The Effects of pH Values on Functional Mechanisms of Nitrite Anions for Q235 Carbon Steels in 0.01 mol L-1 NaNO2-HCl Solutions

Abstract

For Q235 carbon steels in 0.01 mol L-1 NaNO 2-HCl solutions with different pH values, the effects of pH values on the functional mechanisms of nitrite anions (NO 2 −) were studied. In the pH 1 and pH 2 solutions, the Q235 steels showed the electrochemical behavior of activation, and uniform corrosion (UC) was the main corrosion type on the Q235 surface. The presence of NO 2 − did not affect the electrochemical behavior and the corrosion type but only accelerated the anodic reaction of Fe oxidation. In the pH 3 and pH 4 solutions, the Q235 steels presented the electrochemical behavior of activation-passivation transition, and intergranular corrosion (IGC) occurred on the Q235 surface. The cathodic reaction of NO 2 − reduction resulted in the transition of electrochemical behavior from activation to passivation and the change of corrosion type from UC to IGC. In the pH 5 and pH 6 solutions, the Q235 steels exhibited the electrochemical behavior of self passivation-pitting, and pitting corrosion (PC) was present on the Q235 surface. The strong oxidability of NO 2 − played a critical role in the formation of γ-Fe 2 O 3 passive film on the Q235 surface under the combined action of Fe 2+ and NO 2 − .

The relationship between activation–passivation transition and grain boundary dissolution on four steel samples in acidic solutions containing NO2−

“…GBD was observed on the steel surface when the four steels showed the electrochemical behaviors of A-P-T and A-P-P. Before the applied potential reached the transpassivation potential and the pitting potential, the PPCs of A-P-T and A-P-P were similar and composed of an activation (A) region, activation-passivation (A-P) region and passivation (P) region. Furthermore, according to previous studies [27][28][29][30] and the current results obtained in this study, the initiation and propagation of GBD occurred before the four steels were polarized in the P region. Therefore, this study was mainly focused on the anodic and cathodic reactions occurring in the A region and A-P region.…”

“…According to previous studies, [27][28][29][30] for carbon steels in acidic solutions containing NO 2 À , the initiation and propagation of GBD occurs on the steel surface when the carbon steels are polarized in the A-P region. Based on the results shown in Fig.…”

“…The electrochemical characteristic of the A-P transition was due to the effectiveness of NO 2 À and the decrease in the H + concentration; 34 moreover, the presence of Cl À derived from HCl was responsible for pitting. 30 In the pH 5 solutions, the lack of the H + concentration contributed to the sP-T behavior in the HNO 3 -NaNO 2 and HAc-NaNO 2 solutions and the sP-P behavior in the HCl-NaNO 2 solution for the L80 steel, which was attributed to the following reaction: [35][36][37] 2Fe…”

The Comparative Investigation of Corrosion and Passivation for X65 Carbon Steel in pH 1 to 5 HNO3 Solutions without and with 0.01 mol L–1 NaNO2

“…The electrochemical behavior of Q235 carbon steel transferred from the activation in CO 2 saturated solution free of NaNO 2 to the passivation in the same solution containing NaNO 2 ; 19 with the increase of NaNO 2 concentration, the passivation capability of Q235 carbon steel was strengthened obviously until the NaNO 2 concentration was up to 0.05 mol L -1 ; 20 the mechanism of NO 2 on the surface passivation was very closely associated with the formation of Fe 2 O 3 passive film under FeCO 3 corrosion product layer. 21 Further, Zhou et al 22 also investigated the influence of NaNO 2 addition on the electrochemical behavior of Q235 carbon steel in pH 1 to 6 HCl solutions. Due to the absence of strong oxidability and the presence of Clin HCl solutions, the effectiveness of NaNO 2 on the surface passivation was very limited, and the occurrence of pitting corrosion was present when a high potential was applied.…”

“…As summarized above, until by now, the reports involving the effectiveness of NO 2 on the surface passivation of carbon steels in CO 2 saturated solution (a weak and non-oxidizing acid environment) 20 and in pH 1 to 6 HCl solutions (a strong and non-oxidizing acid environment) 22 have been published. However, in strong and oxidizing acid environments, such as HNO 3 solutions, the related investigations are absent.…”