Стойкость против питтинговой коррозии как критерий безопасности эксплуатации металлического оборудования

Abstract

Pitting corrosion is hazardous for personnel of chemical plants operating with tanks, reactors, and pipelines. Pitting is a pinpointed corrosive damage rapidly penetrating a metal and causing its through-perforation. Chemical and thermal burns and poisoning of personnel are also potential. When designing the equipment, it is required to select materials able to resist pitting corrosion in the operational medium. Quick methods of pitting resistance of material include the comparison of critical potentials determined by potentiodynamic polarization curves. The potential of the studied sample is given a value equal to the corrosion value that is increased at the rate of 0.2–2 mV/s. At the same time, values of current density are registered. The gradual increase of current density is followed by its decrease, and the metal turns to a passive state. With a certain value of pitting formation potential, the current density increases acutely; and pittings are formed on the metal surface. When the potential is decreased, current density within a certain range of values remains relevant to the current density at the pitting formation potential but then decreases to the values relevant to dissolution in a passive state. This occurs at the re-passivation potential. Earlier, values of potentials of pitting formation and corrosion were compared. Later, it was proved that the evaluation of the pitting resistance of metals based on the comparison was not correct as pittings could also be formed at the potentials lower than the pitting formation potential. The correct evaluation of metal pitting resistance can be obtained based on the comparison of re-passivation and corrosion potentials. Until now, however, the pitting formation potential has been used for the evaluation. Based on the experimental potentiodynamic polarization curve of stainless steel Х20Н6Г11 and the results of corrosion tests, it has been established that metal corrodes at potentials above the re-passivation potential but lower than the pitting formation potential. When potentials are lower than the re-passivation potential, pittings are not formed.