aramaki and it has been shown that a film composed of Zn(OH)2, ZnSi2O5 and Ce3+/Si2O52– salt can recover the damaged areas[47]. According to the author the precipitation of silicate compounds in the defects of the coating prevents pitting corrosion. Recently the intensive and promising work gave great and new impetus to the design and development of electrochemical method for obtaining ceria-containing films using non-aqueous solutions on stainless steel that significantly improves its corrosion resistance in oxidative and non-oxidative media - solutions of nitric and sulfuric acids[48–51]. In another investigation it is also proved that the ability of cerium oxide films, acting as effective cathodic coatings, to restore the passive state of steel after being disturbed in 3.5% NaCl solution[52]. It was established that cerium ions act as corrosion inhibitors in the case of OC404 stainless steel exposure to non-oxidizing medium of 0.05 mol/L H2SO4 solution[53]. The aim of this investigation was to study the protective and self-healing abilities of electrodeposited CeO2-Ce2O3 films on stainless steel OC404 against corrosion in aerated 0.5 mol/L NaCl solution after the oxide films on the steel substrate were partially scratched before exposure of the system CeO2-Ce2O3/SSt.t. to the investigated aggressive media.

1 Experimental The stainless steel samples (Sandvik OC404 containing 20% Cr, 5.0% Al, 0.02% C, the rest being Fe) were 20 mm×20 mm square plates of steel foil, 50 µm thick. The deposition of the films was carried out in a working electrolyte, consisting of absolute ethanol saturated with 2.3 mol/L LiCl and 0.3 mol/L CeCl3·7H2O salts (Alfa Aesar, 99% purity) as described elsewhere[51]. The deposition time interval was 40 min. The obtained Ce2O3CeO2 layers had thickness of 700 to 1000 nm. The system Ce2O3-CeO2/SS was investigated after thermal treatment (t.t.) at 450 ºC for 2 h in air. The self-healing abilities of CeO2-Ce2O3 film against corrosion of SS were investigated before and after the scratching (by a special knife) spots on the samples surface. The size of the scratched area was 10 mm×10 mm. After scratching the relationship between the revealed surface and the CeO2-Ce2O3 covered surface areas of the investigated samples was approximately 1:7. The dimensions of the open surface sections and covered surface sections of the samples enabled following correctly the changes by SEM, XPS, EDS-observing their surface morphology, the chemical composition, the valence state of the elements and the corrosion damages in the studied sections both prior to as well as after their exposure to the corrosive medium.

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The potentiodynamic polarization curves for stainless steel samples and for the CeO2-Ce2O3/SSt.t. systems in 0.5 mol/L NaCl solution (“p.a.” Merck with distilled water) were recorded using standard three electrode thermostatted cell (100 mL) assembly. A counter electrode, representing a platinum plate, and a calomel (EHg/Hg2Cl2= +0.240 V vs. SHE) reference electrode (SCE), were used. All the potentials listed here are related to SCE. The anodic and cathodic potentiodynamic polarization curves were obtained using an Electrochemical Measurement System-Gamry potentiostat/galvanostat, with a potential sweeping rate of 0.16 mV/s. The recording of the potentiodynamic curves was carried out starting from the open circuit potential (Eocp) measured in the absence of external current in the anode and cathode directions. Individual samples for each recorded curve were used. The open circuit potential of the samples under investigation was determined by direct measurement of the function “Eocp-τ” with respect to the same reference electrode after immersion in 0.5 mol/L NaCl solution as is described[51]. The chemical composition and the state of the elements before and after the partial scratching of the film were monitored inside and outside of the scratched section of the surface using X-ray photoelectron spectroscopy (XPS) as is shown[51]. X-ray powder diffraction patterns for phase identification were recorded in the angle interval 15º– 80º (2θ), on a Philips PW 1050 diffractometer, equipped with Cu Kα tube, scintillation detector and monochromator in the diffracted beam. The morphology and structure of the samples were 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 electron microscope is equipped with an energy-dispersion spectrometer Oxford Instruments INCA x-sight, which enabled carrying out EDХ micro-probe X-ray analyses of the studied samples in a fixed spot, along a line and/or in mapping regime. The presence of Се3+ ions in the corrosive medium after the exposure (for various time intervals) of the studied samples, has been registered by a double-beam spectrometer (Thermo Evolution 300 UV-Vis spectrophotometer, equipped with a Praying Mantis device), using quartz cuvettes with thickness of 1 cm. Aiming at observing and detecting possible influence by the components of the corrosion medium (NaCl solution) on the spot, the shape and the intensity of the absorption peaks of the Се3+ ions, we recorded in advance the spectra of the standard solutions (prepared with distilled water and with 0.5 mol/L NaCl), containing 5, 10 and 100 ppm Се3+ ions, comparing the data with those of a cuvette,

containing only distilled water or 0.5 mol/L NaCl solution. After this type of “calibration” of the experimental run, the working cuvette was washed thoroughly and 3 mL of the corrosive medium was added to it – the same medium, to which the studied scratched samples were exposed (for a determined time interval). During the spectrophotometric recording of these solutions the reference cuvette contained 0.5 mol/L NaCl solution. After the measurements, the investigated solution was returned back to the appropriate beaker. The changes in the pH value of 0.5 mol/L NaCl aggressive media, in which the investigated specimens were exposed for different time intervals, were measured by a standard calibrated pH meter-portable ADWA (Model AD1030) with relative accuracy of ±0.01. Triplicate measurements were made with each system and the respective average value is given here.

2 Results and discussion After preparation of the films and prior to their mechanical scratching, the samples were subjected to thermal treatment (at 450 ºС in air atmosphere for 2 h) – an operation, which is applied in a series of cases, for example in the manufacture of catalytic converters[54]. In order to find out what is the nature of the protective action of cerium-oxide layers and the mechanism of the inhibitory effect of the cerium ions appearing in the corrosion medium (in case of corrosion process of the system CeO2-Ce2O3/SSt.t.), a set of model investigations were carried out with samples, in which the protective layer of CеО2-Ce2O3 on the steel substrate had been partially removed by mechanical scratching (Fig. 1). The structural, phase and chemical transformations occurring with the course of time of exposure to the corrosive medium on the surface of the so treated samples were studied inside and outside of the scratched area, as well as the changes in the corrosion parameters of the system CeO2-Ce2O3/ SSt.t..

Fig. 1 SEM microphotograph of a spot of scratched area next to non-scratched area illustrating the boundary between them prior to exposure to 0.5 mol/L NaCl solution

Desislava Guergova et al., Self-healing effect of ceria electrodeposited thin films on stainless steel in …

In our previous investigations, devoted to the obtaining of functional layers of Ce2O3-CеО2, it has been found that the process of their electrochemical deposition on the surface of stainless steel is associated with the formation of oxides of cerium in two valence states (Ce3+ (~60%) and Ce4+ (~40%))[55–58]. After heating these layers in air atmosphere (for 2 h at 450 ºC) practically complete oxidation of Ce3+ into Ce4+ is achieved, as a result of which the concentration of CeO2 on the surface of the studied samples reaches a value of 95%. It is also important to note that in the course of analogous thermal treatment of the stainless steel used as support the continuity of its native protective film is interrupted due to cracking. The appearance of bare sections on the steel surface as a consequence of the cracking is a prerequisite for the appearance of micro-galvanic couples “oxide film/bare steel surface”, which is leading to an increase in the aptitude of the system to undergo pitting corrosion[50,57]. These two circumstances were taken into account in the course of studying the nature of the processes of self-passivation, respectively “self-healing”, of the studied steel sample. Obviously, besides the selfhealing in the zone of the electrochemically deposited cerium oxide protective layer, which is removed by scratching, the self-healing will occur also in the zones of cracking of the natural passive film (under the layer of CеО2-Ce2O3) of the steel. Moreover we have to take into consideration that the mixed oxides CeO2-Ce2O3 possess different chemical stability, depending on the pH value of the corrosion medium – these are circumstances which hamper substantially the elucidation of the mechanism of their protective action on the steel surface[59]. Similar effect was observed also in Ref. [60] with the systems CeO2/AA6082 and CeO2/TiO2/SS304, obtained either by RF magnetron sputtering or sol-gel deposition[61]. 2.1 Chronopotentiometric studies Fig. 2(а) represents typical chronopotentiometric curves for the thermally treated samples of stainless steel

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(curve (1)) and for the system CeO2-Ce2O3/SSt.t. without (curve (2)) and with partially mechanically removed (curve (3)) protective layer of CeO2-Ce2O3, obtained in the course of 500 h of exposure to 0.5 mol/L NaCl solution. For comparison Fig. 2(b) represents the analogous dependences (obtained for 200 h of exposure to 0.5 mol/L NaCl solution) for samples of non-thermally treated and not covered with a layer of CeO2-Ce2O3 steel, as well as for non-thermally treated metals Fe, Cr and Al, which are the basic components of OC404 SS. It is seen in Fig. 2(b) that during the first 200 h of exposure to the corrosive medium, Еocp of the non-thermally treated steel is not changing substantially and it has a value of ~–0.100 V. In the case of thermally treated steel (Fig. 2(a), curve (1)) the initial value of Еocp is ~–0.250 V. During the first 5 h Еocp shifts strongly in positive direction reaching a value of ~–0.075 V and thereafter it shifted again in negative direction and after 500 h of exposure to 0.5 mol/L NaCl solution it becomes ~–0.450 V – a value close to Еocp of pure Fe (~–0.620 V) in this medium. The juxtaposition of this result with the course of the curve, illustrating the variation of Еocp of the non- treated thermally SS (Fig. 2(b)), is indicative of the disruption of the natural passive protective film on the steel as a result of its thermal treatment, which has also been established in our previous studies, including also aggressive media[50,52,57]. The strong shifting in the Еocp of SSt.t. during the first 5 h can be connected with the processes of formation of films of corrosion products, which in the initial stages of corrosion protect partially the steel surface. Applying longer time intervals of exposure to the corrosive medium, this film is probably being dissolved and its protective action becomes lower, whereupon after ~150 h it practically disappears and the process of corrosion starts to dominate: it is acting basically on the iron component of the steel, i.e. it becomes possible that the reaction Fe–2e–→Fe2+ is occurring in the active anodic sections, which is leading to local corrosion damages on the steel surface.

Fig. 2 Open circuit potential (Eocp) changes vs. time for SSt.t. (curve (1)) and the system CeO2-Ce2O3/SSt.t. without (curve (2)) and with (curve (3)) scratched area after exposure to 0.5 mol/L NaCl solution (a) and the same dependence for non-thermally treated SS, as well as for the basic components of the SS OC404 (b)

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The presence of electrodeposited cerium oxides on the steel surface determines the establishment of more positive value of Еocp at the moment of the immersion (Еocp= –0.150 V) of the sample in the aggressive medium (Fig. 2(а), curve (2)), in comparison to non-covered SSt.t.. Within the range of exposure time intervals to 0.5 mol/L NaCl solution 1–100 h Еocp sharply shifts in negative direction reaching values of ~–0.325 V and thereafter (within the range 100–200 h) it again shifts in the positive direction and it becomes ~–0.045 V. Up to 500 h of exposure time interval this “ennobling” influence of Еocp continues to rise up, although much slower, reaching value of Еocp~ +0.070 V. This value is more positive with ~0.520 V than the value of Еocp of the non-covered with cerium oxides coating SSt.t. and with some ~0.700 V compared to Еocp of pure Fe. These results give evidence that the presence of cerium oxide film influences substantially the kinetics of the corrosion process of steel. It is obvious that the cathodic reaction of reduction of oxygen is occurring mainly on the oxide film, whose surface is significantly larger in comparison to the anodic sections (Fe and Al) of the steel. As is known, the shifting of Еocp in positive direction and the decrease in the corrosion current, as well as the changes in the chemical composition and in the state of the elements of the surface films of the system undergoing corrosion, represent the basic criteria for the occurrence of the processes of “self-healing”. In this connection the impression is that we can follow the course of variation of Еocp for SSt.t. (Fig. 2(a), curve (1)) and for the system CeO2-Ce2O3/SSt.t. (Fig. 2(а), curve (2)). Within the range of time intervals of exposure 1–150 h the course of the curves is just the opposite of inoculation (“ennoblement”) of the Еocp. The variations in the values for SSt.t. are indication of the presence of an effect of self-healing within the range of time intervals 10–15 h and thereafter it is lowering considerably and after ~150 h it disappears completely, whereupon with the course of time in case of continuing further the exposure to 0.5 mol/L NaCl solution. It is approaching the value of Еocp, which is characteristic of pure Fe (Fig. 2(b), curve Fe). Almost symmetrical but opposite effect is observed within this time range, when a protective coating of cerium oxides is deposited on steel (Fig. 2(а), curve (2)). At the moment of immersion of the sample into 0.5 mol/L NaCl solution the value of Еocp is ~–0.150 V (with ~0.100 V more positive than Еocp of SSt.t. at the moment of immersion), and afterwards until the 15th hour it is sharply decreased to ~–0.330 V. Until the 100th hour of exposure of the samples to 0.5 mol/L NaCl solution, the value of Еocp of the system CeO2- Ce2O3/SSt.t. continues to go down more slowly reaching even more negative value of Еocp~–0.350 V. The presence of a layer of cerium oxides on the steel surface in the course of

relatively longer time interval not only gives no improvement in the corrosion behavior of the steel, but it is even deteriorated. Only after the 100th hour the Еocp starts steeply to shift in the positive direction reaching the value ~–0.040 V after 200 h of exposure, respectively ~+0.070 V after 500 h of exposure to 0.5 mol/L NaCl solution. So it can be supposed that the effect of self-healing in case of the system CeO2-Ce2O3/ SSt.t. occurs after ~100 h of exposure and thereafter it becomes stronger with the time of the consecutive exposure to the corrosive medium. This effect differs strongly from the behavior established for non-covered with CeO2-Ce2O3 thermally treated steel, for which the self-healing occurs until about the 100th hour and further it disappears practically completely as a consequence of the dissolution of the formed surface films, probably due to the presence of main chloride compounds of Fe, Cr and Al. The “negative” influence, from the view point of corrosion, of the cerium oxide film, deposited on the steel surface after ~150 h of exposure to 0.5 mol/L NaCl solution can be attributed to the action of the galvanic couples CeO2-Ce2O3/Fe(Cr,Al), whose electro-driving force is higher than 1 V. In our opinion, as a result of the thermal treatment of the steel, the efficient operation of these couples is facilitated due to the appearance of pores and cracks in the native passive film, through which the contact between the cerium oxide film and the metal components of the alloy is being accomplished. After 150–500 h of exposure to the corrosive medium, as noted above, the Еocp of the system CeO2-Ce2O3/SSt.t. reaches much more positive values in comparison to Еocp for SSt.t.. The comparison of the above listed results gives evidence for the occurring of a process of self-healing in the disrupted sections (as a result of the thermal treatment) of the natural passive film on the steel surface, which is determined by the presence of the electrodeposited CeO2Ce2O3 film on it. 2.2 Potentiodynamic polarization curves Aiming at elucidation of the above made conclusion, we carried out model potentiodynamic polarization studies with samples, which have been subjected to 200 h of exposure (at open circuit), prior to recording the potentiodynamic curves, to 0.5 mol/L NaCl solution. The obtained results are represented in Fig. 3. It is seen in the figure that the thermal treatment of the steel samples leads to definite shifting of its Ecor in negative direction (up to ~–0.185 V as shown in Fig. 3(1)), registered also in previous studies[52]. The investigations with the SSt.t. samples, which have some additionally scratched mechanically surface areas, ascertained that their exposure to the aggressive medium determines even stronger shifting of the Ecor of the steel in the negative direction

Desislava Guergova et al., Self-healing effect of ceria electrodeposited thin films on stainless steel in …

Fig. 3 Potentiodynamic curves for the samples: SSt.t. (1) Without scratched surface (curve 1); (2) SSt.t. with scratched part of the surface; (3) The system CeO2-Ce2O3/SSt.t. with scratched part of the surface after 200 h of exposure to 0.5 mol/L NaCl solution

(up to ~–0.250 V, Fig. 3(2)), as well as the enhancement of the corrosion current to values of ~2×10–7 А/сm2. Analogous studies with the system CeO2-Ce2O3/ SSt.t. (i.e. when a protective layer of CeO2-Ce2O3 is deposited on the steel samples, certain parts of which have been removed) have shown that the values for Ecor of the system are strongly shifted in the positive direction (up to ~–0.040 V), while icor is decreased with more than two orders of magnitude (Fig. 3(3)). Thereupon, in a comparatively wide range of potentials (0–0.130 V), a zone appears on the anodic curve, in which the studied CeO2-Ce2O3/SSt.t. system (respectively the steel) is in pseudo-passive state. The results of these studies confirm in a unique way the occurrence of a process of selfhealing in the disrupted sections (including the disruption as a result of the thermal treatment) of the natural passive film on the steel surface, which is determined by the presence of the CeO2-Ce2O3 film electrodeposited on it. Most probably this self-healing effect is connected with the internal anodic polarization, caused by the cerium oxides, (after 200 h of staying in the corrosion medium), which determines the oxidation state of the elements, included in the composition of the steel, respectively the secondary passivation of its surface. We should note here that in the studied corrosion medium no currents of complete passivation are observed (Fig. 3(1)), most probably because of the presence of relatively high concentration of Cl–, therefore availability of soluble chlorides. However, one can observe pseudo-passive behavior of the system CeO2-Ce2O3/SSt.t. with considerably lower currents of anodic dissolution within a relatively wide range of potentials. This result is an indication of the fact that the secondarily formed protective film is not compact. Aiming at more detailed elucidation of the processes of self-healing in the system CeO2-Ce2O3/SSt.t. we plotted chronopotentiometric “Eocp-τ” curves also for samples, in

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which the layer of CeO2-Ce2O3 had been scratched, i.e. comparatively large areas had been revealed (defined based on the area) – the so called “bare” zones of the steel substrate. The typical course of the variation of the Eocp for such type of samples is shown in Fig. 2(а), curve (3). It is seen in the curve that at the moment of immersion of the sample into 0.5 mol/L NaCl solution the value of Eocp is ~–0.315 V. In the course of the next ~5 h of exposure Eocp shifts sharply with 0.200 V in the positive direction and thereafter in the range of 5–150 h of exposure oscillations of Eocp are being registered, which is well known when processes of self-healing are occurring, having amplitude of ~0.150 V. Within the range of 150– 400 h of exposure the oscillations are continuing, whereupon their amplitude is decreasing, while after 400 h the oscillation changes of Eocp are being transformed into gradual shifting of the potential in positive direction. This latter fact is the criterion for occurrence of processes of “self-healing”, as a consequence of the formation of relatively compact and stable protective film both on the “scratched” sections of the steel surface, as well as in the zones of micro-cracks (of the native film) over the entire surface of the studied CeO2-Ce2O3/SSt.t. samples. In our opinion, the registered sinusoidal variations of the Еocp and the thereafter established compromised (mixed), corrosion potentials after 150–200 h (Fig. 2(а), curve (3)) for samples with mechanically removed cerium oxide layers (which are more negative in comparison with the samples having non-disrupted cerium oxide layers (Fig. 2(а), curve (2)) and more positive – in comparison with the non-covered with cerium oxides steel surface (Fig. 2(а), curve (1)) are most probably due to redох processes, occurring on the surface of the system CeO2- Ce2O3/SSt.t.. These are triggered off by an effective cathodic process (reduction of Се4+ to Се3+), determined by internal anodic polarization between the coated (with a layer of CeO2-Ce2O3) and the bare sections of the steel surface[40]. The redох processes, occurrence on the surface of the samples, studied by us, can be illustrated schematically by Fig. 4. This scheme is based on the following concepts. As a result of the occurrence of internal anodic processes in the zone of scratching/removal of the cerium oxides layer, the basic metal components of the steel (Fe, Cr, Al) in the bare section will be corroded, whereupon thermodynamically possible are the following three groups of reactions[62,63]: dissolution of the metal, accompanied by the formation of hydrated metal ions, in accordance with the reaction: Me+xH2O=Men+xH2O+ze– (1) dissolution of the metal, accompanied by the formation of hydrated oxy-anions, according to the reaction: Me+zOH–=MeO2–z +zH++ze– (2) formation of solid hardly soluble oxides or hydroxides on

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Fig. 4 A scheme of the occurring redox processes on mechanically scratched system CeO2-Ce2O3/SSt.t., immersed in corrosive media

the surface of the metals, according to the reaction: Me+zOH–=Me(OH)z↓+ze– (3) If the latter reaction is dominating, on the bare surface of the steel a new solid phase will be formed/precipitated (hydroxides of the basic metal components of the steel), as a result of which a secondary protective film will be formed on its surface and this zone will pass over into passive state again. Thereupon the electrons, donated in the oxidation process, will migrate to the sections being most favorable for the depolarizing processes. As is known, in the corrosive medium studied by us (0.5 mol/L NaCl solution), the conjugated reaction of oxygen depolarization, is connected with the proceeding of reactions of ionization of the oxygen, whose products are ОН– and Н2О2[63]: I step O2+2H2O+2e–→H2O2+2OH– (4) II step H2O2+2e–→2OH– (5) Summing up the two reactions leads to: O2+2H2O+4e–→4OH– (6) Taking into account, however, the strong shift in positive direction corrosion potential of the system CeO2Ce2O3/SSt.t. and the increasing рН of the corrosive medium with the course of time of exposure of the studied samples to it (from 5.85 to 6.42 after 500 h of immersion), it is quite probable[25] that beside the occurrence of the reaction of oxygen depolarization, there proceeds also reaction of reduction of СеО2 (Се4+) to Се2О3 (Се3+) on the steel surface coated with СеО2 (Fig. 4, zone 1). The Се2О3 being formed thereupon, which is chemically soluble, will determine the liberation of Се3+ ions into the

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corrosive medium. During their diffusion they can reach the scratched steel surface (Fig. 4, zone 2). The supply of Се3+ ions to the “bare” sections (revealed as a consequence of the mechanical removal of cerium oxides protective layer and/or disruption of the passive film during the thermal treatment) on the steel surface enables thermodynamically the formation of hydroxides (Се(ОН)4), respectively oxides (CeO2·2Н2О) of cerium upon them, according to the corrected diagrams of Purbe[25]. It is also possible that the Н2О2, formed according to reaction (4), has also some contribution to the formation of Се(ОН)4 and CeO2·2Н2О. As it was shown by Aldykiewicz and coauthors[11] and Li and coauthors[64], in the presence of H2O2 the following reactions are occurring[11,64]: 2Ce3++H2O2+2OH–=2Ce(OH)2+ (7) 2 – 2Ce(OH)2+ (8) 2 +4OH =2CeO2+4H2O Besides this, the increase in the concentration of ОН–, according to reactions (4) and (5), will lead to substantial increase in рН of the corrosive medium in the cathodic sites and therefore to acceleration of the reactions (7) and (8). This increase in рН of the corrosive medium around the cathodic sites, without a doubt, should be taken into account also when considering the Purbe diagrams, respectively the thermodynamic probability for the formation of Се(ОН)4 and CeO2·2Н2О is growing up, as it has been noted above. Another possibility for the formation of Се(ОН)4 and CeO2·2Н2О in the presence of H2O2 has been reported in Ref. [65]. These authors suppose that the oxidation of Се3+ into Се4+ state can be accomplished according to the following reaction scheme: 2Ce(OH)3+3H2O2→2Ce(OH)3OOH+2H2O (9) Ce(OH)3OOH+2Ce(OH)3+H2O→3Ce(OH)4 (10) In a study, devoted to the conversion formation of cerium oxide on aluminum substrate, Decroly and Petitjean have shown and proved that these two mechanisms are possible both in acidic, as well as in alkaline medium[66]. According to the scheme, proposed by them, in the presence of H2O2 the oxidation of Al (the steel studied by us comprises also Fe and Cr) is occurring on the anodic sites. The non-insulating Al(OH)3 being formed thereupon (in our case also Fe(OH)2 or Fe(OH)3 and Cr(OH)3) gel layers allowing Faradeic processes is therefore necessary for CeO2·2H2O deposition to occur. Oxidation of Al on the anodic sites produces Al3+ (in our case also Fe3+ and Cr3+), while the reduction of H2O2 on the cathodic sites induces the pH that is necessary for the precipitation of Се(ОН)3, followed by oxidation into Се(ОН)4. Obviously, in the spontaneous process it is more probable that the electrons for the reduction reactions result from the dissolution of the metal(s) substrate. One cannot exclude also the reaction of decomposition of Н2О2, which is generating free electrons. It follows from the above discussion that as a result of the spontaneously occurring (on the anodic and on the

Desislava Guergova et al., Self-healing effect of ceria electrodeposited thin films on stainless steel in …

cathodic sites) conjugated oxidation-reduction processes on the bare steel surface oxides and/or hydroxides of Fe, Cr, Al and Ce will be formed, determining its self-healing. Thereupon their concentration (in the zone of scratching of the samples), during the time interval of exposure/ corrosion of the system CeO2-Ce2O3/SSt.t. in the corrosion medium, will be growing up. The reasons to suppose such a course and to put forward a hypothesis are given by the following experimentally established facts: (1) The presence of electrodeposited protective layer of cerium oxides on the steel surface shifts strongly its corrosion potential in positive direction. While for the non-coated with cerium oxides SSt.t. the Eocp is changing from ~–0.250 V (at the moment of its immersion into 0.5 mol/L NaCl solution) to more than –0.450 V after 500 h of exposure to the corrosive medium, for the system CeO2-Ce2O3/SSt.t. these values are respectively ~–0.150 V (at the moment of its immersion into 0.5 mol/L NaCl solution) and they become higher than ~+0.070 V after 500 h of exposure to the corrosive medium. Therefore the presence of a layer of cerium oxides on the steel surface determines the shifting of its corrosion potential with more than 0.500 V. (2) During the exposure of the studied samples to the model corrosion medium it was ascertained that its pH is changing substantially. The initial value of pH of the 0.5 mol/L NaCl solution is 5.85. After 200 h exposure of the non-coated with cerium oxide thermally treated steel samples it becomes lower and reaches the value of 5.53. An opposite effect, alkalization of the corrosive medium, was established during the exposure of the system CeO2-Ce2O3/SSt.t.. In this case, after 200 h, the рН is increased up to 6.18, while after 500 h it becomes 6.42. In this aspect it is justifiable to suppose that the value of рН in the cathodic sections is considerably higher. Aiming at verification and proving the correctness of the above assumptions the following studies, listed below, were carried out and the obtained results were discussed.

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tion of the signal, emitted by the surrounding the scratched surface undisrupted zone of the oxide CeO2Ce2O3 layer. The results from the XPS analyses inside the scratched zone showed that only 3 elements are registered in it (prior to the exposure to 0.5 mol/L NaCl solution, Fig. 5, 0 h), Fe, Cr and Al. Thereupon, the iron in this zone exists in the form of Fe0 (1.4 at.%) and oxides of Fe3+ (4.3 at.%), the chromium is in the form of metallic Cr (0.9 at.%) and oxides of Cr3+ (3.9 at.%), while the aluminum is in the form of metallic Al (2.6 at.%) and oxides of Al3+ (5.9 at.%). The exposure of the sample (respectively of the bare zone) to the corrosive medium is leading to substantial changes in the ratio between them and in their chemical state. After the first hour of exposure, the species registered on the scratched surface Cr and Al have passed entirely into oxidic state (Fig. 5, 1 h). The concentration of Fe as a metal on the scratched surface, at small time intervals of exposure (1–6 h), is decreasing at the expense of the increase in the quantity of Fe3+ (probably in the form of Fe(OH)3 Fig. 5, 1–6 h). Thereupon, the ratio Fe3+/Fe0 (compared to what was established before the exposure to the corrosive medium) is practically unchanged. The long duration of exposure (200 h) of the sample to the corrosive medium leads to increase in the concentrations of Fe0 and Fe3+ (Fe(OH)3) on the scratched surface (Fig. 5, 200 h). In view of the results obtained in this study we can draw the following conclusions. Upon increasing the time interval of exposure of the sample to the corrosion medium, beside the changes in the concentrations of Fe0 and Fe3+ (Fe(OH)3) on the scratched surface, there is a substantial enrichment in alumina. There is also no so high enrichment in chromium oxides but to a smaller extend. This effect is obviously connected with the formation of a secondary oxide film, in which the quantities of

2.3 X-ray photoelectron spectroscopy investigations This cycle of measurements, aimed at characterization of the composition and the chemical state of the elements on the surface of the system CeO2-Ce2O3/SSt.t., was carried out with two types of samples of dimensions 20×20 mm. In the first sample, the deposited cerium oxide film covers the entire surface of the steel support, while in the second case, in the center of the sample (area of dimensions 10 mm×10 mm) the cerium oxide layer had been removed (scratched off) mechanically. The occurring changes in the chemical composition and in the state of the elements inside the zone of the removed layer of cerium oxides were monitored prior to and after the exposure to 0.5 mol/L NaCl solution. In this case the XPS studies were carried out using a specially elaborated screen of tantalum, which enabled the isola-

Fig. 5 Variation of the concentrations and the chemical states of elements determined by XPS analysis inside the zone of disrupted corrosion-protection layer of cerium oxides, depending on the time interval of exposure to 0.5 mol/L NaCl

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aluminum, chromium and iron oxides and/or hydroxides (Cr2O3, Cr(OH)3, Al2O3, Al(OH)3, and Fe2О3 and Fe(OH)3) are increased. In the secondary film, formed in the course of the prolonged exposure to the corrosive medium, Cr0 and Al0 are missing completely, while the concentration of Fe0 remains practically the same (Fig. 5). Similar enrichment of the surface of the steel in chromium oxides (in the case of presence of cerium oxide coating) has also been registered in earlier studies, both in chloride ions containing medium, as well as in acidic aggressive media[49,51,67,68]. There it was ascertained, that this effect is resulting in promoting the ratio Cr/Fe in the modified superficial protective passive film, respectively in enhancing the corrosion stability of the Ce oxides/SSt.t. system[49,51]. It was not possible to make definitive conclusion only on the basis of the registered XPS spectra about the availability of cerium oxides/hydroxides inside the scratched surface area even after 200 h of exposure to the corrosive medium, as the obtained photoelectron spectra are integrated (comprising the signals from both the studied and the scratched areas) and these have been registered for the secondarily formed non-homogeneous (having rather an island-like type of nature, Fig. 9(e, f)) cerium oxide phase. For this reason the studied samples were subjected to SEI, BEI and EDX micro-probe X-ray analyses: either in specific point, along a line and/or in mapping regime (The obtained results proved the formation of cerium oxides inside the scratched zone, see below item 2.4. EDХ and SEM investigations). At the same time it is important to note that in the recorded XPS spectra the specific Ce 3d photoelectron line, represented in Fig. 6(а), is undergoing consecutively occurring changes (depending on the time interval of exposure to the corrosive medium). The viewing and the juxtaposing of these spectra with the better expressed changes in the corresponding O 1s photoelectron spectra (Fig. 6(b)) give the reason to accept them as qualitative evidence for the secondary formation of cerium oxide phase inside the zone of mechanically removed cerium oxide film. This conclusion was confirmed also by the additional XPS analyses using monochromatic X-ray irradiation (X-ray spot size of 650 µm) inside the scratched area, after 500 h of exposure of the sample to the corrosive medium. In the spectrum, obtained after soft cleansing the surface using Ar+ ions, revealing the Ce 3d photoelectron line (Fig. 7), the presence of cerium oxide is clearly observable, which is giving evidence in support of the hypothesis, put forward above, that there are processes of self-healing occurrence in the zone of the “bare” steel surface. 2.4 EDХ and SEM investigations The problem of the impossibility to achieve a satisfac-

JOURNAL OF RARE EARTHS, Vol. 33, No. 11, Nov. 2015

Fig. 6 Ce 3d and O 1s XPS spectra of CeO2-Ce2O3/SSt.t. samples taken before exposure to the corrosive media (inserts) and at different time of exposure (6, 36, 93 and 200 h) of the specimens to 0.5 mol/L NaCl solution

Fig. 7 Ce 3d photoelectron spectra taken in the scratched area of the sample after 500 h exposure in the corrosive media

tory quantitative evaluation of the concentration of cerium oxide by means of XPS analysis in the scratched surface zone after 200 h of exposure of the samples to the corrosive medium (Fig. 6), was overcome by means of carrying out EDX and SEM analyses in the same zone (Figs. 8, 9). The results from ЕDX analyses, made with the same samples, in Scan line and Map′s modes, prior to and after 200 h of exposure to the corrosive medium, are

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Fig. 8 EDX scan lines and mapping analyses pictures of the system CeO2-Ce2O3/SSt.t (a) Before exposure to 0.5 mol/L NaCl outside and inside the scratch area; (b) After 200 h exposure to 0.5 mol/L NaCl outside and inside the scratched area (Segments of the scans line 2 mm left and 2 mm right of the border between non-scratched/scratched area of the samples)

Fig. 9 Microphotographs of the SSt.t. and CeO2-Ce2O3/SSt.t. samples (a) SEI image at the scratched zone before exposure to 0.5 mol/L NaCl; (b) SEI image of the SSt.t. at the scratched zone after exposure 200 h to 0.5 mol/L NaCl; (c) SEI image in the non scratched specimen′s area of the CeO2-Ce2O3 layer after 200 h exposure to 0.5 mol/L NaCl; (d) SEI image at the scratched zone of the CeO2-Ce2O3/SSt.t. samples after 200 h exposure to 0.5 mol/L NaCl; (e) BEC image and EDX spectra; (f) detected ceria agglomerate, formed inside the scratched zone after 200 h exposure to 0.5 mol/L NaCl

represented in Fig. 8(a, b). It is seen that the X-ray intensities of Ce, Al, Cr, Fe and O increased to some extent inside the scratch area after 200 h exposure in 0.5 mol/L NaCl solution. This result correlates with the observed increase in the intensity of the photoelectron line in the spectrum of Ce 3d, recorded in the same zone of the samples (Fig. 6(a)). In our opinion, the secondary formation of cerium oxide (oxide and/or hydroxide) on the scratched surface is the consequence of the occurring process of reduction of CeO2 (Ce4++e–↔Ce3+), covering the surface of the samples in the non-disrupted area (zone 1) of the cerium ox-

ide coating (Fig. 4)[25]. This supposition was confirmed experimentally by means of XPS analyses of the surface of the same samples, excluding the area of the mechanically removed cerium oxide layer. (In this case, when carrying out the XPS analysis, we covered the scratched area (Fig. 4, zone 2) of the surface of the scratched sample with a specially elaborated screen of tantalum, which enabled the isolation of the signal, emitted by it). The obtained results are represented in Table 1. It is seen in the table that the change in the concentration of cerium (CeO2) on the non-scratched zone of the studied samples is dynamic (column № 2). After 36 h of exposure to the

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Table 1 Elements contents (at.%) out of scratched area of the samples, versus time of exposure in aggressive media

exposure of the scratched samples to this medium (in particular, carrying out spectrophotometric investigations in regard to the appearance of changes in the concentration of Се3+ in it). The juxtaposition of the data from spectrophotometry with the data established by XPS analyses about changes/decrease in the concentration ofСе4+ (СеО2) on the surface of the non-scratched zone (Fig. 6 and Тable 1, column 2) and proving (by XRD analyses) the appearance of СеО2 on the surface of the scratched zone, in our viewpoint, could supply one more argument in support of the supposed mechanism of the occurring processes of self-healing of the steel. The spectrophotometric data about the changes in the chemical composition of the corrosive medium, which are occurring during prolonged exposure of the scratched samples to it, are represented in Fig. 10. Fig. 10(а) illustrates UV-Vis absorption spectra of standard aqueous solutions, prepared by us, to which we added: 100 ppm Се3+ (spectrum 1) and 10 ppm Се3+ (spectrum 2) and 5 ppm Се3+ (spectrum 3). The spectra have been recorded against a reference cuvette, containing distilled water. Well expressed maxima were obtained at wavelengths 252, 238, 221 and 211 nm, characterizing the presence, respectively the change in the concentration of Се3+ ions in the studied standard solutions. The spectra of the standard solutions, prepared by us, are identical (in view of the positions of the peaks) with those of the solutions based on 0.5 mol/L NaCl, to which we added: 100 ppm Се3+ (spectrum 1′); 10 ppm Се3+ (spectrum 2′) and 5 ppm Се3+ (spectrum 3′), having рН~6.2, Fig. 10(а). In this case the spectra were recorded with respect to a reference cuvette, containing only 0.5 mol/L NaCl. Fig. 10(b) illustrates the UV-Vis absorption spectra of the standard solutions, prepared by us on the basis of 0.5 mol/L NaCl, to which we added: 100 ppm Al3+ (spectrum 1); 50 ppm Fе2+ (spectrum 2) and 100 ppm Fe3+ (spectrum 3) — the ions, which possibly in the course of occurring corrosion process on the scratched zone of the sample, coated with СеО2 film, could be liberated and accumulated in the corrosive medium. The reference cuvette contained 0.5 mol/L NaCl in this case, too. It is seen in the recorded spectra that the maxima for Fе3+ and Fе2+ ions are very close to each other and they appear at ~210 and 330 nm, while the Al3+ ions determine the appearance of poorly expressed wide plateau within the range 215–350 nm. Fig. 10(с) represents the UV-Vis absorption spectra of the corrosive medium (0.5 mol/L NaCl solution), to which the scratched samples CeO2-Ce2O3/SSt.t. have been exposed in the course of 200 h (spectrum 1) and 500 h (spectrum 2). It is seen in these spectra that after 200 h of exposure, two maxima are registered (at wavelengths 221 and 270 nm), although relatively slightly expressed, which confirms the appearance of Се3+ ions in the corro-

Time interval of

Ce/

O/

Cr/

Fe/

Al/

exposure/h

at.%

at.%

at.%

at.%

at.%

0

12.4

74.2

1.3

–

12.1

36

1.5

92.2

0.6

3.5

2.2

93

2.2

87.6

1.1

1.8

7.3

200

1.2

84.3

2.4

2.8

9.3

corrosive medium the surface concentration of cerium is decreased considerably, while simultaneously an increase in the concentration of iron is being registered (column № 5) as well as a decrease in the concentration of chromium and aluminum (columns № 4, respectively № 6). After 93 h of staying in an aggressive medium there occurs slight enrichment of the non-scratched surface area in cerium and aluminum, while in the meantime the concentration of iron is decreasing, and that of chromium is substantially promoted. The further going exposure of the sample to the aggressive medium (200 h) is leading again to a decrease in the concentration of cerium (CeO2, respectively), accompanied by continuing increase in the concentrations of iron, chromium and aluminum (respectively of their oxides). These results support our supposition about the occurrence of redox processes in the system Ce4+/Ce3+. Fig. 9 represents micrographs of the scratches zone of the system CeO2-Ce2O3/SSt.t. prior to (a) and after 200 h of staying in the corrosive medium (c, d, e). The study, carried out in SEI, BEI and EDХ (spot analysis) regimes uniquely visualizes and proves that there are many spots in the scratched zone, upon which secondary cerium oxide sections have been formed (Fig. 9(e), white spots). Moreover, it is important to note that in the secondary passive film, being formed on the mechanically uncovered zone on the steel surface, in addition to the precipitation of hydroxides and/or oxides of cerium (in accordance with the supposed scheme, represented in Fig. 4), also the concentrations of the hydroxides and the oxides of aluminum, chromium and iron are substantially increased (Fig. 5). In our opinion, these secondarily formed phases, leading to the formation of a secondary passive film, are the reason for the registered effect of “selfhealing” and improving the corrosion stability of the steel coated with cerium oxide layers in 0.5 mol/L NaCl, illustrated with the results, discussed in Sections 2.1 and 2.2. 2.5 UV-Vis spectrophotometry and XRD investigations Confirmation of the above given results and assumptions can be obtained, if one studies the changes in the composition of the corrosive medium (0.5 mol/L NaCl solution), which are occurring in the course of prolonged

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Fig. 10 UV-Vis spectra of standard H2O and 0.5 mol/L NaCl solutions, containing 100, 10 and 5 ppm Ce3+ ions registered vs. H2O or 0.5 mol/L NaCl reference cuvette (a), UV-Vis spectra of standard 0.5 mol/L NaCl solutions, containing 100 ppm Al3+, 100 ppm Fe3+ and 50 ppm Fe2+ vs. 0.5 mol/L NaCl reference cuvette (b), UV-Vis spectra of 0.5 mol/L NaCl aggressive media after 200 h immersion of scratched CeO2-Ce2O3/SSt.t. sample (curve (1)) and after 500 h immersion of scratched CeO2-Ce2O3/ SSt.t. sample (curve (2)) vs. 0.5 mol/L NaCl reference cuvette (c)

sive medium. Thereupon, while the coincidence in the maxima at 221 nm corresponds completely to the one registered in the standard solution, the characteristic maximum at 252 nm is shifted with ~18 nm and now it appears at ~270 nm. The relatively slight expression of the maxima, characterizing the appearance of the Се3+ ions in the corrosive medium (spectrum 1), in our opinion, can be connected with: on one side, their very low concentration, determined by the proceeding of the reaction of the reduction of СеО2 (Се4+) into Се2О3 (Се3+) on the steel surface, coated with СеО2 (Fig. 4, zone 1), while on the other side, there is a strong upheaval of the entire spectrum starting from the maximum at 202 nm, connected with the maximum at WL ~197–202 nm, characteristic of the absorbance of aqueous solution of NaCl. The maxima at 221 and 270 nm in spectrum 2, obtained with the corrosion medium after 500 h exposure of the scratched CeO2-Ce2O3/SSt.t. sample, are even more slightly expressed. In our view, this result is due to the continuous increasing with the time of exposure effect of the processes of self-healing, as a consequence of which even larger part of the scratched surface will be covered secondarily with protective oxide film (Cr2O3, Cr(OH)3, Al2O3, Al(OH)3, Fe2О3, Fe(OH)3, and Се(ОН)4/ СеО2). As a result of this, the rate of the corrosion process (respectively of the conjugated cathodic reactions of oxygen depolarization and reduction of СеО2, determining the liberation of Се3+ ions into the corrosive medium) will be decreased. A similar effect has been reported by Joshi et

al. as well as a supposition has been put forward about the influence of the time interval of exposure to the corrosive medium in the case of corrosion of Al in 0.1 mol/L NaCl media, comprising also Се ions, and it is similar to our assumption[69]. Aiming at carrying out one more additional and independent check-up of the results and verification of the conclusions, based on the UV-Vis spectrophotometer investigations, some XRD studies were carried out on the dry residual from the corrosive medium (Fig. 11(a)) and phase analysis of the scratch surface after 500 h of exposure of the studied samples (Fig. 11(b)). For this purpose, 50 mL of the studied by UV-Vis spectrophotometry corrosive medium (the same with which the spectra shown in Fig. 10(с) have been obtained, illustrating the transition of Се3+ ions after 200 h of exposure of the scratched sample, in 0.5 mol/L NaCl solution) were dried up completely and the obtained dry residual was subjected to XRD analysis. The high peak, registered with this dry residual at 2θ=28.63º (Fig. 11(a)), characteristic of a compound of the type СеО2, is an additional evidence that cerium ions were transferred into the corrosive medium, in confirmation of the mechanism proposed by us (Fig. 4) based on reduction of Се4+ to Се3+. At the same time, the appearance of peaks at 2θ=28.60º and 33.10º (Fig. 11(b)), confirming the formation of a phase of СеО2 on the scratched steel surface, could be associated with oxidation reaction of Се3+ to Се4+, conjugated with the oxygen depolarization reaction.

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Fig. 11 XRD patterns of dry residual, obtained after vaporizing the corrosive solution (to which the sample CeO2-Ce2O3/SSt.t. was exposed 200 h) (a), XRD patterns of the non-covered with CeO2-Ce2O3 thermally treated SS OC 404 (1) and the scratched area (2) after 500 h exposure to 0.5 mol/L NaCl (b)

3 Conclusions A study on the processes of self-healing during the corrosion of the thermally treated stainless steel ОС 404, having electrochemically deposited thin protective film of cerium oxides was carried out in corrosive medium of 0.5 mol/L NaCl. In this aspect, a zone was modeled on the surface of the studied samples (defined as location and area), in which the protective layer was removed mechanically. Chronopotentiometric, potentiodynamic, X-ray photoelectron spectroscopy, SEM (in SEI and BEI mode) and EDS (in MAP, scan in LINE and in POINT mode) investigations were carried out with the so prepared samples, with the aim to characterize the changes in the corrosion parameters of the system CeO2-Ce2O3/ SSt.t. with the course of time of exposure of the studied samples to the corrosive medium. In the course of these investigations the qualitative and quantitative changes in the chemical composition were studied and defined in regard to the secondarily formed passive film on the surface of the steel in the zone, where the protective layer was removed mechanically. Thereupon it was ascertained that: (1) The variation of the corrosion potential of the thermally treated steel (Еоср), with the course of time of the exposure to the corrosive medium, showed the course, characteristic of stainless steel, having disturbed passive surface film. Its sharp shifting was observed in the initial stages of exposure to the corrosive medium, occurring in the positive direction (with ~200 mV), owing to the effect of self-healing, while afterwards it started changing gradually to the negative direction, reaching after ~ 500 h the zone of potentials, characteristic of Еоср of the iron – the main component of the studied steel. The course of the Е-τ curves was indicative of the fact that the processes of self-healing in the zones of disturbed native passive film on the surface of the thermally treated steel were not effective;

(2) When a thin protective film of cerium oxides was deposited on the surface of the steel, the curve of the variation of the Еоср of the system CeO2-Ce2O3/SSt.t. was practically the opposite course to that described above for SSt.t.. Thereupon, by increasing the time interval of exposure of the samples to the corrosive medium, the Еоср was shifting strongly in the positive direction, reaching values considerably exceeding the Еоср for the non-treated thermally steel. The course of the Е-τ curve in this case showed the occurrence of effective processes of self-healing in the zones of disturbed native passive film on the surface of the thermally treated steel; (3) For the samples, on which a significant area was formed by scratching i.e. a zone where the surface was “revealed” by removal of the protective cerium oxide film, an oscillating Е-τ curve was registered, which was typical of the processes of self-healing, the course of which during prolonged exposure gave evidence for the occurrence of an effective process of self-healing; (4) The values of the corrosion current for SSt.t., scratched SSt.t. and scratched CeO2-Ce2O3/SSt.t. specimens, determined on the basis of the plotted potentiodynamic curves, confirmed in an unique way the above considerations; (5) The results from the analytical investigations on the changes in the qualitative and quantitative composition of the elements of the surface film on the steel (in the scratched area), prior to and after the exposure of the scratched CeO2-Ce2O3/SSt.t. samples to the corrosive medium, showed the substantial changes, occurring as a consequence of the secondary formation (“self-healing”) of layers of Cе(OH)4, CеO2, Cr2O3, Cr(OH)3, Al2O3, Al(OH)3, Fe2О3 and Fe(OH)3 in the scratched area. This effect had obviously its place inside the micro-cracked sections of the thermally treated CeO2-Ce2O3/SSt.t. system; (6) There are reasons to suppose that beside the oxygen depolarization reaction, there was also reaction of

Desislava Guergova et al., Self-healing effect of ceria electrodeposited thin films on stainless steel in …

reduction of CeO2 (occurring in the zone, where it was not removed), leading to the transformation of CeO2 into Ce2O3. As it was soluble in the corrosive medium, this would lead to liberation of Се3+ ions in it. Diffusing to the revealed steel surface they would be hydrolysed to Cе(OH)3 and/or they would be oxidized to Се4+, as a result of which Cе(OH)4 (CeO2, respectively) was formed, according to the proposed scheme for the proceeding of the conjugated (anodic and cathodic) reactions of the corrosion process; (7) On the basis of the occurring self-healing processes the formed secondary passive films (after exposure to the corrosive media) acted as barriers, which prevented to a great extent the steel surface, hindering the corrosion processes in the active zones.

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