【作者】 唐聿明；

【导师】 左禹；

【作者基本信息】 北京化工大学 ， 材料学， 2005， 博士

【摘要】 碳钢是最重要的工程材料之一,局部腐蚀穿孔则是钢结构的主要破坏形式之一,然而关于碳钢孔蚀早期阶段的研究目前还较少。本文主要研究A3碳钢在含Cl^-的NaNO₂和NaHCO₃溶液中慢速电位极化下的电流波动曲线,并结合回扫极化曲线、交流阻抗及扫描电镜等手段,研究碳钢发生亚稳孔和稳定孔时的电化学行为,以及表面所造成的孔蚀损伤情况。 1.研究了A3碳钢在NaNO₂+NaCl溶液中的电流波动,发现,电流波动具有明显的快速上升、缓慢下降的特点,表明碳钢表面活性点溶解较快,再钝化速率较慢。电流波动的结果,造成碳钢表面微米级的亚稳孔发生,每个小孔均是由若干更小的小孔构成。Cl^-浓度增加,亚稳孔电位E_m往负方向移动,说明Cl^-促进亚稳小孔的发生。恒电位极化时,电位升高,电流波动峰的峰值和峰频均增加,说明电位升高促使孔活性溶解速率增加,并能激活更多的活性点,使孔诱发速率增加。 2.研究发现,A3碳钢在NaHCO₃+NaCl溶液中发生亚稳态孔蚀时电流波动也具有明显的快速上升、缓慢下降的特点。亚稳孔开始出现电位E_m和稳定孔蚀电位E_b均服从正态分布。恒电位极化时,当电位高于E_m而大大低于E_b时,电流波动保持一定时间后会最终消失,产生微米级的小蚀孔。而当电位接近E_b极化时,一段时间的波动后电流会迅速上升,亚稳孔最终转变为稳定蚀孔。亚稳态孔蚀产生的频率随时间延长逐渐降低,与电位变化关系不大。 3.研究了NO₃^-、SO₄^2-、CO₃^2-、NO₂⁺、MoO₄^2-和PO₄^3-6种阴离子对碳钢在NaHCO₃+NaCl溶液中孔蚀及亚稳态孔蚀的影响。发现,NO₃^-大大促进碳钢孔蚀的发生,SO₄^2-、CO₃^2-、NO₂^-、MoO₄^2-和PO₄^3-5种离更多还原

【Abstract】 The electrochemical behaviors of early stage of pitting on A3 carbon steel in NaNO₂ and NaHCO₃ solutions were studied. The current-time measurement, polarization curve measurement and EIS were employed. SEM was also used to observe the corrosion morphology of carbon steel surface. The main results are as follows:1. The current fluctuations of A3 carbon steel in NaNO₂ + NaCl solutions were studied. The current fluctuations showed the feature of quick rise and relatively slow decay, indicating quick breakdown and slow repassivation of the passive film on the surface. The current fluctuations resulted in observable pits on sample surface. A larger pit may be composed of several smaller pits. The occurring potential for metastable pits E_m decreased with the increase of chloride concentration, indicating that Cl- promotes nucleation of metastable pits. Under potentiostatic control, the average peak current and number of fluctuations all increased when appliedpotential was raised. This indicates that the potential increase may accelerate the active dissolution and make more active sites, hence the induction rate of pitting increase.2. Metastable pitting behavior was observed for carbon steel in NaHCO3 + NaCl solution, which was represented by current fluctuations with the characteristic of quick increase and slow decrease. Em and Eb all obey normal distribution. In potentiostatic test, when potential was above Em and far below Eb, the current fluctuations remained for a period then disappeared, resulting in small pits with diameters of microns. When potential was close to Eb, the current may finally rise continuously after a certain time of fluctuations, and metastable pitting was replaced by stable pitting. Fluctuation frequency decreased with time. There is no apparent relationship between potential and the frequency of fluctuations.3 . The effects of CO32", NO2", SO42", MOO42" and PO43" anions on meatstable and stable pitting behaviors of A3 carbon steel in NaHCC}? + NaCl solutions were studied. It was found that these anions all made Eb and Em move to positive direction, indicating that they retard not only nucleation of pitting corrosion but also nucleation of metastable pitting. NO2" ions showed the strongest inhibition on nucleation of metastable pitting. The effects of these anions on nucleation and growth of metastable pits are very similar to those on stable pits.4. A larger pit on carbon steel surface may be composed of severalsmaller pits, especially in NaNO₂ + NaCl solutions, which suggests that a repassivated metastable pit may still be active site for following metastable pits to nucleate. Repeated local dissolution events around a certain active site caused accumulated corrosion damage. The pit size distribution calculated from the current fluctuations was very close to that observed with SEM, indicating that there is good correlation between current fluctuations and growth process of metastable pits. It is possible to make prediction for accumulated pitting damage of carbon steel in certain systems according to the current fluctuations and the calculation of current value. Corrosion products may pile up at pit mouth and lead the pit to an occluded state, thus metastable pitting process was replaced by stable pitting. Once the pit mouth was completely blocked up by corrosion products, the pit would stop growing.5. The pitting behaviors of A3 carbon steel in NaHCO₃ and NaNO₂ solutions were compared. It was found that many metastable pits occurred before stable pits for carbon steel in NaHCO₃ solution. While in NaNO₃ solution, a few metastable pits occurred and they would be replaced by stable pits soon. Many individual current fluctuations of carbon steel in NaBCO₃ solution leaded to many stochastic pits on surface. While in NaNO₂ solution it was found that a larger metastable pit was usually composed of several smaller pits, which was consistent with the overlapped currents on current-time curves. The potential step measurements, EIS and更多还原