E. Stoyanova

Electrochemical aspects of the immersion treatment of aluminium

The processes occurring during zinc immersion plating of aluminium have been studied in alkaline medium. The mechanism of immersion zinc deposition on aluminium has been elucidated by studying the kinetics of the partial electrode reactions involved in this complex process. The diffusion of zincate ions has been shown to play a decisive role on the cathodic reaction kinetics as well as on the global (cathodic and anodic) reaction rate.

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.

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers

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).

Deposition of thin tin coatings on aluminium alloys

Abstract A systematic investigation of the immersion and electrodeposition processes of thin tin coatings used as an underlayer for the deposition of the multilayer system Cu/Ni/Cr on a typical aluminium alloy, applied in the automobile industry and other branches of mechanical engineering, was carried out. To elucidate the parameters affecting the adhesion of the tin coating and the adhesion of the protective-decorative coatings, the following characteristics were studied: the thickness of the tin coating (the current efficiency for electrodeposited tin coatings); the amount of aluminium dissolved during the deposition of the coating; the overpotential for the deposition of tin (for electrolytically deposited tin coatings) on pure aluminium, on the aluminium alloy and on pure zinc, magnesium, chromium, manganese and iron (the main alloying components of the alloy); the distribution of the alloying components and their influence on the initial stages of the tin deposition; the morphology of the alloy surface before and after the deposition of the tin coating. It is shown that iron in the aluminium alloy is distributed preferentially in the form of segregates and provides active sites for the formation of the thin tin coatings on the aluminium alloy. It is suggested that the iron segregates in the stannate treatment of iron-containing aluminium alloys determine the adhesion of the electrodeposits.

Influence of the surface pre-treatment of aluminum on the processes of formation of cerium oxides protective films

It is known that there is special interest in the contemporary investigations on conversion treatment of aluminum aimed at promoting its corrosion stability, which is focused on electrolytes on the basis of salts of metals belonging to the group of rare-earth elements. Their application is especially attractive, as it enables a successful substitution of the presently applied highly efficient, but at the same time toxic Cr6+-containing electrolytes. The present paper presents a study on the influence of the preliminary alkaline activation and acidic de-oxidation of the aluminum surface on the processes of immersion formation of protective cerium oxides films on Al 1050. The results obtained show that their deposition from simple electrolytes (containing only salts of Ce3+ ions) on the Al surface, treated only in alkaline solution, occurs at a higher rate, which leads to preparing thicker oxide films having a better protective ability. In the cases when the formation of oxide films is realized in a complex electrolyte (containing salts of Ce3+ and Cu2+ ions), better results are obtained with respect to the morphology and protective action of cerium oxides film on samples that have been consecutively activated in alkaline solution and deoxidized in acidic solution. Electrochemical investigations were carried out in a model corrosion medium (0.1 M NaCl); it was shown that the cerium protective films, deposited by immersion, have a cathodic character with regard to the aluminum support and inhibit the occurrence of the depolarizing corrosion process -- the reaction of oxygen reduction.

Protective ability of cerium-containing conversion films on aluminium

Abstract The options for electroless deposition of conversion films of lanthanum group metal oxides, in particular cerium, as an alternative to chromate conversion films (containing Cr6+) have been studied on aluminium. The main task of the study was to establish the influence of copper ions as a component of electrolytes for conversion treatment on the processes of formation and on the corrosion protective ability of cerium oxide films. It has been ascertained that the electrochemical activation of the aluminium surface in the presence of Cu2+ in the electrolyte, leads to formation of oxide layers richer in cerium and possessing a better protective ability. The presence of active cathodic sections of electroless, contact-deposited Cu has been shown to promote the corrosion activity of the system, due to the possibility of the appearance of micro-galvanic couples Al/Cu. A model experiment has been carried out to establish the inclination of the system to pitting corrosion. It has been shown that at the studied concentration of Cu2+ in the electrolyte the character of the corrosion does not change observably but there is a catalysing effect of the copper ions in the working electrolyte. The copper islands on the Al surface also have a catalysing effect, which is dominating in the processes of cerium oxide film formation. The chemical state and the composition of the conversion films have been defined by XPS measurements.