The corrosion of carbon steel in chloride media has been investigated extensively in literature [1, 2]. However, the role of cations in carbon steel corrosion behavior is not widely reported. In the present work, the effect of Na⁺, K⁺ and NH₄⁺cations on the corrosion of carbon steel in chloride media is investigated via. potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and field emission scanning electron microscopy (FESEM) measurements. All the electrochemical measurements were carried out using a three electrode system, and after open circuit potential reaches a stable value. The carbon steel was used as working electrode. The reference and counter electrode were Ag/AgCl (3M KCl) and platinum wire respectively. The concentration of chloride is 10 wt.% for all the solutions. The polarization curves were obtained by scanning the electrode potential from -250 mV to 500 mV w.r.t OCP at a scan rate of 1 mV/s. The impedance spectra were obtained by applying an AC perturbation signal of 10 mV in the frequency range 100 kHz to 10 mHz.

The polarization measurements (Fig. 1) reveals that the corrosion rate is higher in case of ammonium chloride (NH₄Cl) solution following the order NH₄⁺> K⁺ > Na⁺. Further, the polarization measurements, in case of chloride solutions containing Na⁺ and K⁺ cation shows the formation of passive layer on the carbon steel surface (the regime of insignificant current increase with potential) followed by breakdown of the same (the regime of increase in current) indicating susceptibility to pitting corrosion. While the anodic polarization curve in the presence of NH₄⁺cation does not show any sign of passivation. This implies that the carbon steel is more prone to uniform corrosion. These results suggest the formation of passive layer on the carbon steel surface and the mechanistic reaction pathway of carbon steel dissolution in chloride media is strongly influenced by the cations present in the solution.

The EIS measurements performed at open circuit potential, shows two time constant for all the cations (Fig.2). However, for NaCl and KCl system, the two time constants are merged and appears to be a single capacitive loop. The impedance data is analysed by an appropriate electrical equivalent circuit (EEC) and the EEC parameters are retrieved. Impedance results also confirms that the corrosion rate is higher in NH₄Cl solution while lower in NaCl solution. So the presence of cations affect both the corrosion rate and the corrosion mechanism.

Keywords: Carbon steel, cations, chloride media, polarization, EIS References

1. K. Toba, M. Ueyama, K. Kawano and J. Sakai, "Corrosion of carbon steel and alloys in concentrated ammonium chloride solutions," NACE Intl., 68, 1049 (2012)

2. O. Forsen, J. Aromaa and M. Tavi, "Corroison reistance of different materials in dilute ammonium chloride bearing environments," Corros. Sci., 35, 297 (1993)