Statistical investigations of the pitting of passive iron

Abstract

The surface concentration of pits at passive pure iron in borate buffer solutions containing chloride grows at a maximum rate shortly after the appearance of the first pit. Later on, a steady state surface concentration of pits is established after some characteristic time decreasing with the electrode potential and with the chloride concentration. The steady state surface concentration of pits increases linearly with the difference of the electrode potential vs the critical pitting potential, with the concentration of chloride and with the activity of hydrogen ions, but also grows with the surface concentration of emerging dislocations. The probability of finding a pit was the same on any part of the iron surface with uniform mechanical preparation. The Poisson distribution for the probability of finding a certain surface concentration of pits in a given area was nearly symmetrical. The distribution of the nearest distances of pits around a central pit also was of the Poisson type, but with an exclusion zone in which no pits nucleated. The width of the exclusion zone was independent of the electrode potential, but decreased with the activities of chloride and hydrogen ions.