J. Banaś

Anodic dissolution of silicon monocrystals in anhydrous organic solutions of chlorides

Abstract The electrochemical investigations of p- and n-silicon monocrystals were performed in anhydrous organic solutions of LiClO 4 , LiCl and HCl to examine the possibility of etching and passivation of silicon semiconductors. The results obtained by means of linear sweep voltammetry (LSV), potentiostatic and galvanostatic transient technique, as well as XPS surface analysis allow us to give an explanation of the mechanism of silicon dissolution in these media. Silicon dissolves in anhydrous organic solvents (methanol, N , N -dimethyloformamide, formamide) which contain chloride ions according to the consecutive two-step mechanism. The Si(II) ad intermediate inhibits the anodic dissolution at low overpotentials. The presence of this intermediate was confirmed by means of XPS measurements on potentiostatically etched surface. The increase in chloride concentration in organic solvents stimulates desorption of the intermediate and therefore increases the rate of surface etching. The best results in anodic etching of silicon monocrystals have been obtained in anhydrous HCl solutions. Microscopic observations of surface morphology of Si monocrystals after etching show anisotropy of anodic dissolution.

Passivity of chromium in sulphuric acid solutions

The passivity of pure chromium has been studied in sulphuric acid solutions as a function of concentration (1–18 M), acidity, activity of water and the nature of the anions in the electrolyte. The passivity phenomena of this metal have been investigated using anodic polarization measurements and X-ray photoelectron spectroscopy. Chromium undergoes passivation in the whole area of the acid concentration. Its passivation parameters are closely connected with the structure of the electrolyte. In dilute solutions of acid (M < 6) the passivation of chromium proceeds with the participation of water molecules. The XPS measurements reveal that the average composition of the passive film is nearly equal to chromium (III) oxy—hydroxides with a small degree of hydration. The passivity of chromium in concentrated sulphuric acid proceeds with the participation of undissociated acid molecules or HSO−4 ions. The anodic layer formed in concentrated solutions also contains sulphur species in lower oxidation states ((S2−). The presence of these species shows that acid molecules undergo reduction in the passivation processes.

Passivity of metals in concentrated and anhydrous solutions of sulphuric acid

Abstract Passivity of iron, nickel and chromium has been investigated in anhydrous and concentrated solutions of sulphuric acid. The oxide-hydroxide film can be formed on the metal surface only in solutions with the molar ratio of acid to water lower than 1:4. In concentrated and anhydrous environments passivation proceeds with participation of undissociated acid molecules, which can become a source of oxygen necessary to oxide formation, and a thick oxide-salt layer is formed on the metal surface.

Anodisches verhalten von chrom in wasserfreien und wasserhaltigen dimethylformamidH2SO4-und methanolH2SO4-Lösungen

Zusammenfassung Auf der Chromoberflache bildet sich sowohl in wasserfreien als auch in wasserhaltigen organischen H 2 SO 4 -Losungen eine Schicht, die stark die Metallauflosung wahrend anodischer Polarisation hemmt. Die Passivierung des Chroms verlauft wahrscheinlich durch gleichzeitige Festkorperreaktionen zwischen Metall, Wasser und Sulfationen. In wasserfreien Losungen bildet sich auf der Chromoberflache eine amorphe Salzschicht. Mit wachsender Wasserkonzentration geht diese Schicht in eine Salz-Hydroxidschicht und schliesslich in eine Hydroxidschicht uber. Die kritische Wasserkonzentration, bei der die Passivschicht einen Hydroxidcharakter hat, wurde durch den Zusammenhang zwischen Passivierungspotential und Wasserkonzentration bestimmt.

The anodic properties of FeCr alloys in organic solvents

The influence of the water concentration on the passivation of FeCr alloys in sulfuric acid—organic solvents solutions has been investigated with the aid of potentiodynamic curves of the anodic polarisation. The dependence of the passivation potential or the break potential from the chromium concentration in the alloys and the water concentration in the solution has been determined. At low water concentrations the passivation potential decreases with increasing water content. This fact can be accounted for by the strengthening of the electrolyte structure and the resulting decrease of the solvolytic capabilities of solvents. In this range of water concentrations the salt—hydroxid type of the film is formed on the surface of the alloy. The increasing concentration of chromium in the alloy facilitates the formation of the films. At high water concentrations, when water molecules are used for the solvation of cations, only the alloys containing more than 18 per cent of Cr can form a stable anodic film below the passivation potential of iron. The passivation potential for the alloys which contain lower amounts of chromium has been found to increase as the water concentration increases. This is due to the fact that Fe+2 compounds from the anodic layer are exhibiting higher solubility in the solutions.

Electrochemical study of the corrosion behaviour of carbon steel in fracturing fluid

Corrosion behaviour of carbon steel (K-55) in fracturing fluid was studied with a rotation cylinder electrode, under static and rotation conditions by means of several electrochemical techniques which are as follows: open circuit potential (OCP) decay, potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). The corrosion rate was determined by weight loss measurements. The electrode surface after a prefixed immersion time was characterised by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that carbon steel showed anodic dissolution behaviour that increased under rotating condition. The cathodic polarisation current density also increased with the electrode rotation due to the increased oxygen diffusion on the electrode surface. Two different oxide layers were formed: a dark, thin layer of magnetite tightly adhering to the electrode surface, characterised by localised corrosion spots, and a porous reddish layer of poorly adhering hematite (Fe2O3) and maghemite (γ-Fe2O3). Under higher rotation rate, the developed oxide layer was not so stable, owing to the shear stress induced between the solution and the specimen surface, enhancing the corrosion rate.

The Effect of CO2 and H2S on the Passivation of Chromium and Stainless Steels in Aqueous Solutions at Elevated Temperature and under High Pressure

The effect of temperature, CO 2 and H 2 S pressure on passivation of chromium and on Fe-Cr and Fe-Cr-Ni-alloys was investigated in H 2 O-Na 2 SO 4 and H 2 O-NaCl solutions by means of linear sweep voltammetry, electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). The investigations have been performed in the temperature range 20–150°C under the pressure of 20 bars. The growth of the temperature from 20°C to 150°C increases the (Fe 2+ +Fe 3+ )/Cr 3+ and O 2- /OH − ratio in the passive film formed on Fe-Cr alloys in aqueous solutions of 0.1M Na 2 SO 4 . It seems that the presence of dissolved CO 2 also leads to dehydration of the passive film. The stability of passive layer formed on the surface of Fe-Cr and Fe-Cr-Ni alloys decreases in the solutions saturated with CO 2 and CO 2 + H 2 S mixture. The effect is more distinct for austenitic Fe-Cr-Ni alloys than for ferritic Fe-Cr ones. The presence of nickel deteriorates the formation of the passive film in solutions saturated with H 2 S and CO 2 + H 2 S mixture because of the high susceptibility of nickel to sulfide formation.

Inhibition Effect of Guar Gum on the Corrosion Behaviour of Carbon Steel (K-55) in Fracturing Fluid

The inhibition effect of guar gum on carbon steel (K-55) in the simulated fracturing fluid under static conditions was studied. The electrochemical experimental results indicated that the carbon steel shows active dissolution behaviour in the absence and the presence of the inhibitor. However, guar gum, acting as mixed-type inhibitor, was swiftly able to promote the formation of a product layer onto the metal surface, lowering both cathodic and anodic current density. Electrochemical impedance spectroscopy (EIS) measurements were consistent with the polarisation curves, showing an improvement of the corrosion resistance of the carbon steel in the presence of the inhibitor. However, for long exposure times, the corrosion activity of the metal increases, as result of the breakdown of the corrosion product layer and initiation of localised corrosion spot. After both polarisation and EIS measurements, two different layers were observed. A thin, dark stable layer of magnetite and, for long holding times, a porous red layer, poorly adhered, of hematite. The hematite layer, due to its porous nature, was characterised by poor corrosion ability.