Local electrochemical studies of the microstructural corrosion of AlCu4Mg1 as-cast aluminium alloy and influence of applied strain

The microstructure of AlCu4Mg1 as-cast aluminium alloy was first determined by means of field emission-scanning electron microscope with an integrated electron dispersion spectrometer, secondary ion mass spectroscopy and atomic force microscopy. Large precipitates (Al₂Cu, Al–Si–Mn–Fe–Cu, oxides) were located at grain boundaries, whereas small particles (aluminium, magnesium and copper) were present in grains. The electrochemical response and pitting susceptibility of sites containing precipitates were then investigated after polishing using the electrochemical microcell technique. After straining, big scattering was observed in the electrochemical response. The most active places corresponded to the sites containing wide microcracks and severe damages in the matrix. In this case, the corrosion potential was around −1,000 mV vs. Ag/AgCl, and the current in the passive domain was five times higher than on the strained matrix. In the absence of severe damage in the matrix or wide microcracks, the corrosion potential was more anodic and the current density in the passive range was around 0.5 mA cm⁻². Local polarisation curves carried out in sites containing large precipitates and no defects induced by straining were very close to those obtained in grains far from precipitates.     

The inhibition mechanism of pitting corrosion of pure aluminum by nitrate and sulfate ions in neutral chloride solution

The inhibition mechanism of pitting corrosion of natural oxide film-covered pure aluminum by NO⁻ ₃ and SO²⁻ ₄ ions has been examined in 0.1 M NaCl solution using potentiodynamic polarization experiments, a.c. impedance spectroscopy, Auger electron spectroscopy and a combination of the potentiostatic current transient method and optical microscopy. It was found that NO⁻ ₃ ions can be incorporated into the natural oxide film on pure aluminum at open-circuit potential, but the incorporation of SO²⁻ ₄ ions into the film hardly occurs. The incorporation of NO⁻ ₃ ions lowered the pitting susceptibility of pure aluminum in 0.1 M NaCl solution. Based upon the experimental results, it is suggested that the pitting corrosion inhibition mechanism by anions can be classified into two different groups: inhibition by competitive adsorption of anions (SO²⁻ ₄) with Cl⁻ ions and inhibition by the incorporation of anions (NO⁻ ₃) into the film rather than competitive adsorption. Both cases may impede the incorporation of Cl⁻ ions into the film, thus inhibiting pitting corrosion of natural oxide film-covered pure aluminum in chloride solutions. Received: 16 October 1998 / Accepted: 6 January 1999     

Performance of some thiophene derivatives as corrosion inhibitors for 304 stainless steel in aqueous solutions

The inhibitive effect of 2-cyano-3-hydroxy-4(Ar)-5-anilino thiophene derivatives on the corrosion of 304 stainless steel (SS) in 3 M HCl solution has been investigated by weight loss, galvanostatic polarization techniques, and potentiodynamic anodic polarization in 3.5 % NaCl. The results indicate that these compounds act as inhibitors retarding the anodic and cathodic corrosion reactions. The presence of inhibitors does not change the mechanism of either hydrogen evolution reaction or SS dissolution. The activation energy and some thermodynamic parameters are calculated and discussed. These compounds are mixed-type inhibitors in the acid solution, and their adsorption on the SS surface is found to obey the Temkin adsorption isotherm. The results suggest that the percentage inhibition of these thiophene derivatives increases with increasing inhibitor concentration and decreases with increasing temperature. The synergistic parameter (S) was calculated and found to have a value greater than unity, indicating that the enhanced inhibition efficiency caused by the addition of I^?, SCN^?, and Br^? is only due to a synergistic effect. The relationship between molecular structure and inhibition efficiency was elucidated by quantum-chemical calculations using semi-empirical self-consistent field (SCF) methods.     

Layered double hydroxides as supports for intercalation and sustained release of antihypertensive drugs

Zn/Al layered double hydroxides (LDHs) were intercalated with the anionic antihypertensive drugs Enalpril, Lisinopril, Captopril and Ramipril by using coprecipitation or ion-exchange technique. TG-MS analyses suggested that the thermal stability of Ena⁻, Lis⁻ (arranged with monolayer, resulted from X-ray diffraction (XRD) and Fourier transform infrared spectra (FT-IR) analysis was enhanced much more than Cap^- and Ram^- (arranged with bilayer). The release studies show that the release rate of all samples markedly decreased in both pH 4.25 and 7.45. However, the release time of Ena^-, Lis^- were much longer compared with Cap^-, Ram^- in both pH 4.25 and 7.45, it is possible that the intercalated guests, arranged with monolayer in the interlayer, show lesser repulsive force and strong affinity with the LDH layers. And the release data followed both the Higuchi-square-root law and the first-order equation well. Based on the analysis of batch release, intercalated structural models as well as the TG-DTA results, we conclude that for drug-LDH, stronger the affinity between intercalated anions and the layers is, better the thermal property and the stability to the acid attack of drug-LDH, and the intercalated anions are easier apt to monolayer arrangement within the interlayer, were presented.     

Electrochemical and computational studies of an expired vilazodone Drug as environmentally safe corrosion inhibitor for aluminum in chloride medium

Through using chemical and electrochemical methods, the theoretical and experimental investigation of the expired vilazodone drug's ability to prevent corrosion on aluminium (Al) in a corrosive medium of HCl (1 M) has been examined. Weighing tests (WL), electrochemical (impedance spectroscopy (EIS), potentiodynamic polarization (PDP)), atomic force microscopy (AFM), scanning electron microscope (SEM), and Fourier transform infrared spectroscopy (FTIR) tests at 25 °C have all been used to investigate Vilazodone's capability to prevent corrosion of Al in 1 M HCl in the concentration in the range of 25–150 ppm. The corrosion inhibition effect of the investigate Vilazodone's against Al in acid environment was investigated weight loss and electrochemical methods. The highest % inhibition efficiency (%IE) was 95% resulted from weight loss technique at the highest concentration for inhibitor. According to the PDP data, this examined vilazodone function as a mixed-type inhibitor, impacting both the anodic and cathodic reactions. The inhibitors covered the active points of the metal surface, according to electrochemical impedance spectroscopy (EIS), to prevent corrosion. It was discovered that the inhibitor adsorption on the Al surface obeyed the Langmuir adsorption isothermal model. AFM, SEM, and FTIR surface examinations proved the inhibitor had a significant protective effect against Al dissolution in 1 M HCl. The outcomes from chemical and electrochemical methods are relatively consistent. Vilazodone acted as an effective corrosion inhibitor, according to all of the experimental data.     

Amino acids and their derivatives as corrosion inhibitors for metals and alloys

In the last two decades, research in the field of corrosion inhibitors had been directed toward the goal of using cheap effective molecules of low or non-negative environmental impact to replace the environmentally hazardous compounds. One of the encourager compounds which can be used as safe corrosion inhibitors are amino acids. They are environmentally friendly, non-toxic, biodegradable and relatively cheap. On other hand, the development of computational modeling helps to understand the inhibition mechanism of those compounds and to develop the newly designed inhibitors. In this review, most of contribution made in literature on the use of amino acids and their derivatives as corrosion inhibitors for metallic alloys materials were presented and discussed.     

Chromatographic separation and determination of Zr in bauxites and aluminum alloys

Summary A simple thin-layer chromatographic method for the complete separation of Zr from aluminum is described. The optimum solvent composition is evaluated by means of the simplex method of optimization. Quantitative determination and regression analysis is carried out.     

Energy of the surface segregation of Ag atoms in Ag–Au alloys in an aqueous solution

1. Download : Download high-res image (320KB)
2. Download : Download full-size image

     

Use of rare earth metal salt solutions for corrosion protection of aluminum alloys and mild steel

The rare earth metal salt (REMS) compounds which are non-toxic and inexpensive have been used successfully for the corrosion protection of commercial aluminum alloys as well as of mild steel. The resistance to localized corrosion of aluminum alloys such as Al 2024, Al 6061 and Al 7075 has been greatly improved by immersion in hot cerium salt solutions. Cerium oxides/hydroxides were formed at sites where intermetallic compounds containing copper were located and eliminated the formation of local cathodes. REMS solutions were also used for sealing of anodized layer on Al alloys replacing toxic chromates. In addition to increased corrosion resistance excellent paint adhesion was observed. Cerium nitrate and yttrium sulfate solutions produced the most satisfactory results. Cerium salts were also applied successfully as inhibitors and as pretreatment for the corrosion protection of carbon steel in hot NH₃/water solutions used in absorption heat pumps. Factorial design experiments have been used to determine the optimum concentrations of CeCl₃ and H₂O₂ as well as the treatment time used in the cerating process for mild steel. The corrosion protection provided by the cerated layer was further improved by cathodic polarization in cerium chloride. A dual corrosion protection strategy employing cerating and addition of a REMS to the working solution is expected to provide long-lasting corrosion protection of mild steel. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth. This article was submitted by the author in English.     

Apparent molar volumes and adiabatic compressibilities of aqueous solutions of amphiphilic drugs

The apparent molar volumes and adiabatic compressibilities of aqueous solutions of the amphiphilic tricyclic drugs, chlorpromazine, promethazine, promazine and imipramine have been determined from measurements of density and ultrasound velocity. Positive deviations of the apparent molar volume from the Debye-Hückel limiting law in dilute solution indicate possible premicellar association. The changes of molar volume and compressibility accompanying aggregate formation were appreciably smaller than those of typical surfactants, suggesting a more tightly packed aggregate. The magnitude of the increase in molar compressibility on micellisation of imipramine decreased with temperature rise between 20 and 35°C. The results are discussed in terms of the structure and hydration of the drug aggregates.     

The passivation current density as a parameter for a non-destructive test on plants of the pitting corrosion resistance of welded NiCrMo alloys

The test methods used so far for the pitting corrosion resistance of weldments of high-alloyed NiCrMo alloys are not suitable for a test directly on plants. Therefore, the passivation current density was qualified as a parameter for the pitting corrosion resistance and its correlation with critical pitting temperatures examined by varying the heat treatment, the weld heat input and the surface treatment. The measuring cell developed for this test can be fitted fast and safely on surfaces with an inclination up to 90° and on cambered surfaces (r≥750 mm). Afterwards, it can be loosened again without any deposits.Dedicated to Prof. Dr.-Ing. habil Dr. h.c. mult. Kurt Schwabe on the occasion of his 100th birthday.     

Kinetic analysis of photoelectrochemical hydrogen evolution over p-type silicon in acidic aqueous solutions of electrolytes

The kinetics of photoelectrochemical hydrogen evolution at p-Si single crystals in acidic aqueous solutions of electrolytes under pulse photoexcitation was studied. Despite a low stability of the silicon surface under the experimental conditions, a distinct interrelation between the characteristic time of interfacial charge transfer and stationary current was found. The determination of the characteristic transfer time does not need the detailed elaboration of generation-recombination processes in the semiconductor. The steady-state current density was shown to be determined both for the dark current and photocurrent by the surface charge density. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1724–1728, October, 2000.     

A fluorescent and colorimetric chemosensor for selective detection of aluminum in aqueous solution

We have synthesized a new Schiff base 1, which detects Al³⁺ through fluorescence and naked eye in aqueous solution. The sensor 1 exhibited selective and sensitive recognition toward Al³⁺ via significant fluorescence enhancement (31-fold). Moreover, it showed a significant color change from colorless to yellow. The complex formation was proposed to be 1:1 ratio, based on the Job plot, ESI-mass spectrometry analysis, ¹H NMR titration, and IR analysis. The detection limit was 1.00 μM, which is below the WHO acceptable limit (1.85 μM) in drinking water. In addition, the sensor 1 could be recyclable simply through treatment with a proper reagent such as EDTA.     

Spectroscopic studies of aluminum and gallium complexes with oxalate and malonate in aqueous solution

The local structures of Ga(III) in aqueous oxalate and malonate complexes were studied by means of Ga K-edge EXAFS spectroscopy. Irrespective of the number and type of coordinated ligands, the EXAFS results showed very regular first coordination shells consisting of six oxygen atoms. Scattering paths from more distant atoms revealed that both oxalate and malonate form mononuclear chelate structures where one oxygen from each carboxylate group binds to Ga(III). Again, very little variation in bond distances and no changes in coordination modes were detected as the number of ligands coordinated to Ga(III) was varied. Based on the very close resemblance of IR spectra of oxalate and malonate complexes of Al(III), and the corresponding complexes of Ga(III), it is believed that the local structures of the Al(III) complexes are similar to those of the Ga(III) complexes in terms of ligand coordination modes and distortions. This conclusion was corroborated by results from theoretical frequency calculations.     

Protection of aluminum hydroxide during lyophilisation as an adjuvant for freeze-dried vaccines

Lyophilisation of an aluminum hydroxide adjuvant containing formulation for use in vaccine development has been investigated. Novel formulation approaches using disaccharides, polysaccharides, polyols and polymers have been investigated as stabilising excipients to prevent aluminum hydroxide adjuvant from aggregation during a freeze–thaw cycle or the lyophilisation process itself. Amorphous character of the lyophilisates was investigated by wide angle X-ray diffraction. Sedimentation assay, particle size analysis and optical microscopy could show that aluminum adjuvant gel character of selected formulations could be maintained within acceptable quality or stayed the same in some cases. In principle, it could be demonstrated that not less than 10% excipient is necessary to yield good results within applied analytics evaluating the excipients’ protecting qualities. Based on results obtained by this study, the formulations containing trehalose at a concentration of 15 or 20%, dextran, PVP K 25, HES 450 and 200, saccharose and sorbitole along with polysorbate 80 and buffer promise to be good candidates for the protection of aluminum adjuvanted vaccines during exposure to coldness stresses as during freezing or lyophilisation.