局部碳酸钙沉积对N80碳钢腐蚀行为的影响

Effect of Local Calcium Carbonate Deposition on Corrosion Behavior of N80 Carbon Steel

目的研究局部碳酸钙沉积对N80碳钢腐蚀行为的影响,为解决碳酸钙沉积引发的腐蚀问题提供新思路。方法采用恒电位阴极极化法诱导碳酸钙沉积。采用电化学阻抗谱、丝束电极、扫描电镜和能谱仪对N80碳钢的均匀腐蚀、局部腐蚀、电偶腐蚀、微观形貌和化学组成进行表征,揭示局部碳酸钙沉积对N80碳钢腐蚀行为的影响规律。结果碳酸钙沉积会使电极的腐蚀电位正移约100m V。相同条件下,碳酸钙覆盖电极的容抗弧直径远大于裸电极。随着浸泡时间的延长,裸电极的容抗弧直径不断增大,而碳酸钙覆盖电极的容抗弧先增大后减小。电偶测试中,碳酸钙覆盖电极作为阴极,裸电极作为阳极,在168 h内,二者没有发生极性反转,电偶电流密度最终稳定在–0.4μA/cm^(2)左右。WBE结果显示,浸泡开始时,碳酸钙覆盖区域的电位较高且均为阴极电流,24 h后,该区域逐渐出现阳极点。浸泡72 h后,电位最负的位置开始向碳酸钙覆盖区域转移。微观分析结果表明,碳酸钙覆盖电极表面有一层完整且致密的腐蚀产物膜,可能对基体起到了一定的保护作用。去除腐蚀产物后,基体存在明显的点蚀坑。结论局部沉积的碳酸钙虽然对基体有一定的保护作用,但会导致电偶腐蚀的发生。随着浸泡时间的延长,覆盖碳酸钙的区域会由于氧浓差电池和酸化自催化效应发生局部腐蚀。

This paper studied the effect of local calcium carbonate deposition on the corrosion behaviors of N80 carbon steel in order to provide new ideas for the petroleum industry to solve the corrosion problem caused by calcium carbonate deposition.The author adopted the constant potential cathodic polarization method to induce the calcium carbonate to deposit at the local position of the electrode.Electrochemical impedance spectroscopy(EIS)and wire beam electrode technology(WBE)were used to study the corrosion behavior of N80 carbon steel during local calcium carbonate deposition.The morphology and composition of corrosion(uniform,local,and galvanic corrosion)products were analyzed using scanning electron microscope(SEM)and energy dispersive spectrometer(EDS).The electrochemical test results showed that the deposition of calcium carbonate would positively shift the corrosion potential of electrode by about 100 mV.Under the same conditions,the capacitive arc diameter of the electrode covered by CaCO_(3) deposit was much larger than that of the bare electrode.After the electrode was immersed for a longer time,the capacitive arc radius of the bare electrode became larger,while the capacitive arc of the electrode covered by CaCO_(3) deposit first increased and then decreased over time.The results of the galvanic electrode showed that the electrode covered by CaCO_(3) deposit acted as the cathode in the galvanic corrosion and the bare electrode was the anode.Polarity reverse did not occur during the 168 h immersion.The galvanic current density finally stabilized at-0.4μA/cm2.The WBE results showed that the potential at the calcium carbonate deposition area at the beginning of the immersing was higher than other areas and all was cathode current.After 24 h immersion,the anode spots gradually appeared in the calcium carbonate deposition area.After 72 h immersion,the position with the most negative potential began to shift to the calcium carbonate deposition area.The morphology analysis showed that corrosion products on the surface of the electrode covered by CaCO_(3) deposit were complete and dense,providing a better protective effect for the carbon steel.Local corrosion pits were observed on the substrate after the removal of corrosion products.In summary,the locally deposited calcium carbonate shows protective effect on the electrode in a certain extent,but it will induce galvanic corrosion.As the immersion time becomes longer,the locally deposited calcium carbonate will cause local corrosion due to the presence of the oxygen concentration difference battery and the autocatalytic effect.