Corrosion Potential Oscillation of Iron in H₃PO₄ + H₂O₂ Solutions

Abstract

Iron is used in large quantities in various industries. For example, it is utilized as a material for structures, vehicles, and daily necessities. However, iron has the disadvantage of being easily corroded. Iron corrosion causes economic and resource losses, and if it progresses without being noticed, it triggers severe problems and accidents. Therefore, a deep understanding of corrosion is necessary to create a safe society; hence, much research has been conducted on the fundamental principles of corrosion, its behavior, and corrosion protection. Although iron dissolves in a dilute nitric acid solution, the dissolution does not occur in a concentrated nitric acid solution because of surface passivation. It is known that when a small amount of water is added to the concentrated solution, the dissolution and passivation of iron occur alternatively. During this corrosion, the corrosion potential of iron oscillates spontaneously, and the amount of gas products generated by nitric acid reduction also oscillates, as reported in our previous paper [1]. Also, when iron is immersed in a phosphoric acid solution containing an appropriate amount of hydrogen peroxide (e.g., 0.5 M H 3 PO 4 + 1.5 M H 2 O 2 ), its corrosion potential oscillates spontaneously [2]. This oscillation is interesting because the amount of oxygen produced by the oxidation of H 2 O 2 on the iron surface oscillates synchronously with the oscillation [3]. Potential oscillations are frequently observed in various electrochemical reactions under current flowing conditions, and their mechanism can be explained based on negative differential resistance (NDR) characteristics. However, little has been known about the mechanism of the corrosion potential oscillations. In our previous paper [1], we studied the corrosion potential oscillation of iron in nitric acid solutions and concluded that the oscillation was caused by an N-shaped NDR (N-NDR) characteristic and local cell action. In the present study, to generalize this conclusion, we studied the corrosion potential oscillation in the “iron- H 3 PO 4 - H 2 O 2 ” system. When the current ( I )–potential ( E ) relationship was measured under current-controlled conditions, a potential oscillation appeared at I = 0 mA, that is, a corrosion potential oscillation appeared (Figure 1). Notably, potential oscillations appeared even when I < 0 mA. A current oscillation appears at around 0.3 V under potential-controlled conditions (Figure 2a). The electrochemical impedance measurements indicated the presence of N-NDR characteristics (Figure 2b). Based on these results, we can conclude that the oscillation is a “hidden” N-NDR (HN-NDR) type oscillator. In this presentation, the mechanism of the corrosion potential oscillation is discussed based on the framework of an HN-NDR-type oscillator. References [1] R. Sato, T. Kuge, S. Nakanishi, and Y. Mukouyama, J. Electrochem. Soc. , 170 , 081505 (2023). [2] A. Ishihara and S. Asakura, Sensors and Actuators A: Physical , 39 , 231 (1993). [3] R. Sato and Y. Mukouyama, The 74th Annual Meeting of the ISE, S16-P-026 (2023). Figure 1