PROCESSES OF 12X18H10T STEEL PITTING IN SELF-DILUTION SIMULATION
Most of electrochemical studies of pitting corrosion are performed under potentiostatic or potentiodynamic conditions, while the conditions of natural pitting corrosion are closer to the galvanostatic steel dissolution regime. As the object, we chose 12X18H10T stainless steel which is widely used in chemical industry. The research was carried out with galvanostatic and potentiostatic polarization in an electrochemical cell in chloride-containing environments: chloride solutions, mol/l: xNaCl (x = 0.01; 0.1; 0.5; 1). Galvanostatic tests conducted possibly closely to self-dissolution conditions at low current densities (0.2-1.0A/cm2) showed that in solutions of 0.01-0.5M NaCl the surface is in a passive state, since the maxima of the potentials on the chronopotentiograms did not exceed the values of stable potential for pitting formation characteristic of the stage of formation of a stable pitting. All potential oscillations were attributed to nucleation phenomena. To study the change in the nature of potential oscillations, long 24 and 72 hours’ experiments were conducted. They showed that experiments lasting more than 24 hours in galvanostatic studies at current densities simulating the self-dissolution process do not lead to a significant change in the number of peaks in time. To confirm the possibility of samples self-dissolution from the passive state, we performed potentiostatic studies at maximum potentials obtained in the galvanostatic experiment. It was found that on chronamperograms with a solution concentration of 0.01-0.5M, there was no sharp increase in current density, at a concentration of 1.0M, a linear increase in current was observed at which pittings are formed in the increasing portion of the curve, and a critical current of pitting appears. Long-lasting experiments were proved inexpedient, which is also confirmed by metallographic studies. We recommend the mode simulating the conditions of self-dissolution when studying pitting processes - a solution of 0.01-0.5M NaCl, a current density of 0.2 - 0.5A/cm2, the duration of one hour.