Potential oscillations induced by the local breakdown of passive iron in sulfuric acid media. An evaluation of the inhibiting effect of nitrates on iron corrosion

Abstract

Chemical perturbation of the Fe|H2SO4 electrochemical system caused by adding a critical chloride ion concentration induces sustained potential oscillations under a galvanostatic control. The potential oscillates between the active and passive states of Fe due to a local breakdown/rebuilding of the iron oxide. This work investigates how nitrates modify and eventually suppress the chloride-induced potential oscillations, which can be used as a probe to sense and evaluate the nitrate inhibiting effect. It is shown that adding an appropriate concentration of nitrates into the Fe|0.75 M H2SO4 + x M Cl− system causes distinct differences in system temporal patterning revealed in galvanodynamic and galvanostatic curves. Depending on the chloride concentration and the current imposed to the system, the number of potential oscillations decreases, while the oscillatory waveform alters by increasing gradually the nitrate concentration. Above a critical concentration, nitrates suppress completely potential oscillations resulting in a stable passive state of Fe. This situation resembles that of the chloride-free system, which under current-controlled conditions shows only bistability, without potential oscillations. A quantitative characterization of the inhibiting effect of nitrates is carried out using diagnostic criteria related to the oscillation features. The inhibiting effect of nitrates and the suppression of oscillations are discussed by considering a competitive adsorption of nitrates and chlorides along with nitrate redox reactions.