Naslov
Investigation of ascorbic acid influence on steel passivity in sat. Ca(OH)2

Autori
Valek, Lidija ; Martinez, Sanja ; Serdar, Marijana

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Cd of Abstracts of 59th Annual Meeting of the International Socitey of Electrochemistry / International Society of Electrochemistry - Sevilla : International Society of Electrochemistry, 2008

Skup
59th Annual Meeting of the International Society of Electrochemistry

Mjesto i datum
Sevilla, Španjolska, 07.09.2008. - 12.09.2008

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
passivity; steel; ascorbic acid

Sažetak
Characteristics of the passive film formed on steel in alkaline solutions are of major importance when considering the susceptibility to pitting corrosion. Presence of chelating agent may improve the performance of such film by competitive adsorption through formation of surface chelates that force chloride ions to interact with less energetic favourable sites, therefore requiring greater activation energy for pitting to occur. The effect of the complexing agent, ascorbate anion, on the electrochemical behaviour and semiconductive properties of passive film formed on non alloyed steel in deaerated saturated Ca(OH)2 was investigated in this study. Cyclic voltammetry measurements showed that presence of ascorbate in the solution suppressed the oxidation and reduction peaks, lowering the current in the passive region. Electrochemical impedance spectroscopy revealed that the processes at the interface at the open circuit potential are charge transfer controlled with significantly higher charge transfer resistance in ascorbate presence. ATR FTIR measurements confirmed presence of ascorbate chelate at the steel surface after exposure to the sat. Ca(OH)2 with addition of ascorbate. It has been reported in the literature that passive film formed on steel consists of inner barrier layer (Fe3O4) and precipitated outer layer (γ -Fe2O3). Film formed at anodic potentials in the passive region (-100 to 300 mV) was removed layer by layer by galvanostatic cathodic reduction. Potential arrest at approximately -900 mVSCE commonly attributed to the reduction of γ -Fe2O3 to Fe(II) appeared only in sat. Ca(OH)2. Reduction of film formed in the presence of ascorbate appeared at more negative cathodic potentials with less charge due to the lower concentration of ferrous ion in the surface layer. Growth of anodic films by galvanostatic oxidation starting from the same pre-existing films as in the case of reduction was studied. In the presence of ascorbate, longer formation times were observed before the oxygen evolution potential was reached. Presence of chelating agent adsorbed at the oxide surface probably hinders formation of the precipitated outer layer and thus results in shorter reduction times and longer formation times. Mott Schottky experiments were performed in order to investigate semiconducting properties of passive film. The electronic character of passive film formed in sat. Ca(OH)2 with and without ascorbate was found to be n-type which is consistent with the dominant defects being oxygen vacancies and cation interstitials. Using dielectric constant assumption, semiquantitative defect densities and flatband potentials were calculated. The defect concentration was found to decrease with increasing the film formation potential. Mott Schottky analysis showed lower defect density of the passive film formed in the presence of ascorbate.

Izvorni jezik
Engleski

Znanstvena područja
Kemijsko inženjerstvo

POVEZANOST RADA