Effect of chromium content and sweep rate on passive film growth on iron-chromium alloys studied by EQCM and XPS

Growth of passive films on Cr and Fe-Cr alloys with Cr concentrations ranging from 15% to 54% was studied in situ with the electrochemical quartz crystal microbalance (EQCM). Mass change and current transients were measured as the response to a potential change in the passive region from 0 to 800 mV (SHE) in a pH 1.5 sulfate electrolyte. From these measurements, the thickness change as well as integral and differential growth fractions were calculated. The growth fraction expresses the fraction of oxidized metal that remains in the film. It was found that higher Cr contents gave thicker films and an increase in the growth fraction. The thickness change curves were compared to film growth models assuming rate control at either a film interface or in the film itself through high field conductivity. The integral growth fraction for the potential change experiment was found to vary approximately linearly with the Cr bulk composition. The fraction of dissolved Cr calculated from EQCM data matched well with previous solution analysis results.     

A mixed-conduction model for oxide films on Fe, Cr and Fe-Cr alloys in high-temperature aqueous electrolytes—II. Adaptation and justification of the model

The aim of this two-part work is to propose a model for the corrosion mechanism of ferrous alloys in high-temperature aqueous environments. In this second part, the modifications to the mixed-conduction model (MCM) are discussed on the basis of experimental data presented in the first part for Fe, Cr and two Fe-Cr alloys (12 and 25 wt% Cr) in an aqueous solution at 200 °C. Application of the MCM to fit and predict experimental behaviour both at room temperature and at 200 °C is demonstrated. The major difference between the behaviour of films at room temperature and at 200 °C is that the mobility of ionic defects is much higher at the higher temperature. Estimates show that the ratio of the electronic and ionic diffusion coefficients (D_e/D_i) is of the order of 10⁵ at room temperature and ≈30 at 200 °C for pure Fe. Such a large difference explains the higher growth rate and thickness of films formed on Fe at the higher temperature. It is also in agreement with the higher defect content and lower field strengths in high-temperature films. The application of the MCM to Fe-Cr alloys indicates that the diffusion coefficient of major ionic current carriers is smaller for the alloys than for pure Fe. Alloying with Cr thus lowers the ionic mobility in the passive film on a ferrous alloy also at 200 °C.     

Passivation Behavior of 2507 Super Duplex Stainless Steel in Hot Concentrated Seawater: Influence of Temperature and Seawater Concentration

The electrochemical behavior and passivation properties of 2507 super duplex stainless steel (SDSS) in the hot concentrated seawater with different concentrations at different temperatures are characterized by electrochemical methods and X-ray photoelectron spectroscopy. The passive film formed on 2507 SDSS belongs to n-type semiconductor and mainly composed of hydroxides and oxides of Fe and Cr under all circumstances. The protectiveness of the passive film decreases with the increase in temperature mainly because of the increase in defect density in the passive film. The corrosion resistance of 2507 decreases with the increase in seawater concentration, and the critical pitting temperature also decreases.     

Semiconductor properties and protective role of passive films of iron base alloys

Semiconductor properties of passive films formed on the Fe-18Cr alloy in a borate buffer solution (pH = 8.4) and 0.1 M H₂SO₄ solution were examined using a photoelectrochemical spectroscopy and an electrochemical impedance spectroscopy. Photo current reveals two photo action spectra that derived from outer hydroxide and inner oxide layers. A typical n-type semiconductor behaviour is observed by both photo current and impedance for the passive films formed in the borate buffer solution. On the other hand, a negative photo current generated, the absolute value of which decreased as applied potential increased in the sulfuric acid solution. This indicates that the passive film behaves as a p-type semiconductor. However, Mott-Schottky plot revealed the typical n-type semiconductor property. It is concluded that the passive film on the Fe-18Cr alloy formed in the borate buffer solution is composed of both n-type outer hydroxide and inner oxide layers. On the other hand, the passive film of the Fe-18Cr alloy in the sulphuric acid consists of p-type oxide and n-type hydroxide layers. The behaviour of passive film growth and corrosion was discussed in terms of the electronic structure in the passive film.     

Comparison Study on the Semiconductive and Dissolution Behaviour of 316L and Alloy 625 in Hydrochloric Acid Solution

The corrosion behaviour of 316 L and Alloy 625 was investigated using cyclic polarization,electrochemical impedance spectroscopy,X-ray photoelectron spectroscopy,Auger electron spectroscopy and induced coupled plasma-optical emis sion spectrometer.The results indicated that Alloy 625 showed better corrosion resistance than 316 L and the prolonging immersion time could enhance corrosion resistance of the two alloys.The passive film formed on the surface of 316 L exhibited an electronic structure of p-p heterojunction,with Fe_3 O_4 and Cr_2 O_3 enriched in the outer and inner layers,respectively.However,Alloy 625 presented the electronic structure of n-p heterojunction dominated by the outer Fe_2 O_3/NiFe_2 O_4 and inner Cr_2 O_3.This resulted in the opposite semiconductive properties of the passive films formed on the two materials.In the acid solutions,Fe and Mo suffered from selective dissolution while Cr and Ni were relatively stable.The corrosion rates were mainly dominated by the dissolution of iron.Alloy 625 presented better corrosion resistance than 316 L due to the obviously lower content of Fe and the higher content of Cr and Ni in the passive film.The continuously selective dissolution of iron resulted in the increase in Cr/Fe ratio in the passive film,which was responsible for the enhancement in corrosion resistance.     

Passivity of 316L stainless steel in borate buffer solution studied by Mott-Schottky analysis, atomic absorption spectrometry and X-ray photoelectron spectroscopy

The passivity of 316L stainless steel in borate buffer solution has been investigated by Mott-Schottky, atomic absorption spectrometry (AAS) and X-ray photoelectron spectroscopy (XPS). The results indicate that the polarization curve in the passive region possesses several turning potentials (0 V_SCE, 0.2 V_SCE, 0.4 V_SCE, 0.6 V_SCE and 0.85 V_SCE). The passive films formed at turning potentials perform different electrochemical and semiconductor properties. Further, the compositions of the passive films formed at turning potentials are investigated. The results reasonably explain why these potentials appear in the passive region and why specimens perform different properties at turning potentials.     

Microstructure and electrochemical properties of high entropy alloys—a comparison with type-304 stainless steel

High entropy alloys (HEAs) are a newly developed family of multi-component glassy alloys composed of several major alloying elements, such as copper, nickel, aluminum, cobalt, chromium, iron, silicon, titanium, etc. The HEA studied had a nearly amorphous structure as proven by X-ray diffraction (XRD), selected area diffraction (SAD), and differential scanning calorimetry (DSC) analysis. The dendritic phase was composed mainly of a non-crystalline phase with a little body centered cubic (BCC) structure whereas the interdendritic phase had an amorphous structure containing small amounts of nano-scale precipitates. The HEA had a high degree of atomic disorder with mechanical properties comparable to that of glass and it was therefore hard but brittle. Its hardness (Hv860) was much higher than that of type-304 stainless steel (Hv265). The anodic polarization curves of the HEA, obtained in aqueous solutions of NaCl and H₂SO₄, clearly indicated that the general corrosion resistance of the HEA at ambient temperature (∼25 °C) is superior to that of 304S, irrespective of the concentration of electrolyte in the range 0.1-1 M. On the other hand, the HEA’s resistance to pitting corrosion in a Cl⁻ environment is inferior to that of 304S, as indicated by a lower pitting potential and a narrower passive region for the HEA. Tests in 1 N sulfuric acid containing different concentrations of chloride ions showed that the HEA has least resistance to general corrosion at a chloride ion concentration of 0.5 M (close to the concentration in seawater). The lack of hysteresis in cyclic polarization tests confirmed that the HEA—like 304S—is not susceptible to pitting corrosion in chloride-free 1 N H₂SO₄.     

Electrochemistry of conductive polymers 44: A comparative study on electrochemically polymerized polythiophenes from thiophene, bithiophene, and terthiophene

Polythiophenes obtained by electrochemical polymerization of thiophene, 2,2′-bithiophene, 2,2′:5′,2″-terthiophene in 1.0 M LiClO₄ propylene carbonate solutions have been studied using various experimental techniques including UV–vis absorption spectroscopic, electrochemical quartz crystal microbalance, and current sensing atomic force microscopic (CS-AFM) experiments. The polythiophene films were obtained by repeated potentiodynamic voltage scans on gold electrodes. Clear differences were observed from the polymer films obtained from the monomers in their electrical, electrochemical, and optical properties due to various factors such as degradation of the polymer at high anodic potentials, conjugation lengths of polymers, incorporated oligomers, and solvated ion pairs in the polymer matrix. The poly(bithiophene) displayed the best conductivity, while the polythiophene showed almost no electrical conductivity.