Electrochemical Behavior Of Passive Films Research Articles

Effect of secondary passivation on corrosion behavior and semiconducting properties of passive film of 2205 duplex stainless steel

The effects of secondary passivation on the electrochemical behavior of passive films on the surface of 2205 duplex stainless steel were studied by potentiodynamic polarization, EIS and Mott–Schottky analysis. The composition and surface morphology of the passive film were analyzed by ICP-MS, ARXPS and SEM. These results manifested that the passive film formed by secondary passivation has good pitting resistance, due to the increase of the content of oxidized Fe3+, Cr3+ and Mo4+ in the passive film, which improves and increases the dissolution and migration rate of metal cations in the passive film, thus improving the compactness and stability of the passive film. The Mott–Schottky plots demonstrated that the five kinds of passive films have different semiconductor properties. The estimated doping density shows that the donor density of the passive film was significantly reduced by secondary passivation. In addition, Cl− and SO42− are permeated in the passivation layer and their adsorption on the surface of the passive film can be inhibited by the secondary passivation.

In Vitro Electrochemical Behavior of Aged Ti-6Al-4V Alloy: A Trial of the Point Defect Model

This article deals with the electrochemical behavior of passive films that were formed on the surface of age hardened Ti-6Al-4 V alloy in Ringer’s physiological solution at 37 °C. The influence of age hardening at 740 °C for different ageing times up to 8 h on the microstructural features and electrochemical properties were characterized. To do so, the microstructural evolution was studied using X-ray diffraction technique, Vickers microhardness test and scanning electron microscopy, while the electrochemical characteristics were investigated by electrochemical impedance spectroscopy, Mott-Schottky analysis and point defect model (PDM). The experimental results indicated less microhardness and microstrain values due to proceeding phase transformation of β to α at prolonged ageing times, which exerted an adverse effect on the passive film properties as a result of the formation of more defective passive films upon the surface. The PDM results revealed that deteriorated corrosion protection ability of the passive films on the surface of overaged specimens can be attributed to higher diffusivity of the point defects in their respective crystal structure.

Electrochemical Behavior of Passive Films Formed on the Surface of Coarse-, Fine- and Ultra-fine-Grained AA1050 Based on a Modified PDM

Electrochemical impedance spectroscopy (EIS) and Mott–Schottky analysis were carried out to evaluate the electrochemical behavior of the passive films formed on the surface of coarse-grained (CG), fine-grained (FG) and ultra-fine-grained (UFG) 1050 Al alloy (AA1050) samples in alkaline media (pH value of 8.0) based on a modification of point defect model (PDM). The EIS results revealed that the polarization resistance increased from about 22.71–120.33 kΩ cm2 for UFG sample when compared to CG sample (annealed sample). The semiconductor properties of the passive films formed on CG, FG and UFG AA1050 samples in the test solution were investigated by employing Mott–Schottky analysis in conjunction with PDM. The results indicated that donor densities were in the range of 2.19 × 1021–0.61 × 1021 cm−3 and decreased with grain refinement. Finally, all electrochemical tests showed that the electrochemical behavior of AA1050 alloy was improved by decreasing the grain size, mainly due to the formation of thicker and less defective oxide films.

Electrochemical Behaviour of Passive Films Formed on Inconels 600 and 690

We have verified through an electrochemical technique (capacity, impedance and photo-electro- chemical measurement) and also through an analytic technique of auger spectrometry that passive films formed on inconels 600 and 690 are comprised of two layers. ...

Corrosion Behaviour of a Highly Alloyed Austenitic Alloy UB6 in Contaminated Phosphoric Acid

The influence of temperature (20–80°C) on the electrochemical behaviour of passive films anodically formed on UB6 stainless steel in phosphoric acid solution (5.5 M H3PO4) has been examined by using potentiodynamic curves, electrochemical impedance spectroscopy, and Mott-Schottky analysis. UB6 stainless steel in contaminated phosphoric acid is characterised by high interfacial impedance, thereby, illustrating its high corrosion resistance. The obtained results show that the films behave as n-type and p-type semiconductors in the potential range above and below the flat band potential, respectively. This behaviour is assumed to be the consequence of the semiconducting properties of the iron oxide and chromium oxide regions which compose the passive film.

An Electrochemical Impedance Spectroscopic Study of the Passive State on AISI 304 Stainless Steel

The passivity and protective nature of the passive films are essentially related to ionic and electronic transport processes, which are controlled by the optical and electronic properties of passive films. In this study, the electrochemical behavior of passive films anodically formed on AISI 304 stainless steel in sulfuric acid solution has been examined using electrochemical impedance spectroscopy. AISI 304 in sulphuric acid solution is characterized by high interfacial impedance, thereby illustrating its high corrosion resistance. Results showed that the interfacial impedance and the polarization resistance () initially increase with applied potential, within the low potential passive. However, at a sufficiently high potential passive ( V), the interfacial impedance and the polarization resistance decrease with increasing potential. An electrical equivalent circuit based on the impedance analysis, which describes the behavior of the passive film on stainless steel more satisfactorily than the proposed models, is presented.

An Electrochemical Impedance Study of AISI 321 Stainless Steel in 0.5 M H2SO4

The electrochemical behavior of passive films formed on AISI 321 has been examined using electrochemical impedance spectroscopy. AISI 321 is characterized by high interfacial impedance, thereby illustrating its high corrosion resistance. Results showed that the interfacial impedance and the polarization resistance initially increase with applied potential, within the low potential. However, at a sufficiently high potential ( V), the interfacial impedance and the polarization resistance decrease with increasing potential. The impedance data were adequately represented by an equivalent electrical circuit model based on point defect model, which described the behavior of the passive film on stainless steel more satisfactorily than the proposed models.

Electrochemical behavior of passive films on Al–17Si–14Mg (wt.%) alloy in near-neutral solutions

The anodic polarization behavior of alloy Al–17Si–14Mg in borate solutions with and without 0.01 M NaCl was compared to that for pure Al. Results showed that, for the alloy, the passive current density increased but the pitting susceptibility decreased. The first effect was ascribed to a significant electrochemical activity of the Mg2Si intermetallics and the second to improved stability of the oxide film. X-ray photoelectron spectroscopy analysis of potentiostatically formed passive film on the alloy showed that it consisted of aluminum oxyhydroxide with incorporation of silicon in its elemental and two oxidized states (+3 and +4). Mott–Schottky analysis showed that trivalent silicon ion acted as an n-type dopant in the film. The interrelationship between passive film composition, electronic properties, and pitting behavior has been discussed.

Electrochemical behaviour of passive films on molybdenum-containing austenitic stainless steels in aqueous solutions

Passivation and its breakdown reactions have been studied on Mo-containing stainless steel specimens using different electrochemical techniques. Mo-containing stainless steel specimens were polarized in both naturally aerated NaCl and Na 2SO 4 solutions of different concentrations at 25 ± 0.2 °C between −1000 and 1500 mV versus saturated calomel electrode (SCE). The results of potentiodynamic polarization showed that i corr and i c increases with increasing either Cl − or SO 4 2− concentration indicating the decrease in passivity of the formed film. EIS measurements under open circuit conditions confirmed that the passivity of the film decrease with increase in either Cl − or SO 4 2− concentration.

Capacitance behaviour of passive films on ferritic and austenitic stainless steel

The electrochemical behaviour of passive films formed on one austenitic stainless steel (AISI 304) and one ferritic stainless steel (AISI 446) in solutions with pH between 0.6 and 8.4 was studied by capacitance measurements and photocurrent spectroscopy. Compositional characterization of the passive films was done by X-ray photoelectron spectroscopy. The capacitance increases with decreasing pH. Doping densities evaluated from Mott–Schottky plots are in the range 2–6 × 10 20 cm −3 and increased with the pH in the neutral/alkaline range while in pH 0.6, values above 10 21 cm −3 were found. The bandgap energy indicates two transitions, at 2.5–2.8 and 3.2 eV. The analytical data reveal that, as the pH increased, the films become enriched in Fe(II) and Fe(III), whereas the Cr(III) gradually decreases. The films formed at very low pH had a behaviour that contrasts with that of the films formed in the neutral/alkaline media. The films are described by a bilayer structure, with hydroxides in the outer layer and a spinel type oxide in the inner layer.

Influence of alloying on the passive behaviour of steels in bicarbonate medium

The electrochemical behaviour of passive films on carbon steel, chromium steel and high speed steel (HSS) was studied in 0.5 M NaHCO 3. The influence of the presence of 10 mM potassium chloride on passive film stability, and of the heat treatment of chromium steel and HSS on the respective film properties, was also investigated. Polarization curves showed that the alloy composition exerts a significant influence on the onset of passive film formation, chromium and HSSs passivating more easily as a result of the higher alloying content. Values of passive current, obtained both potentiodynamically and at fixed potential, were higher for HSS than for carbon and chromium steels, which was attributed to the contributions of Mo and W to the passive film composition. The presence of chloride ion delays the passivation of chromium and HSSs, but does not interfere with passive film stability, as shown by the chronoamperometric profile and passive current values. In the case of carbon steel, with no alloying elements, chloride ion diminishes film stability but does not influence passive film formation. Heat treatment, which alters the microstructure, does not alter the properties of the passive films except for a small effect on those formed potentiodynamically on HSS.

An electrochemical impedance spectroscopy study of passive zinc and low alloyed zinc electrodes in alkaline and neutral aqueous solutions

The electrochemical behaviour of passive films on Zn and low alloyed Zn-Co coatings (Co < 1.0%) has been studied in alkaline and neutral aqueous solutions (8.4 ≤ pH ≤ 12.9) by means of electrochemical impedance spectroscopy (EIS). For comparison, measurements on ZnO single crystals were also performed. From Mott-Schottky plots the flat band potential E FB and the donor concentration N D were determined as a function of pH and Co content. A Nernstian dependence of E FB on pH and an increase of N D proportionally to the Co content were found. The passive current of Zn coatings decreases with decreasing pH value and increasing Co content. Photocurrent and photovoltage measurements are in satisfactory agreement with the electrochemical results. The decrease of the passive current density observed with increasing Co content can be explained in terms of a chemical stabilization of the passive film.