Maja Kliškić

Aqueous extract of Rosmarinus officinalis L. as inhibitor of Al-Mg alloy corrosion in chloride solution

The first neutral phenol subfraction of the aqueous extract of rosemary leaves was studied as corrosion inhibitor for the Al–2.5Mg alloy in a 3% NaCl solution at 25 ∘C. Experiments were carried out over a wide range of concentrations. The results show that the additive adsorbs on the alloy according to the Freundlich isotherm. The polarization curves show that the first neutral subfraction acts as a cathodic-type inhibitor. The impedance spectrum obtained was interpreted in terms of change in characteristics of the natural oxide film at the surface of the alloy. Equivalent circuits, which represent the system, are proposed and individual circuit elements are defined.

Study of passivation of Al and Al–Sn alloys in borate buffer solutions using electrochemical impedance spectroscopy

Abstract Properties of thin oxide films on Al and Al–Sn alloys (with Sn content of 0.02, 0.09, 0.20 and 0.40 wt.%) formed either naturally or anodically in borate buffer solutions were investigated by means of electrochemical impedance spectroscopy. Equivalent circuits have been proposed that completely illustrate the Al(Al–Sn alloy)/oxide film/electrolyte systems examined, and properties of oxide films were determined. The stability (thickness and resistance) of oxide films has been found to increase with increased Sn content in the alloy, with increased passivation potential, and with longer time of anodising. The increase in temperature of anodising significantly reduces impedance in systems observed.

Abnormal yields of hydrogen and the mechanism of its evolution during cathodic polarization of aluminium

Abstract The yield of hydrogen formed during cathodic polarization of aluminium in neutral salt solutions was measured volumetrically as a function of current density and temperature. It was established that at any current density above 0.5 mA cm−2 and at any temperature the yield is larger than 200%; implying that a simultaneous dissolution of aluminium is an inherent part of the cathodic process.

Impedance and transient study of aluminium barrier-type oxide films

The effect of the passivation potential on the properties of the barrier films formed on high purity aluminium has been examined. The current-time responses were recorded, the charge used during formation of the barrier film was determined, and the steady-state currents were discussed. Impedance measurements provided an insight into the characteristic sizes of barrier films. Equivalent circuits which illustrate the examined aluminium/oxide film/electrolyte systems were proposed and individual circuit elements were defined. The resistance and thickness of the barrier film were shown to increase linearly with increase in the passivation potential, while increase in solution pH caused a marked decrease in the total system impedance. The paper also defines current efficiency in the formation of the barrier film. The value of ∼20% indicates that the formation of barrier films on aluminium is a process that takes place with considerable current losses. The mechanism of barrier film growth has been examined within the framework of two models suggested by different authors. Both agreements with and differences from the theories proposed have been indicated.

Processes on aluminium on the negative side of the open-circuit potential

Abstract Cathodic current—time transients were recorded on high-purity aluminium immersed into a neutral 2 M NaCl solution, when subjected to a potential pulse from a potential around the open-circuit potential, to different negative values. Hydration of the oxide layer occurs at potentials more negative than −1700 mV vs. SCE, whereupon hydrogen is evolved at the metal—hydrated oxide interface. A linear Tafel function is obtained for the maximum current attained in that potential region with a slope of 110 mV dec−1 indicating that the Volmer reaction is the rate-determining step, and the exchange current density for hydrogen evolution at aluminium is estimated at 5 × 10−11 A cm−2. Anodic current responses to the return of the potential to the rest values were also recorded at three different time scales. The presence of some substances, formed at the more negative potentials, other than gaseous hydrogen, was detected and their dependence on potential and cathodic pulse duration is discussed.

Pyridine and its derivatives as inhibitors of aluminium corrosion in chloride solution

Pyridine and its selected derivatives (symmetric collidine and 2,5-dibrompyridine) have been studied as corrosion inhibitors for high purity aluminium in a 2m NaCl solution at 25°C and 35°C using d.c. polarization techniques. The investigations were carried out with a wide range of concentrations of the compounds examined (from 0.5×10−5m to 1×10−3m). The results show that the derivatives are adsorbed on aluminium according to the Freundlich isotherm. The values of the standard free energy of adsorption suggest that the adsorptive properties of the compounds under consideration approach the physical type. The polarization curves show that the compounds act as cathodic-type inhibitors.Pyridine and its selected derivatives (symmetric collidine and 2,5-dibrompyridine) have been studied as corrosion inhibitors for high purity aluminium in a 2m NaCl solution at 25°C and 35°C using d.c. polarization techniques. The investigations were carried out with a wide range of concentrations of the compounds examined (from 0.5×10−5m to 1×10−3m). The results show that the derivatives are adsorbed on aluminium according to the Freundlich isotherm. The values of the standard free energy of adsorption suggest that the adsorptive properties of the compounds under consideration approach the physical type. The polarization curves show that the compounds act as cathodic-type inhibitors.

Anodic film growth on aluminium and Al-Sn alloys in borate buffer solutions

Abstract The mechanism, kinetics of growth and properties of thin oxide films on Al and Al–Sn alloys (containing up to 0.40% Sn) in borate buffer solutions have been studied by means of potentiodynamic, galvanostatic and impedance techniques. The results obtained indicate that the growth of oxide films on Al and Al–Sn alloys takes place due to ionic conductivity under the influence of the high electric field through the oxide film according to an exponential law like on valve metals. The constants of the exponential law (A and B), ionic conductivity through the oxide film, field strength and half barrier width have been calculated. Impedance measurements provided an insight into the characteristic sizes of oxide films. The resistance and thickness of the oxide film has been found to increase with the increase in tin content in the alloy. The paper also defines the current efficiency in the formation of the oxide film. An increase of tin content in the alloy increases the value of current efficiency. It has been established that an Sn content of 0.02% in the alloy has no effect on the growth and properties of the oxide film on Al. However, an Sn content ≥0.09% has a significant effect on the anodic film growth on Al.

Behaviour of Al-Sn alloy on the negative side of the open-circuit potential

The influence of the addition of small quantities of tin to high-purity aluminium (99.999%) on its electrochemical behaviour at high cathodic potentials was investigated. Cathodic current-time transients were recorded on Al-Sn alloys immersed in neutral 2m NaCl, when subjected to a potential pulse from a potential around the open-circuit potential to different negative values. Hydration of the oxide layer occurs at potentials more negative than −1600 mV vs SCE (for alloys with 0.02% and 0.09% Sn), −1750 mV (Al−0.2% Sn) and −1900 mV (Al−0.4% Sn), whereupon hydrogen is evolved at the metal-hydrated oxide interface. Anodic current responses to the return of the potential to the rest values were also recorded, and the charges required for oxidation of the substances created during the cathodic processes are discussed.

CORROSION INHIBITION OF CuNi10Fe ALLOY WITH PHENOLIC ACIDS

The purpose of this study was to investigate the efficiency of two phenolic acids, namely p-hydroxybenzoic acid (HBA) and protocatechuic acid (PCA), as corrosion inhibitors for CuNi10Fe alloy in a 0.5 mol dm−3 NaCl solution. In these investigations, open circuit potential measurements, potentiodynamic polarization measurements, and the linear polarization method have been used. It was found that both phenolic acids had similar corrosion inhibition effects on the CuNi10Fe alloy dissolution. The inhibition efficiency (IE) increased with increase in inhibitor concentration. The polarization shows that both compounds functioned as cathodic corrosion inhibitors by adsorption on the surface of CuNi10Fe alloy according to the Freundlich adsorption isotherm. The numerical values of free energies of adsorption indicate physical adsorption of the compounds on the electrode surface. DFT-based quantum chemical computations and molecular dynamics (MD) simulations revealed identical electronic and adsorption structures for both HBA and PCA, which could account for the similarities in the experimentally observed inhibiting effects.

Some new studies of sacrificial anodes

Aluminium and its alloys play an important role as anodic materials in sacrificial protection. Many well-known zinc sacrificial anodes are prone to passivation, which reduces their protective activity. For that reason it was necessary to find an anodic material in which the passive state of the protector has been disrupted. Such capabilities have been found with aluminium when a small quantity of an alloying element is added to it, making the ever-present protective oxide film porous and providing small resistance to anodic dissolution of aluminium. The contents of the alloying element in the total alloy mass prepared out of technical purity aluminium (99, 8%) and the alloying element must be as small as possible so as not to affect the basic properties of pure aluminium. Aluminium is the anode of choice for saltwater environments. The purpose of this study is to examine the effect of addition of small quantities of alloying element, which will not affect the basic advantages of aluminium, electrochemical properties and homogeneity of the alloy with the commercial name Akalpro. Due to high output current densities, the Akalpro electrodes are designed so that the outer surface is as small as possible. This is an advantage when protecting parts of objects (e.g. ships) where it is important to ensure protection with small masses of protector. Akalpro does not sparkle when hit, and does not release significant quantities of hydrogen, so that it can be used without danger as sacrificial anode in ballast tanks, for transportation of oil and inflammable liquids.