Корозійна тривкість гальванічних покривів Fe-Co-Mo

Abstract

A comparative analysis of the results of the morphology, phase composition and corrosion resistance study for samples with coatings Fe(Co)-Mo and Fe-Co-Mo deposited by direct current is carried out. It is shown that Fe-Co-Mo coatings are characterized by the globular structure with spheroid size of 2‒2.5 μm, and surface is more uniform compared to the bimetallic ones. According to the results of X-ray diffraction analysis, the formation of mixed amorphous-crystalline structure of a ternary alloy with the formation of intermetallides Fe7Mo, Fe7Co, FeCo was established. Increase in molybdenum content from 13 at. % to 38 at. % does not change the phase composition of the alloy, but decreases the size of the coherent scattering zone from 6.6 nm to 5.3 nm. The corrosion behavior of the coatings was investigated by methods both the electrode impedance spectroscopy and polarization dependence analysis. Fe-Co-Mo alloys exhibit high corrosion resistance in all model media due to the chemical properties of alloying metals. The corrosion current of ternary coatings is 1.3 and 1.6 orders of magnitude lower than that of mild steel substrate and binary Fe(Co)-Mo alloys respectively. According to the SEI results, an equivalent circuit representing the corrosion of coated with Fe-Co-Mo samples was constructed, which contains two imperfect capacitors (CPE1 = 0.90, CPE2 = 0.66) and the inductance resistance. The corrosion resistance of samples coated with Fe-Co-Mo alloy in 2% NaCl solution is 3400 Ω·cm2, which is explained by the formation of variable valence molybdenum oxide layer on the coating surface. Diagrams «composition ‒ depth corrosion index kh, mm/year» for the Fe ‒ Со ‒ Mo system are constructed, which allow to determine the optimal composition of alloy coatings Fe(Co)-Mo, Fe-Co-Mo with the required resistance for corrosion protection, depending on the operating conditions.