U. Lelek-Borkowska

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.

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.