Corrosion behaviour of steel in the presence of rare earth salts: synergistic effect

Abstract

In this work, the inhibition behaviour of rare earth (RE) salts (Y(NO 3 ) 3 .6H 2 O, CeCl 3 .7H 2 O) in conjunction with metasilicates (Na 2 SiO 3 .5H 2 O) for carbon steel A37 and in aerated 3%NaCl solutions has been investigated. Corrosion inhibition power evaluation and surface layer characterisation have been carried out by electrochemical measurements (potentiodynamic polarisation and impedance) followed by X-ray photoelectron spectroscopy. It was demonstrated that the binary RE/SiO 22 3 improved the protective role of oxide/hydroxide films formed on the steel. Electrochemical tests showed that the cathodic and anodic reactions were both inhibited to some extent, leading to a decreased rate of the global corrosion process. The maximum protective inhibition against corrosion attack was evident when an RE/SiO 22 3 ion mixture was added. Surface analysis revealed the diverse nature of inhibitor coverage on the steel.

Effect of Nd3+ Ion Concentration on the Corrosion Resistance of API X70 Steel in Chloride‐Rich Environments

“…In the absence of inhibitor, oscillations in the polarization resistance values are observed in the first 10 hours of immersion, and a quasistationary state is subsequently observed. ese variations are consistent with the observations established in the OCP measurements; that is, the initial behavior corresponds to a reorganization of the surface layer due to the acceleration of the anodic dissolution process [2,4]. In the presence of Nd 3+ ions in all cases, a significant increase in the polarization resistance values was observed in the first 10 hours of immersion and, subsequently, a less pronounced ascendant trend was observed with slight oscillations.…”

“…When an inhibitor affects both processes (corrosion current density and corrosion potential), it is considered as a mixed inhibitor, and its net effect in the polarization curves will be determined by the inhibition magnitude of the anodic and cathodic reactions [6]. e previous analysis shows that the addition of Nd 3+ ions changed the corrosion process of the API X70 steel, affecting both the anodic and cathodic branches (different current densities); this suggests that both the local anodic dissolution and the cathodic reduction of oxygen are affected simultaneously [2,4]. However, based on the negative displacement of the corrosion potential and the decrease of the cathodic current, it can be said that the Nd 3+ ions act as mixed inhibitors with a predominant cathodic effect.…”

“…On the other hand, in the presence of Nd 3+ ions, it is also possible to observe an abrupt drop in the OCP values at the beginning of the test, and subsequently after 50 hours of immersion, it is possible to observe a quasistationary state. It is known that the displacement of the OCP values to more active values besides being associated with an increase in the anodic reaction rate (metallic solution) can also correspond to the presence of an inhibition process of the cathodic reaction [2]. In the case of rare earth-based corrosion inhibitors in chloride-rich solutions, this displacement has been associated with the formation of a layer of rare earth products onto cathodic sites of the metal surface, blocking the reduction of dissolved oxygen [2,23].…”

“…It is known that the displacement of the OCP values to more active values besides being associated with an increase in the anodic reaction rate (metallic solution) can also correspond to the presence of an inhibition process of the cathodic reaction [2]. In the case of rare earth-based corrosion inhibitors in chloride-rich solutions, this displacement has been associated with the formation of a layer of rare earth products onto cathodic sites of the metal surface, blocking the reduction of dissolved oxygen [2,23]. It is possible to observe that, regardless of the concentration of Nd 3+ ions, the OCP values are within a band of 25 mV.…”

“…It has been recognized that rare earth salts are low-toxicity compounds whose average lethal dose (LD50) is similar to that of sodium chloride (NaCl) [1][2][3]; therefore, the interest in the use of rare earths has emerged as an alternative to replace those toxic and highly carcinogenic corrosion inhibitors that affect both people and the environment, and many of these dangerous inhibitors are based on chromates (CrO 2− 4 ), mercurate, nitrite, and alkynol [4]. Most of the studies have been reported based on the use of both Ce and La salts as corrosion inhibitors, in the form of either chlorides, nitrates, or other salts [2,[4][5][6][7][8][9]. Many of these studies have been carried out in chloride-rich solutions, and the excellent corrosion inhibiting capacity of rare earth salts has been attributed to the high reactivity of rare earths.…”