Passivity of AISI 321 stainless steel in 0.5 M H2SO4 solution studied by Mott–Schottky analysis in conjunction with the point defect model

The passivity of AISI 321 stainless steel in 0.5 M H2SO4 solution, in the steady-state condition, has been explored using electrochemical impedance spectroscopy (EIS) and Mott–Schottky analysis. Based on the Mott–Schottky analysis in conjunction with the point defect model (PDM), it was shown that the calculated donor density decreases exponentially with increasing passive film formation potential. The thickness of the passive film was increased linearly with the formation potential. These observations were consistent with the predictions of the PDM, noting that the point defects within the passive film are metal interstitials, oxygen vacancies, or both

The Mechanism of Transpassive Dissolution of AISI 321 Stainless Steel in Sulphuric Acid Solution

The transpassive dissolution mechanism of AISI 321 stainless steel in a 0.5 M sulphate solution was studied using electrochemical impedance spectroscopy (EIS). On the basis of the experimental results, surface charge approach, and point defect model, a kinetic model of the transpassive dissolution process is proposed. The transpassive film is modeled as a highly doped n-type semiconductor—insulator-p-type semiconductor structure. Injection of negative defects at the transpassive film/solution interface results in their accumulation as a negative surface charge. It alters the nonstationary transpassive film growth rate controlled by the transport of positive defects (oxygen vacancies). The model describes the process as dissolution of Cr as Cr (VI) and Fe as Fe (III) through the transpassive film via parallel reaction paths

EFFECT OF IMMERSION TIME ON THE ELECTROCHEMICAL BEHAVIOUR OF AISI 321 STAINLESS STEEL IN 0.1 M H2SO4 SOLUTION

In this study, effect of immersion time on the electrochemical behaviour of AISI 321 stainless steel (AISI 321) in 0.1 M H 2SO 4 solution under open circuit potential (OCP) conditions was evaluated by potentiodynamic polarization, Mott–Schottky analysis and electrochemical impedance spectroscopy (EIS). Mott–Schottky analysis revealed that the passive films behave as n-type and p-type semiconductors at potentials below and above the flat band potential, respectively. Also, Mott–Schottky analysis indicated that the donor and acceptor densities are in the range 1021 cm-3 and increased with the immersion time. EIS results showed that the best equivalent circuit presents two time constants: The high-medium frequencies time constant can be correlated with the charge transfer process and the low frequencies time constant has been associated with the redox processes taking place in the surface film. According to this equivalent circuit, the polarization resistance (interfacial impedance) initially increases with the immersion time (1 to 12 h), and then it is observed to decreases. This variation is fully accordance with potentiodynamic polarization result

Effect of solution pH on the electrochemical behaviour of AISI 304 austenitic and AISI 430 ferritic stainless steels in concentrated acidic media

In this study, the effect of solution pH on the electrochemical behaviour of AISI 304 austenitic and AISI 430 ferritic stainless steels at open circuit potential (OCP) in concentrated acidic solutions was studied. Polarization curves showed that the corrosion current density of both stainless steels increased with decreasing pH. Mott–Schottky analysis revealed that passive films formed on AISI 304 and AISI 430 stainless steels behave as n-type and p-type semiconductors and the donor and acceptor densities increased with decreasing pH. Electrochemical impedance spectroscopy (EIS) results showed that the reciprocal capacitance of the passive film is directly proportional to its thickness which decreases with decreasing pH. Thus for both austenitic and ferritic stainless steels in acidic solutions, increasing the solution pH offers better conditions for forming passive films with higher protection behaviour, due to the growth of much thicker and less defective films

Semiconducting behavior of the anodically passive films formed on AZ31B alloy

This work includes determination of the semiconductor character and estimation of the dopant levels in the passive film formed on AZ31B alloy in 0.01 M NaOH, as well as the estimation of the passive film thickness as a function of the film formation potential. Mott–Schottky analysis revealed that the passive films displayed n-type semiconductive characteristics, where the oxygen vacancies and interstitials preponderated. Based on the Mott–Schottky analysis, it was shown that the calculated donor density increases linearly with increasing the formation potential. Also, the electrochemical impedance spectroscopy (EIS) results indicated that the thickness of the passive film was decreased linearly with increasing the formation potential. The results showed that decreasing the formation potential offer better conditions for forming the passive films with higher protection behavior, due to the growth of a much thicker and less defective films

Investigation of the passive behaviour of AZ31B alloy in alkaline solutions

In this work, the passivity of AZ31B alloy in NaOH solutions was studied by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and Mott–Schottky analysis. Potentiodynamic polarization results indicated that decreasing NaOH concentration leads to decrease the corrosion rate of this alloy. EIS results showed that the reciprocal capacitance (1/C) of the passive film is directly proportional to its thickness which increases with decreasing NaOH concentration. Therefore, it is clear that dilute NaOH solutions offer better conditions for forming the passive films with higher protection behaviour, due to the growth of a much thicker and less defective films. The Mott–Schottky analysis revealed that the passive films displayed n-type semiconductive characteristics, where the oxygen vacancies and interstitials (over the cation vacancies) preponderated. Also, Mott–Schottky results showed that the donor densities evaluated from Mott–Schottky plots are in the range of 1020 cm−3 and decreased with decreasing NaOH concentration

PHASE STABILITY AND SEMICONDUCTING BEHAVIOUR OF LOW ACTIVATION AUSTENITIC MN-CR STEELS

Four compositions of austenitic Mn-Cr steels have been developed successfully for in-vessel component materials in power plant industry. The phase stability of these Mn-Cr steels was studied by and X-ray diffraction (XRD) patterns. XRD patterns have shown that the matrix of these Mn-Cr steels is a single γ-phase structure. The potentiodynamic polarisation curves suggested that these fabricated Mn-Cr steels showed passive behaviour in 0.1M H2SO 4solution. Therefore, semiconducting behaviour of passive film formed on these fabricated Mn-Cr steels in 0.1M H2SO 4 solution was evaluated by Mott–Schottky analysis. This analysis revealed that passive films behave as n-type and p-type semiconductors. Based on the Mott–Schottky analysis, it was also shown that donor and acceptor densities are in the order of 1021 cm -3 and are comparable for other austenitic stainless steels in acidic environment

Diffusivity of Point Defects in the Passive Film on Stainless Steel

The semiconductor properties of passive films formed on AISI 316 stainless steel in sulfuric acid solution were studied by employing Mott-Schottky analysis in conjunction with the point defect model. The donor density of the passive films, which can be estimated by the Mott-Schottky plots, changes depending on the film formation potentials. Based on the Mott-Schottky analysis, an exponential relationship between donor density and the film formation potentials of the passive films was developed. The results showed that the donor densities evaluated from Mott-Schottky plots are in the range 2-3 × 1021 cm−3 and decreased with the film formation potential. By assuming that the donors are oxygen ion vacancies and/or cation interstitials, the diffusion coefficient of the donors, (), is calculated to be approximately 3.12 × 10−16 cm2/s

AN INVESTIGATION ON CORROSION BEHAVIOUR OF A356-10 VOL.% SiC COMPOSITES IN HCl SOLUTIONS

In this study, corrosion behaviour of A356-10 vol.% SiC composites casted by gravity and squeeze casting is evaluated. For this purpose, prepared samples were immersed in HCl solution for 1h at open circuit potential. Tafel polarization and electrochemical impedance spectroscopy (EIS) were carried out to study the corrosion resistance of composites. The Tafel polarization and EIS studies of the corrosion behaviour of the A356-10 vol.% SiC composites showed that the corrosion resistance of the composite casted by squeeze casting was higher than that of the composites casted by gravity in selected corrosion media. Also, the Tafel polarization and EIS studies revealed that the corrosion current densities of both composites increase with the increase in the concentration of HCl. The micrographs of scanning electron microscope (SEM) clearly showed the squeeze casting composite exhibits a good dispersion/matrix interface compared to that of the composites produced by gravity castin

Corrosion Behavior of A356-10 Vol.% SiC Composites Cast by Gravity and Squeeze Casting in H2SO4 Solutions

Corrosion behavior of A356-10 vol.% SiC composites cast by gravity and squeeze casting is evaluated. For this purpose, prepared samples were immersed in H2SO4 solution for 2 hrs. at open circuit potential. Tafel polarization and electrochemical impedance spectroscopy (EIS) were carried out to study the corrosion resistance of composites. The results showed that corrosion resistance of composites cast by squeeze casting is higher than that of the gravity cast composites. The micrographs of scanning electron microscope (SEM) clearly showed the squeeze casting composites exhibit a good dispersion/matrix interface when compared with composites produced by gravity casting