Desislava Guergova

Self-healing effect of ceria electrodeposited thin films on stainless steel in aggressive 0.5 mol/L NaCl aqueous solution

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.

Investigation of the Inhibiting Effect of Cerium Ions on the CorrosionBehavior of OC404 Stainless Steel in Sulfuric Acid Medium

The inhibiting effect of Ce ions, introduced in the form of Ce(SO4)2, on the corrosion electrochemical behavior of stainless steel OC404, has been studied in sulfuric acid corrosion medium (0.05 M H2SO4). On the basis of the obtained data on the values of the density of the corrosion current (icorr), evaluated using the dependences E-lgi, both in inhibited and uninhibited corrosion medium, the degree of corrosion protection in the presence of the inhibitor was calculated, as well as the fraction of surface coverage (Q) the electrode surface with inhibitor. These studies were carried out with samples of non-treated thermally steel (SS) and thermally treated steel (SSt.t.). The cathodic reaction of reduction of the oxidative component of the medium Ce was found to be the basic driving force of the conjugated anodic process, leading to a transition of SSt.t. from active into passive state. Thereupon an option is created to form an efficient protective film of phase type on the SSt.t. surface reaching a degree of inhibition efficiency of about 98 %. In order to elucidate the nature of the inhibiting action of cerium ions the influence of the value of the equilibrium oxidation-reduction potential of the Ce/Ce ion pair was investigated as well as that of the exchange current of the oxidation-reduction process upon the processes of passivation of the studied steel. The obtained results give the reason to classify the cerium ions as cathode type of inhibitor having oxidative action.

Preparation and Characterization of Al2O3 Thin Films for Catalytic Activity Studies

In the present work, the surface properties of various Al oxide films were investigated. The oxide films were produced on a stainless steel by spray pyrolysis and cathodic deposition methods. The films obtained represent typical layers that can be used as a support in model systems to investigate alumina-based catalysts. Information about the chemical environment of the Al and O ions in the oxide films depending on the preparation conditions has been deduced from the binding energies of the Al 2p and O 1s electron core levels and corresponding Auger parameters.

A Study of the Kinetics of the Electrochemical Deposition of Ce3+/Ce4+ Oxides

The kinetics of cathodic electrodeposition of Ce3+ and/or Ce4+ oxides from alcoholic electrolytes on gold substrates has been studied. It was found that, depending on the oxygen content in the CeCl3-based electrolyte, Ce2O3 (in oxygen atmosphere) or CeO2 (in an inert atmosphere), respectively, were obtained. XPS studies clearly separated the two valence states of Ce ions in the oxide layers. The microstructure of the coatings was analyzed by atomic force microscopy (AFM).