Rearrangement of Oxide Scale on Ni-11Fe-10Cu-6Al Pre-Oxidized at 950 °C during Anodic Polarization in Molten Carbonate

The effect of anodic polarization in molten Na2CO3-K2CO3 at 750 °C was investigated on the structure of oxide scale formed by pre-oxidation of Ni-11Fe-10Cu-6Al alloy at 950 °C in air. The pre-formed oxide scale evolves and rearranges under anodic polarization related to melt corrosion and non-uniformly distributed electric field. Both of pre-oxidized and as-rearranged electrodes can serve as inert anodes with oxygen evolution. Anodic polarization exhibits a negative rearrangement-destructivity effect for the pre-formed oxide scale with corrosion protection of the rearranged oxide scale decreasing. The structure rearrangement of pre-formed oxide scale is also discussed during anodic polarization in the melt.

Electrode processes during electrodeposition, electropolishing and oxidation of niobium

The electrode processes occurring during the electrodeposition, electropolishing and oxidation of niobium are considered. The discharge of Nb(IV) complexes during Nb electrodeposition was studied by cyclic voltammetry. The anodic polarization curve on niobium in a mixture of acids H2SO4:HF (9:1) was obtained by chronopotentiometry method, the potential range at which the highest quality and speed of electropolishing is achieved was found. The film formation mechanism of niobium pentoxide Nb2O5 on niobium was studied by cyclic voltammetry.

Selection of rational electrochemical methods for controlling the efficiency of machining of corrosion-resistant steel

The article discusses methods of controlling the processes of mechanical processing based on electrochemical effects. The corresponding anodic polarization curves of 1X18H9T steel obtained in electrolyte solutions without and with stirring are presented. The article discusses methods of machining processes control based on electrochemical effects. Lubricating and cooling technological media (LCTM) used in machining are in most cases electrolytes, therefore, electrochemical processes and phenomena actively occur during contact dynamic machining. It is possible to control the processes of machining by acting on the system elements of the tool - LCTM- part, in particular by activating the LCTM and reducing the strength characteristics of the processed steel in the cutting zone. A reserve for increasing the efficiency of mechanical processing can be the composition selection of the applied LCTM, combined with the simultaneous electrochemical polarization of the treated surface of friction pair parts. It was found that when cutting, the efficiency of machining and the chip shapes are changed, which is explained by the influence of the current density on the strength of the processed steel. In the conditions of machining, complex dynamic processes occur due to the rotation of the work piece and/or tool, so it is necessary to take into account the hydrodynamic phenomena and processes that arise in this case. Electrode potentials are considered to be the most important characteristic of the metal cutting process. The potential of the system can regulate such processes and indicators as wear and surface micro hardness. Anodic polarization curves of the steel 1X18H9T obtained in various electrolyte solutions without stirring and with stirring on a rotating disk electrode are given. The study allowed determining the factors affecting the processes occurring in the cutting zone and to identify rational current densities due to simulating the conditions of real technological processes of the combined steel processing. The increase in the processing intensity of the steel 1X18H9T with the cutting zone polarization is associated with the action of factors activating the selective anodic dissolution of the processed alloy.

Pitting resistance evaluation of ternary Ni-Cr-Mo alloys in flue gas environment

After the appearance of pitting corrosion, the corrosion rate reaches very high values, which leads to a deterioration in the quality of the product in a short time. It may happen that the pitting does not continue its penetrating effect: if there are no necessary conditions for its growth, a pitting of greater activity is not formed, which will absorb all the current supplied from the area surrounding the cathode. Results of potenciodynamic investigations of four Ni‐base alloys are presented. The tests were conducted in 1 percent sulphuric acid, containing 0.2 percent chlorides at temperature 353K. Gravimetric test, performed in the same conditions, revealed excellent properties of alloy signed A3. Pitting corrosion of alloy A4 at the test conditions after long exposure at 353K was observed and was confirmed by the applied tests. The multiple anodic polarization (MAP) method is proposed to control alloys’ susceptibility to pitting corrosion.

NIOBIUM’S BEHAVIOR IN AQUEOUS HYDRO­FLUORIC ACID SOLUTION

Thanks to the unique combination of physicochemical properties, niobium and its compounds are widely used in various fields of science and technology. The main areas of niobium’s applications are the production of superconductors, nuclear energy, chemical engineering, metallurgy, manufacture of optically active materials, thin-film lithium batte­ries, fuel cells. The aim of this work is to study the processes that take place on the niobium electrode in aqueous solutions of hydrofluoric acid, as well as to establish the composition of niobium compounds that are formed. The paper presents the results of studies the behavior of the niobium electrode in aqueous solutions 0.25 N. hydrofluoric acid. The kinetic para­meters of the processes occurring at the phase boundary are determined. It was found that the anodic polarization of the niobium electrode is accompanied by the formation of a passive layer, the destruction of which is facilitated by increasing the polarization potential and fluorine anions, in the presence of which complex fluoroiobate anions [NbF7]2- and [NbOF5]2-are formed. Cathodic polarization of niobium is accompanied by the formation of hydrides on its surface, which causes an increase in the overvoltage of hydrogen evolution. The anodic polarization of the niobium electrode in a solution of hydrofluoric acid causes the formation on its surface of a passive layer, which is destroyed with increasing potential. In the Nbo–NbO2–0.25 –0.25 n HF system, [NbF7]2-anions are formed, as evidenced by bands in the region of 500 nm on the electron absorption spectra. The rate constants of [NbF7]2- and [NbOF5]2- formation are estimated at 3.78 • 10-3 s-1 and 5.18 • 10-3 s-1, respectively. The reduction of hydrogen at the niobium cathode from a solution of hydrofluoric acid is accompanied by the formation of hydrides, which causes an increase in the overvoltage of hydrogen evolution and high values of the angular coefficients of the Tafel dependence.

Controlling Atmospheric Corrosion of Weathering Steel Using Anodic Polarization Protection Technique

The atmospheric corrosion of weathering steels varies as a function of geographic zone, season, and other environmental variables related to that region which the experiments have been done. Meanwhile, rusting is a continuous process, and it is the main corrosion product of atmospheric corrosion. The current study investigates the effects of rust on weathering steel in the localized region of Digha, a sea resort of West Bengal, India. The investigations have been performed by purposely accelerating the rusting of weathering steel in a laboratory within one week in order to simulate approximately 18 months of actual rusting that can be achieved at field exposure. Anodic polarization of weathering steel comparable to potentiostatic passivation is obtained by shorting weathering steel with nobler metals, such as copper or graphite. The effect of rust formation on corrosion resistance after being immersed in 0.01 M KCl solutions for polished and unpolished samples has been investigated using electrochemical techniques, such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The rusted surfaces’ morphology and composition were characterized using field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray analysis (EDX). Based on the obtained results, it is concluded that the progressive rusting of weathering steel leads to a decrease in corrosion rate.