S. M. Reshetnikov

Electrochemical properties of iron silicocarbide and cementite in acidic and neutral environments

Voluminous specimens of individual Fe5SiC iron silicocarbide and Fe3C iron carbide phases were produced by mechanical alloying with subsequent pressing and considered as models of nonmetallic inclusions in carbon steels and siliceous cast irons. In an acidic sulfate solution, silicocarbide is highly active in hydrogen reduction and iron ionization in the active dissolution range. Therefore, the corrosion resistance of silicocarbide in acids is lower compared to iron and cementite, which is caused by the peculiarities of its crystal structure. In a neutral borate solution both on silicocarbide and cementite, two anodic peaks are observed that are caused by the dissolution of the corresponding phase and the additional oxidation of the passive film, which is characterized by the heightened defectiveness because of the carbon accumulation. Silicocarbide has lower dissolution currents compared to cementite and a higher resistance to the local activation and depassivation, which is caused by the presence of a superficial layer enriched in SiO2.

Corrosion-electrochemical properties of α-Fe + Fe3C + TiC nanocomposites in neutral environments

The corrosion-electrochemical properties of α-Fe + Fe3C + TiC three-phase nanocrystalline composites in borate solutions with pH 6.3–9.0 both with and without NaCl are studied. α-Fe + Fe3C + TiC composites are found to have heightened resistances at active-oxidation potentials of α-Fe and cementite due to the formation of xFeO · yTiO2 mixed surface oxides. The protective properties of passive films based on xFeO · yTiO2 and their resistance to local activation are worse than those of Fe3O4/γ-Fe2O3 (γ-FeOOH) passive films formed on iron and α-Fe + Fe3C composites.

Electrochemical properties of α-Fe + Fe3C nanocrystalline composites in acidic environments

Regularities of the physicochemical behavior of α-Fe + Fe3C nanocomposites in a concentration range of cementite from 9 to 92 wt % in sulfate and chloride solutions are studied. Nanocrystalline state (∼40 nm) is found to intensify the active dissolution of α-Fe + Fe3C alloys and not noticeably affect the hydrogen evolution in acidic environments. With an increase in the cementite content or when it forms a reticular structure, the catalytic activity of α-Fe + Fe3C nanocrystalline composites with respect to the hydrogen evolution increases. Cementite manifests a higher overpotential of dissolution in both sulfate and chloride solutions.

The influence of the ultrafine-grained structure on passivation of copper

The influence of ultrafine-grained structure on passivation of copper in borate solutions was studied. Ultrafine-grained cooper was prepared by the equal channel angular extrusion. It was shown that, under the conditions of anodic oxidation, an oxide film with improved protective properties is formed on the ultrafine-grained sample. This film is characterized by a small thickness, low roughness, and high content of copper(I) oxide.

Cathodic evolution of hydrogen on carbides of iron-family metals

The activity of carbides of iron-family metals with respect to hydrogen evolution in acidic solutions is found to be much higher than the activity of the corresponding metals and increase in the sequence Ni3C < Co3C < Fe3C. Of the carbides studied, cementite has the highest activity, which is comparable to that of a Pt electrode.

Corrosion-Electrochemical Behavior of Composite Layers Produced by Laser Sintering of Nanoscale Iron-Nickel Powders

Corrosion-electrochemical behavior of composite layers formed by high-speed laser sintering of nanoscale Fe-Ni powders (3.2 and 10 wt % Ni) is studied with the use of anodic potentiodynamic curves recorded in a borate buffer solution in a neutral environment. Composite layers produced are shown to retain nanostructure properties of the original iron and nickel powders. The layers formed exhibit better passivation properties than do iron and nickel separately. Changes in the composition and properties of the surface layers formed by laser sintering before and after corrosion studies are confirmed by X-ray diffraction and X-ray photoelectron analyses.

The effect of the structure-phase state of iron-cementite nanocomposites on local activation processes

Studies of the local activation of iron-cementite nanocomposites (NC) revealed the step-wise increase in their resistance to pitting with an increase in the Fe3C Content (∼50 mass %) or with the formation of an ordered cementite network structure. The corrosion process is determined by the chemical state of carbon, namely, free or bound into carbides. The free carbon evolution leads to depassivation and the transition to structure-selective corrosion

Corrosion-electrochemical behavior of nanostructured chromium oxide layers obtained by laser irradiation of unalloyed steel by short pulses

Laser short-pulse treatment of ultradispersed powdered chromium(III) oxide supported on the surface of steel 20 was used to obtain nanosize surface layers containing chromium and iron oxides dispersed in α- and γ-iron, as well as mixed oxides with a FeO · Cr2O3 spinel structure. Reduced metallic chromium is also found. Measurement of anodic potentiodynamic curves in neutral and weakly basic borate buffer solutions shows that the formed surface layers promote enhancement of corrosion stability of steel due to transition of the surface into a passive state.

Passivation and local activation of α-Fe + MeC + Fe3C (Me = Ti, V, or Nb) nanocrystalline composites in neutral environments

The electrochemical behavior of α-Fe + MeC + Fe3C (Me = Ti, V, or Nb) nanocrystalline carbide-steel analogs, which were produced by mechanical activation in an organic environment followed by compaction, is studied. Passivation of the composites in borate solutions is found to be determined by the formation of FeO · xTiO2, FeO · xNb2O5, and FeO · xV2O3 (FeO · xVO2) mixed oxides, which substantially increase the resistance against oxidation at potentials of the active dissolution of ferrite phase. Resistance against local activation increases with an increase in the content of MeC and Fe3C carbide phases.

Corrosion-electrochemical properties of nanocomposites of α-Fe+Fe3C+VC in acidic and alkaline sulfate solutions

This work studies corrosion-electrochemical properties of three-phase composites of α-Fe+ Fe3C + VC in acidic and alkaline sulfate solutions. Nanosize carbide inclusions are characterized by a high level of activity in the reaction of cathodic hydrogen evolution and result in a high rate of composite dissolution in acidic solutions. The presence of carbides and carbon has a negative effect on passivation of the iron matrix. The resistance of vanadium carbide inclusions to anodic oxidation increases upon a decrease in the amount of carbon in the carbide.