On the Potential-pH Diagram of Carbon Steel under Corrosion Fatigue

Abstract

Observation has been enacted of behaviours of corrosion fatigue of carbon steel under potential controlled conditions in buffer solutions of various pH values, and electrochemical studies have been made on the characteristics of the materials with respect to its passivation and immunity, as well as on the variation in the potential-pH diagram under the repetition of stress cycles. Cathodic protection is found as not efficient enough for the protection of metals against corrosion fatigue in the solutions of pH1, while the perfect efficiency can be expected at E=-1200mV in pH4 and at E=-1000mV in pH6, being more easy in the solutions of higher pH values. On the other hand, anodic protection is fairly efficient in the solution even of lower pH value, though perfect efficiency is not reached in this instance for protection of metals irrespective of the pH value of the solution. The cathodic protection at the electrode potential which is efficient against the static corrosion is not efficient enough when applied to the material under the repetition of stress cycles in corrosive environments, since the domain of corrosion in the direction of the catholic protection in the potential-pH diagram is expanded, due to the increase in metal activity, in the direction of negative electrode potential in the course of repetition of stress cycles. Perfect catholic protection is inhibited when 1N H2SO4 of pH1 is used, since metal dissolution is observable from the very beginning of stress cycles throughout in spite of the low electrode potential. On the other hand, the domain of corrosion remains in the direction of positive electrode potential in spite of the corrosion fatigue, but in the anodic protection, even at the sufficiently positive potential of higher grade than the Flade potential (EF), metal dissolution is inevitable with stress cycles. The EF is immaterial after all so far as corrosion fatigue is concerned.