Effect of pH on the formation-dissolution processes of anodic films on bismuth

Interrupted polarization measurements indicate that among Na₂SO₄, buffer (pH 7) and NaOH, the most passivating anodic film is formed on bismuth in the buffer solution. Under opencircuit conditions, film growth occurs on the metal surface in buffer solutions of pH 1.81 to 11.25. The rate of growth increases with decrease in pH. The passive metal undergoes dissolution in buffer solutions according to a zero-order mechanism; the most effective pH values for film dissolution lie between 8 and 10. The results indicate the simultaneous importance of the anion type and pH of the electrolyte in determining anodic film stability.     

Role of some thiadiazole derivatives as inhibitors for the corrosion of C-steel in 1 M H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

Inhibition of C-steel corrosion by some thiadiazole derivatives (I–VI) in 1 M H₂SO₄ was investigated by weight loss, potentiodynamic polarization, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) techniques. The presence of these compounds in the solution decreases the double layer capacitance, increases the charge transfer resistance and increase of linear polarization. Polarization studies were carried out at room temperature, and showed that all the compounds studied are mixed type inhibitors with a slight predominance of cathodic character. The effect of temperature on corrosion inhibition has been studied and the thermodynamic activation and adsorption parameters were calculated and discussed. Electrochemical impedance was used to investigate the mechanism of corrosion inhibition. The adsorption of the compounds on C-steel was found to obey Langmuir’s adsorption isotherm. The synergistic effect brought about by combination of the inhibitors and KSCN, KI and KBr was examined and explained. The mechanism of inhibition process was discussed in the light of the chemical structure and quantum-chemical calculations of the investigated inhibitors.     

Influence of halides on the dissolution and passivation behavior of AZ91D magnesium alloy in aqueous solutions

The electrochemical behavior of AZ91D in various aqueous sodium halide solutions was investigated using open-circuit potential (E_oc), potentiodynamic polarization and ac impedance (EIS) techniques. Generally, the results reveal that during immersion a protective layer of a salt film is formed on the alloy surface whose passivation performance depends on the halide nature, its concentration and temperature. E_oc shifts positively with time until attaining a steady (E_st) value, which becomes less noble with increasing concentration or temperature of the test solution. At any given conditions, self-passivation was found to be favored in the order F⁻ > I⁻ > Br⁻ > Cl⁻. This sequence is consistent with that for surface film resistance (R_T) and its relative thickness (1/C_T). Nevertheless, in F⁻ medium each of the above parameters increases with [F⁻] up to a critical value of 0.3 M then decreases. Increasing concentration above 0.3 M induces large change in the microstructure of the outermost layer of the fluorinated extremely protective film and depassivation behavior predominates. In Br⁻ and I⁻ solutions, as well as the lower Cl⁻ concentrations (≤0.01 M), AZ91D exhibits pseudo-passive state over the polarization range from the corrosion potential (E_corr) to the knee point (E_pt) in the anodic scan, at which passivity breakdown occurs with rapid increase in the anodic current and hydrogen gas reaction. At Cl⁻ concentrations >0.01 M the negative difference effect (NDE) occurs under cathodic polarization where E_pt lies negative to E_corr. Addition of F⁻ to the Cl⁻ solution can induce large changes in the behavior of AZ91D. Equal concentration addition (1:1) produces the highest propensity of the surface to form passivating layer that can afford better protection.     

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 Na₂SO₄ 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 SO₄²⁻ 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 SO₄²⁻ concentration.     

Investigation on the corrosion and hydrogen evolution for AZ91D magnesium alloy in single and anion-containing oxalate solutions

Corrosion characterization of AZ91D alloy was studied in aqueous sodium oxalate solutions with various concentrations using different electrochemical techniques (open circuit potential, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV)). The corrosion rate and consequently the rate and extent of hydrogen evolution were found to increase significantly with increasing oxalate anion concentration and temperature or with decreasing the pH of solution. Increasing additions of various anions over the lower concentration range (0.001-1.0 mM) in the blank oxalate solution increases to a varying extent the corrosion rate of the alloy and hence increases the hydrogen evolution rate and decreases surface film stability in the following order Cl⁻ > SO₄²⁻ > F⁻. On the other hand, addition of phosphate anion exhibits a reverse trend, where the active corrosion rate decreases with increasing PO₄³⁻ anion concentration, implying that this anion acts as a passivator for AZ91D alloy. The obtained electrochemical results are further confirmed by scanning electron microscopy (SEM) analysis.     

Corrosion characterization of new tin-silver binary alloys in nitric acid solutions

Electrochemical techniques were used to characterize the corrosion behavior of four new binary alloys xSn-Ag (x = 26, 50, 70 and 96.5 wt%) alloys and their individual metal components in nitric acid solutions. The experimental data were collected by using open-circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Over the concentration range studied (0.075-4.5 M), each of the two corrosion parameters (E_corr and i_corr) shows a regular dependency on both the alloy composition and the solution concentration. In general, for all studied samples, especially pure Ag and those with lower Sn contents (26 and 50 wt%), increasing the acid concentration increases i_corr, meanwhile causes a shift of the corresponding E_corr towards more positive values. This is probably due to the increase in the effect of cathodic depolarizer as the nitric acid concentration is increased. EIS results at the free corrosion potential confirmed well this behavior, where at concentrations ?1.5 M the thickness of the surface film increases while its resistance decreases with increasing tin wt%, indicating formation of less protective thicker film. However, at higher concentrations all samples exhibit identical behavior.     

Electrochemical behaviour of Ti–6Al–4V alloy and Ti in azide and halide solutions

Electrochemical techniques including open circuit potential measurement, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion and passivation behaviour of Ti–6Al–4V alloy in sodium azide (NaN₃) solutions compared to the behaviour of its pure base metal Ti. The results showed that increasing azide concentration increases the rate of corrosion (i_corr) and shifts negatively the rest potential (E_f), as well as decreases the spontaneous thickening rates of the inner and outer layers constituting the passive oxide film on each sample. These effects are more accentuated for the alloy than for the metal. Moreover, the electrical resistance (R_ox) and the relative thickness (1/C_ox) of the oxide films on the two samples exhibit an almost linear decrease vs. NaN₃ concentration. The results suggested that addition of Al and V to Ti, although improves its machinability, yet it decreases the performance of its surface oxide film to protect the degradation of the metal. The alloy was found to be more susceptible to corrosion than its base metal, since Ti expresses higher apparent activation energy (E_a) for the corrosion process than Ti–6Al–4V. Electrochemical behaviour of Ti in azide medium was also compared with that in various halide solutions. It was found that Ti has a stronger propensity to form spontaneous passivating oxide layers in bromide more than in azide and other halide media, where the positive shift in the value of E_f and the simultaneous increase in the oxide film resistance (R_ox) follow the sequence: Br⁻ > > Cl⁻ > I⁻ > F⁻.     

Optimizing parameters affecting electroless Ni-P coatings on AZ91D magnesium alloy as corrosion protection barriers

Magnesium alloys are materials difficult to plate due to their high reactivity. The surface passive layer formed in air or water must be first removed prior the plating process. Generally, it was shown that electroless Ni–P coatings on AZ91D alloy are directly related to its surface pretreatment parameters and the various plating conditions. Micro-image morphology and chemical composition of the different coats were characterized by SEM, EDX, XRF and XRD techniques. Electrochemical polarization, porosity and adhesion tests were also performed to study the corrosion resistance of the samples. The plating step was accomplished either after a zincating treatment or directly after the fluoride activation treatment using basic nickel plating bath. Successful direct EN coatings obtained are good corrosion barriers for AZ91D alloy in aggressive environments.     

Impedance studies of the inhibitive effect of benzotriazole on the corrosion of copper in sodium chloride medium

Impedance studies of a copper electrode in chloride medium with or without benzotriazole (BTA) were made over a wide frequency range (10^?2?10²KHz). The impedance characteristics of the Cu-BTA surface film were established under various experimental conditions. The results indicate that the film was of a dielectric nature having a dielectric constant of ca. 20. It was found that the film thickness increased and the corrosion rate decreased by increasing the BTA-concentration or with ageing. It was assumed that, at the stationary state, the rate of dissolution of Cu-BTA film blanced that of the slow transport of Cu ions through the film. Good support for these results was obtained from the data of potentiostatic polarization and weight loss measurements.     

Formation and dissolution of anodic oxide films on zirconium in NaOH: Kinetic studies

The open circuit behaviour of a mechanically polished zirconium electrode in NaOH solution revealed that the oxide building or dissolution process depended on NaOH concentration. Anodic oxide films formed at and below 30 V are fairly stable in NaOH solutions below 1 M. The oxide film is highly unstable, however, when immersed in NaOH at concentrations higher than 1 M. In such cases, oxide dissolution occurs preferentially over oxide building although its extent is much lower if compared with that occurring in acid medium. The results are in accord with the duplex nature of the oxide formed on zirconium.