Studies on the Pitting Corrosion of the Delta‐52 Steel in aqueous solutions. 2. Variation of the pitting potential in relation to the concentration of the aggressive anion

Abstract

The validity of the above model requires verification by further experimental research.Assumption is made that f depends upon the cathodic current density, but is independent of pH and promoter present. For the given cathodic material, N depends upon both pH and promoter. Therefore, the relationship between f and i, is complex. This might be the reason for the observed (1) change from the linear relationship between i, and i, at low cathodic current densities into a linear relationship between i, and i, 'I2 at higher i, values and, eventually, into independence of i, on 4 at high degrees of cathodic polarization. This, in turn, involves a reduction of the relative permeation efficiency, ip/ic.It is assumed, furthermore, that in aqueous electrolytes there occurs competition between the dissociative chemisorption of promoting hydrides and adsorption of inhibiting hydroxyl ions. With increasing pH over 7 the hydroxyl ions begin to overweight the action of the promoting hydrides. However, at high concentrations of arsine (most probably of phosphine and stibine as well, but not in that of hydrogen sulfide, selenide, and telluride), the promoting effect of hydrides can override the action of OH-.formula The relationship shown in Fig. 1 1 can be expressed by thewhere a and k are constants and C~H -is the concentration of hydroxyl ions at the surface of the cathode. Constant k depends upon the promoter used, and constant a expresses the slope of the log i, versus log C~O H -curve. Under conditions applied in the case of results shown in Fig. 11, a = Assuming that i, is proportional to the coverage of the metal surface by species responsible for the transmission of hydrogen atoms through the electrolyte/metal interface, equation 2 can be considered as adsorption isotherm.-0.1 15. Conclusions 1.Compounds of phosphorus, arsenic, antimony, sulfur, selenium and tellurium are able to enhance the permeation of electrolytic hydrogen through steel exclusively under conditions under which hydrides of these elements are produced at the cathode and exhibit a sufficient stability. Elements and anions do not enhance permeation.2. Hydrides of the above elements do not qualitatively modify the permeation process; they only affect its velocity by assisting the transmission of hydrogen atoms across the electrolyte/metal interface.3. The relatively slow permeation of hydrogen evolved from alkaline solutions is thought to be caused by the inhibitive action of hydroxyl ions. 4. A tentative model of the hydrogen transmission mechanism based on assumptions taking into consideration the dissociative chemisorption of hydrides is proposed. (Eingegangen: 27. 10. 1975) References 1. T. Zakroczymski, Z Szklarska-Smialowska and M. Smialowski, Werkstoffe und Korrosion, 26 (1975) 617-624. 2. E. Lunarska, Z. Szklarska-Smialowska and M. Smialowski: Werkstoffe und Korrosion. 26 (1975) 624-628. 3. M Smialowski and Z. ' Szkiankabnialowska: Roczniki Chem. 29 (1955) 85-94. 4. Pourbaix: Atlas of Electrochemical Equilibria in Aqueous Solutions Pergamon Press an...