The Measurement of Transient pH Values Near the Surface during the Pitting Corrosion of AISI 1020 Steel

The pitting corrosion is one of the most dangerous forms of corrosion and its mechanism is not yet fully understood. The existence of stable pits when the pitting corrosion occurs in dissolved CO₂ environments can be attributed to the precipitation of FeCO₃ which occurs inside the pits and induces the acidification of the electrolyte in this region. Consequently, Cl^- ions tend to accumulate at the interfacial pit boundary to counterbalance the H⁺ produced during the formation of siderite film and, as a consequence, pits grow on the metal surface. Therefore, it is expected that transient pH changes could be directly related to the processes of nucleation, growth, passivation and stabilization of pit. In this context, this work presents the development of a ring-shaped sensor built around a steel sample to determine the interfacial pH changes during the pitting corrosion. AISI 1020 steel is the constituent material of the disc in the center of the electrode surrounded by a ring of IrO_x, a sensitive material to pH changes. The Pechini method was employed to generate the ring on a glass tube (gap between the ring and the disc) with thickness of 7 ± 2 μm. A disc area of 680 μm x 544 μm (steel sample) was observed in situ by temporal series of micrographs (TSM) with an optical microscope. At the same time, the sensor detected the interfacial pH changes during the initiation of pitting corrosion, its evolution and during the pit passivation. The study of pitting corrosion of carbon steel was performed in 0.1 mol dm^-3 Na₂HCO₃ (pH 8.3) with Cl^- ions during the open circuit potential (OCP), polarization curves (PC) and chronoamperometric (CA) measurements. Before the corrosion experiments, acid-base titration of NaOH 0.1 mol dm^-3 by H₃PO₄ 1.0 mol dm^-3 were performed for characterization of pH sensor together with pH measurements on solution through a combined glass electrode. The behavior of sensor is very close to the pH determined with conventional pH electrode, e.i., it was observed the same profile for the first and second dissociations constants for H₃PO₄. Two regions that exhibit a linear response were observed for ring shaped sensor. The first one is the 2-3.5 pH range and the other is the 3.5-12.5 pH range. These two linear bands had slops of 52.1 mV pH^-1 and 24.9 mV pH^-1, respectively. The TSM obtained in the corrosion experiments was quantified using the ImageJ software to get the number of pits, during the passivity breakdown (nucleation of several pits over the metal surface), and to access the total pit area during the localized corrosion evolution. It was observed a slowly decrease and increase at pH in near surface solution during the passivation of steel sample. At passivity breakdown was observed approximately 6000 pits on the TSM and this value remained approximately constant after 740 s during the CA measurements performed up to 500.0 mV more positive than OCP. Besides, it was observed a slight decrease at pH during this process. Analyzing the total pit area changes during the breakdown of passive film, we noticed a shift when the number of pits becomes constant where two growth rates were calculated based on TSM. This behavior is due to nucleation of metastable and stable pits. This explains the reduction on this second growth rate after the number of pits gets constant. After the pitting potential the greatest variation on the sensor potential was observed. The pH in the solution near the electrode decreases one order of magnitude at the end of the anodic polarization due to anolyte released from pits. A surface mapping shows approximately 30 big pits on metal surface which could be related to this pH change.