CALCULATION METHOD OF IMPEDANCE MODULE FOR CORROSION MONITORING OF SURFACE STATE OF CHROMONICKEL STEELS

Abstract

In discussed issue the additional criterion for chromium-nickel steels surface state assessment is proposed. It is used in periodic potentiostatic regime of stainless steels surface state monitoring under the condition of pitting corrosion origin. Electrochemical investigations were carried out with pitting corrosion resistant austenitic (12Х18Н10Т) and austenitic-ferritic chromium-nickel steels 10Х17Н13М2Т in chloride solutions of 0.1 mol/L NaCl and 0.5 mol/l NaCl with the additional oxidant K2Cr2O7. Polarization curves were recorded under potentiostatic mode of polarization in the electrochemical three-electrode cell using the ZIVE SP2 workstation and a personal computer. The fast Fourier transformation method was used for conducting the spectral analysis, as this method is more effective in terms of the algorithmic complexity when processing a large amount of the initial data. It is established that scalar impedance angular coefficient shows the current oscillation frequencies change concerned with the pitting occurrence and passivation process. The scalar impedance angular coefficient allows to estimate the pitting corrosion evaluation processes boundaries: passivity; metal dissolution due to the development of metastable pittings; the metastable nucleation and passivation processes concurred with the stable pitting development. The algorithm based on the statistical method of spectral analyses for scalar impedance angular coefficient calculation is developed. It is established that the decrease in passive film total resistance corresponds to the breakdown process and first metastable, and then stable local surface destructions that reflect on the higher the scalar impedance angular coefficient value. The boundary values of an additional criterion for surface state assessment were proposed, on the basis of which an interval scale was constructed to determine the current state of the surface of chromium-nickel steels. It is shown that the boundary values of the scalar impedance angular coefficient correspond to the metallographic studies of investigated steels.