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

Yong Zhou,Huiju Huang,Pei Zhang,Jinping Xiong,Fuan Yan

doi:10.21577/0103-5053.20190070

Abstract

For Q235 carbon steels in 0.01 mol L-1 NaNO2-HCl solutions with different pH values, the effects of pH values on the functional mechanisms of nitrite anions (NO2-) 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 NO2- 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 NO2- 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 NO2- played a critical role in the formation of γ-Fe2O3 passive film on the Q235 surface under the combined action of Fe2+ and NO2-.

Highlights

Carbon steels are used in the different fields of production and living widely,[1,2,3] but it is unavoidable that the application and development of carbon steels are limited by the disadvantage of corrosion and failure.[4,5,6] For carbon steels, the addition of inhibitors into service environments is one of main methods to decrease the corrosion rate,[7] and nitrite anions (NO2−) are kind of effective inhibitors to restrain the corrosion process.[8]At present, the studies on the inhibitive effect and mechanism of NO2− for carbon steels are mainly focused on alkaline and neutral media, on the corrosion environments of reinforced concrete[9,10,11] and chloride.[12,13,14] It is generally accepted that in alkaline and neutral media, the presence of NO2− promotes the surface passivation of carbon steels, which is attributed to the mechanism of NO2− for repairing the defects in the air‐formed oxideVol 30, No 8, 2019 acidic media, Zuo et al.[19] studied the combined influences of NO2− and thioureido imidazoline (TAI) on the passivation behavior and the pitting corrosion (PC) of X70 carbon steels in acidic NaCl solution (pH 5.5) and reported that the mechanisms of NO2− and TAI were both interactive and superimposed, and the addition of NO2− was beneficial to restrain the initiation and propagation of PC
The authors reported that the addition of NO2− showed a limited inhibition for the pit propagation, but the excessive addition even increased the uniform corrosion (UN) rate
Similar results were reported in our previous study: for Q235 carbon steel in NaNO2-CO2 solution with 0.01 mol L-1 NO2− and pH 3.7, the occurrence of intergranular corrosion (IGC) was observed on the Q235 surface when the Q235 steel was polarized into activation-passivation potential region.[17]

Summary

Introduction

Carbon steels are used in the different fields of production and living widely,[1,2,3] but it is unavoidable that the application and development of carbon steels are limited by the disadvantage of corrosion and failure.[4,5,6] For carbon steels, the addition of inhibitors into service environments is one of main methods to decrease the corrosion rate,[7] and nitrite anions (NO2−) are kind of effective inhibitors to restrain the corrosion process.[8]At present, the studies on the inhibitive effect and mechanism of NO2− for carbon steels are mainly focused on alkaline and neutral media, on the corrosion environments of reinforced concrete[9,10,11] and chloride.[12,13,14] It is generally accepted that in alkaline and neutral media, the presence of NO2− promotes the surface passivation of carbon steels, which is attributed to the mechanism of NO2− for repairing the defects in the air‐formed oxideVol 30, No 8, 2019 acidic media, Zuo et al.[19] studied the combined influences of NO2− and thioureido imidazoline (TAI) on the passivation behavior and the pitting corrosion (PC) of X70 carbon steels in acidic NaCl solution (pH 5.5) and reported that the mechanisms of NO2− and TAI were both interactive and superimposed, and the addition of NO2− was beneficial to restrain the initiation and propagation of PC.

Results

Conclusion