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

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-.

针对0.01 mol·L⁻¹ NaNO₂-HCl溶液中不同pH值的Q235碳钢，本研究考察了pH值对亚硝酸根阴离子（NO₂⁻）作用机制的影响。在pH为1和2的溶液中，Q235碳钢表现出活化电化学行为，其表面主要腐蚀类型为均匀腐蚀（uniform corrosion, UC）；亚硝酸根的存在并未改变其电化学行为与腐蚀类型，仅加速了铁氧化的阳极反应。在pH为3和4的溶液中，Q235碳钢呈现活化-钝化转变的电化学行为，其表面发生晶间腐蚀（intergranular corrosion, IGC）；亚硝酸根还原的阴极反应，促成了电化学行为从活化向钝化的转变，以及腐蚀类型从均匀腐蚀向晶间腐蚀的转变。在pH为5和6的溶液中，Q235碳钢表现出自钝化-点蚀的电化学行为，其表面出现点蚀（pitting corrosion, PC）；在Fe²+与亚硝酸根的协同作用下，亚硝酸根的强氧化性对Q235碳钢表面γ-Fe₂O₃钝化膜的形成起到关键作用。