The initiation and development of pitting corrosion of steel 20Cr13 in the NaCl
solutions with various concentrations, temperatures, and pH values are studied under the
potentiostatic conditions and at the free-corrosion potential. The pitting and repassivation potentials
are determined using the method of cycling voltammetry. In spite of the fact that thus determined
pitting potential is more positive than the corrosion potential (the open-circuit potential Eo.c.), the
long-term experiments, which were performed at the free-corrosion potential, showed that pitting
corrosion takes place without imposing a potential using an external power source. It is concluded
that the probability of pitting corrosion of steel should be determined by comparing the corrosion
potential (the open-circuit potential) with the repassivation potential Erp. Steel 20Cr13 is prone to
the pitting corrosion, because Erp is more negative than Eo.c..
In the potentiostatic experiments, the variation of the depth and diameter of pits and their
number with the time and the effect of temperature and electrode rotation on the pit propagation are
studied.
The results, which were obtained at the free-corrosion potential, are much less reproducible.
In this case, in contrast to the potentiostatic conditions, the pit depth increased only slightly and the
pit width increased to a larger extent. The effect of concentration, pH value, and temperature of
NaCl solutions on the pit propagation is considered.
It is concluded that the data on the development of pitting corrosion under the potentiostatic
conditions can be hardly extended to the conditions of free corrosion potential.
Solubility Data of Potential Salts in the MgO-CaO-SO2-H2O-O2 System for Process Modeling