The electrochemical behavior of corrosion and passivation for Q235 carbon steel in acidic phosphate buffer solutions without and with NO_2⁻

Abstract

Purpose The purpose of this paper is to reveal the mechanism of nitrite (NO2−) for the surface passivation of carbon steels in acidic environments through investigating the influences of 0.01 mol/L NaNO2 addition on the corrosion and passivation behaviors of Q235 carbon steel in acidic phosphate buffer (APB) solutions (pH 2 to 6). Design/methodology/approach The electrochemical techniques including open circle potential evolution, potentiodynamic polarization, electrochemical impedance spectroscopy and cyclic voltammetry were applied. Findings In APB solutions without NO2−, the Q235 steel presented the electrochemical behaviors of activation (A), activation-passivation-transpassivation and self-passivation-transpassivation at pH 2 to 4, pH 5 and pH 6, respectively; the corrosion rate decreased with the up of pH value, and the surface passivation occurred in the pH 5 and pH 6 solutions only: the anodic passivation at pH 5 and the spontaneous passivation at pH 6. Originality/value In APB solutions without NO2−, the corrosion rate decreased with the up of pH value, and the surface passivation occurred in the pH 5 and pH 6 solutions only: the anodic passivation at pH 5 and the spontaneous passivation at pH 6. With the addition of 0.01 mol/L NaNO2, into APB solutions, the variation of corrosion rate showed the same rule, but the surface passivation occurred over the whole acidic pH range, including the anodic passivation at pH 2 to 4 and the spontaneous passivation at pH 5 to 6.

The one-step preparation and corrosion resistance of a Ca–Mn–Zn ternary phosphate conversion coating on magnesium alloy AZ91D

“…All EIS comprise a capacitive semicircle at highfrequency region and an inductive semicircle at low-frequency region, which is regardless of immersed time. For the lowfrequency inductive semicircle of EIS, it is generally accepted that it may be attributed to the occurrence of unstable localized corrosion or the adsorption relaxation of Figure 8 Substrate SEM morphologies of bare AZ91D sample and Ca-Mn-Zn@TPCC-coated AZ91D sample immersed in NaCl solution for 8 h Figure 7 Surface macroscopic images of bare AZ91D sample and Ca-Mn-Zn@TPCC-coated AZ91D sample immersed in NaCl solution with different times adsorbed species (Hu et al, 2020) on the surface of working electrode. From the results of Figures 7 (f)-(j) and 8(b), no obvious localized corrosion occurs on the surface of Ca-Mn-Zn@TPCC-coated AZ91D sample when it is immersed in the NaCl solution, implying that the appearance of inductive semicircle is because of species adsorption rather than localized corrosion.…”