Zoran Grubač

The study of aluminium corrosion in acidic solution with nontoxic inhibitors

The inhibitory activity of some substituted N-arylpyrroles on aluminium corrosion in hydrochloric acid was studied in relation to inhibitor concentration, using potentiodynamic and impedance spectroscopy techniques. All investigated compounds were found to act as cathodic-type inhibitors and inhibition was ascribed to the adsorption of inhibitor onto the electrode surface. The inhibiting efficiency of the additives depended on the inductive power of the groups attached to the benzene and/or pyrrole ring. The carbaldehyde group showed better inhibiting power due to additional condensation on the electrode surface. The impedance results analysed in terms of the polarization resistance showed that the EIS technique can be successfully applied in the determination of corrosion resistance in systems where the corrosion kinetics are not simple.

Impedance investigation of corrosion inhibition of armco iron by thiourea

The inhibitive action of thiourea on the corrosive behaviour of ARMCO iron was investigated in deaerated 0.5 M H2SO4 solution, by means of electrochemical impedance spectroscopy. The inhibitor effectiveness increases with concentration, reaches a maximum (at about 1 mm) and then decreases. The adsorptive behaviour of thiourea on the electrode surface up to its peak follows a Frumkin-type isotherm with lateral repulsion, where the molecules are vertically adsorbed on the iron surface via the sulfur atom. Thiourea acts as a mixed inhibitor up to the critical concentration. It decreases the dissolution of iron and the hydrogen evolution reaction by blocking the electrode surface. The free energy of adsorption ΔGad = −39 kJ mol−1 and the attraction constant a = −4.4.

Impedance spectroscopic study of aluminium and Al-alloys in acid solution: inhibitory action of nitrogen containing compounds

The adsorption behaviour of triethanolamine (TEA) on aluminium, AlMgSi and AlZnMg alloys covered with naturally formed oxide films was investigated in 0.5 M NaCl solution, pH 1.3, by means of potentiodynamic and electrochemical impedance spectroscopy techniques. The results of polarization measurements show that in all cases the addition of TEA induces a decrease in the cathodic currents without affecting the anodic polarization behaviour, and, accordingly, TEA can be treated as a cathodic type inhibitor. The adsorption behaviour of TEA on the electrode surface follows a Frumkin isotherm with constants:a = 1.74,B = 18.41 dm3 mol−1. The results of impedance measurements presented in Nyquist plots show a high frequency capacitive loop related to the dielectric properties of the oxide film and a low-frequency inductive part which was determined by the faradaic process and attributed to the localized corrosion.

Corrosion protection of aluminium in acidic chloride solutions with nontoxic inhibitors

Corrosion kinetics of 99.6% aluminium covered by a thin spontaneously formed oxide film in hydrochloric acid solution with and without the presence of substituted N-aryl pyrroles was studied using electrochemical impedance spectroscopy and quasi steady-state polarization. Measurements were performed on a rotating disc electrode in an argon-deaerated solution in the temperature range 20 to 50°C. The addition of inhibitor considerably increases overvoltage of the cathodic process (HER) and shifts Ecorr to negative potential values. The activation energy of the hydrogen evolution reaction was Ea=50±5kJmol−1 and was not affected by the presence of inhibitor. The inhibitory action occurs by π-bonding between the adsorbed inhibitor molecules and the electrode surface. The electrode coverage follows the Langmuir adsorption isotherm with an adsorption equilibrium constant K=1.1–2.64×105dm3mol−1. The adsorption of organic compound prevents the adsorption of chloride ions and slow down the rate of corrosion.

The corrosion behavior of sputter-deposited aluminum-tungsten alloys

Abstract Thin films on aluminum–tungsten alloys were prepared by co-deposition of pure aluminum and pure tungsten, each sputtered by an independently controlled magnetron source, on glass and sapphire substrates. Completely amorphous films were obtained in the Al 80 W 20 –Al 67 W 33 composition range. Passivity and corrosion behavior of amorphous Al–W alloys were investigated in 1 M deaerated hydrochloric acid solution using polarization and impedance spectroscopy measurements and have been correlated with the properties of pure alloy components. Tungsten and sputter-deposited Al–W thin films are inherently passive materials while aluminum undergoes pitting corrosion in hydrochloric acid solution. The passive film formed at the OCP on each alloy possesses excellent electric and dielectric properties comparable to those of the isolating film on tungsten. The absolute impedance increases with increasing tungsten content in the alloy. According to electrochemical polarization measurements, alloying Al with W in solid solution significantly enhances the materials resistance to pitting corrosion by shifting the breakdown potential above 2000 mV (Al 67 W 33 ) and lowering the corrosion rate at the OCP by more than two orders of magnitude. The most likely mechanism explaining the passivity of amorphous Al–W alloys, the Solute Vacancy Interaction Model (SVIM), involves the formation of complexes between highly oxidized solute atoms (W +6 ) and mobile cation vacancies, which restrict the transport of Cl − through the oxide film and inhibit its breakdown in hydrochloric acid solution. The role that film stress relaxation effects and microscopic defects in amorphous Al–W films, of the some composition, and deposited on various substrates play in their corrosion resistance is discussed.

Electrodeposition of thin sulfide films: nucleation and growth observed for Bi2S3

The potentiostatic pulse technique proved to be the most powerful electrochemical technique for in situ studies of the early stages of Bi2S3 electrodeposition on bismuth. The current–time response recorded at the overpotentials in the vicinity of Er (Bi2S3/Bi) in dependence of hydrogen sulfide ion concentrations, was described by a theoretical expression for three-dimensional progressive nucleation and growth under diffusion control. Through the application of relations which hold true for such a nucleation model, the following parameters were determined: (i) the anodic nucleation potential (EAN); (ii) the rate of nucleation (AN∞); (iii) the diffusion coefficient (D); and (iv) the density of the growing sites (Ns).

Application of substituted N-arylpyrroles in the corrosion protection of aluminium in hydrochloric acid

The inhibiting action of N-arylpyrroles on aluminium in 0.17 mol dm−3 hydrochloric acid solution, in the temperature range 20–60 °C, was studied using potentiodynamic and electrochemical impedence spectroscopy techniques. The inhibiting efficiency of both investigated compounds 1-(2-fluorophenyl)-2,5-dimethylpyrrole (compound A) and 1-(2-fluorophenyl)-2,5-dimethylpyrrole-3-carbaldehyde (compound B) slightly increases with increasing temperature of the corrosive solution. Inhibitor adsorption on the elecrode surface decreases the apparent activation energy of the hydrogen evolution reaction. Compound A follows the Temkin adsorption isotherm, while its carbaldehyde derivative follows the Langmuir isotherm. The results of the apparent energy of activation and the standard free energy of adsorption point to stronger chemisorption of the compound A. However, due to additional condensation of molecules with carbaldehyde groups on the electrode surface, the carbaldehyde derivative exhibits even better inhibiting efficiency than compound A. The kinetic corrosion parameters, analysed in terms of the impedance data, show a satisfactory agreement with those obtained by the potentiodynamic method.

Nucleation and growth of anodic oxide films on bismuth

Abstract Combined voltammetry and potentiostatic transient techniques have been used to study nucleation, formation and growth of thin oxide films on high purity polycrystalline bismuth in a borate buffer solution, pH=9.2. It was shown that the initial step in the formation of the continuous anodic layer of bismuth oxide on bismuth is a nucleation process. The potentiostatic technique was a valuable tool in its study. The oxide film nucleation kinetics were explained reasonably well through a 3D progressive nucleation and growth mechanism under diffusion controlled growth. Nucleation potential ( E AN ), steady-state nucleation rate ( AN 0 ) and the number density of growing centres ( N s ) are determined. A detailed mechanistic interpretation of the nucleation process and thickening of the anodic layer under potentiodynamic conditions was obtained using the criteria of cyclic voltammetry.

Functionalization of biodegradable magnesium alloy implants with alkylphosphonate self-assembled films

Mg and Mg-alloys are promising materials for biodegradable implants. In order to slowdown the Mg-alloy (AZ91D) degradation and enhance its biocompatibility, the alloy surface was modified with alkylphosphonate self-assembling films. The binding configuration and the structural organization of alkylphosphonate monolayers on the Mg-alloy surface were investigated using contact angle measurements, FTIR, and XPS. Combination of FTIR and XPS data indicated the presence of several different bonding modes (mono-, di-, and tri dentate) of phosphonate head groups with the alloy surface. The existence of well organized and ordered self-assembled alkylphosphonate monolayers with good barrier protecting properties in a physiological solution is a key step in the development of biocompatible Mg-alloy implants.

Kinetics and mechanism of electrocrystallization of bismuth in oxide matrix

The kinetics and mechanism of cathodic decomposition of potentiodynamically formed Bi2O3 layer in a borate buffer solution, pH 9.2, were studied in-situ using cyclic voltammetry, potentiostatic transient and electrochemical impedance spectroscopy (EIS) techniques. The electrocrystallization of bismuth in the matrix of the oxide film occurred by injection of electrons from the underlying metal into the film/electrolyte interface. Impedance spectra demonstrated that the film/electrolyte interface had a close-to-perfect RC feature, because of intrinsically good electronic conductivity of bismuth oxide. Potentiostatic current–time transients revealed more details of the reduction process, giving further insight into the mechanism of reductive decomposition of Bi2O3 up to metallic Bi. The initial stage of reductive decomposition of Bi2O3 at relatively low cathodic potentials (close to the reversible potential of the Bi2O3/Bi couple) was described by equations valid for progressive nucleation and 3D growth mechanism under charge transfer control. At higher negative potentials, where the metal phase segregation took place, the electroreduction was described by a complex equation which included progressive nucleation and 3D growth mechanism under charge transfer control, as well as instantaneous nucleation and 2D growth mechanism controlled by diffusion of redissolving OH− ions away from the reacting interface. A local increase in the pH at the interface from pH 9.2 up to pH 11.14, during the diffusion stage of OH− ions resulting from the electroreduction process, the density of nuclei instantaneously formed (N0), the rate constant of crystal growth parallel to the electrode surface (k1) and the roughness factor (σ), have been determined.