大气腐蚀性等级分类的低碳钢电化学阻抗探针方法研究

The essence of atmospheric corrosion is the electrochemical reaction under the dynamic thin liquid film. The environmental factors such as temperature, relative humidity, the composition and concentration of pollutant and other environmental factors determine the thickness of the thin liquid film on the surface of the material and the wetting time. The concentration of pollutants and the thickness of the liquid film can reflect the changes of the conductivity of the thin liquid film. While both temperature and the conductivity and the wetting time of the thin film affected the corrosion degree of the material. Therefore, in the study of atmospheric corrosivity categroy classification, the conductivity of thin liquid film has a relationship with the atmospheric environmental factors. In addition, the polarization resistance of standard test samples during atmospheric corrosion exposure process corresponds to the factors of corrosion weight loss. The comb like bi-electrode electrochemical impedance probe made of mild steel will be used in this project, aiming at the corrosivity categoru classification standard of ISO 9223 determined by environmental factors and ISO 9224 based on the corrosion loss on standard sample. By calculating the electrolyte concentration, high frequency and low frequency impedance as well as the wetting time by measuring the electrochemical impedance spectra and the change of the thickness of the thin liquid film with time of low carbon steel impedance probes in 3 typical simulated atmospheric thin film simultaneously, which will be used for calibrating the corrosivity level of the environmental factors in the real atmospheric environment and establishing the relationship between the high frequency impedance and the pollutant concentration, temperature and relative humidity. By finding out the relationship between the weight loss results of atmospheric corrosion and the classification of the corrosivity category, low frequency impedance is conversed into the corrosion rate, and it is compared with the weight loss of samples in outdoor atmosphere. This project will achieve the scientific classification of atmospheric environmental corrosivity category and provide reference and basis for the rational development of the classification of atmospheric corrosion grade standard by using the electrochemical impedance probe.

在大气腐蚀性等级分类方法研究中，薄液膜的电导率与大气环境的各项因子之间存在对应关系。标准试样在大气环境曝晒过程中的极化阻抗与腐蚀失重因子相对应。针对ISO 9223依据环境因子所确定的腐蚀性等级划分标准和ISO 9224依据标准试样腐蚀失重所建立的腐蚀性等级划分方法，本项目拟采用梳齿型双电极低碳钢电化学阻抗探针技术，通过系统地同步测量3种典型模拟大气薄液膜下低碳钢阻抗探针的电化学阻抗谱及薄液膜厚度随时间的变化，求取电解质浓度、高频和低频阻抗以及润湿时间，由此标定真实大气环境中的腐蚀性环境因子，建立高频阻抗与污染物浓度、温度和相对湿度等大气腐蚀环境因子的对应关系；将阻抗探针所测低频阻抗转换为腐蚀速度，对比试片户外曝晒失重结果，找出大气腐蚀失重结果与腐蚀性等级分类的对应关系。实现用电化学阻抗探针技术对大气环境腐蚀性等级进行科学分类的方法，为更为合理地制定大气腐蚀性等级标准分类提供参考及依据。

大气腐蚀评价主要采用实场挂片曝晒实验，虽然实场曝晒实验方法可靠，可以如实地反映材料在服役过程中的腐蚀规律，但此法缺点是工作量大、周期长、计量不准、误差大、取样间隔时间长、尤其是难以即时地反映材料的腐蚀演化规律等缺点。这使得钢铁大气腐蚀实场曝晒实验仍停留在数据积累阶段，难以描述钢铁大气腐蚀机理。大气腐蚀是一种发生在液膜下的电化学反应，因此通过电化学方法研究金属与液膜的作用及其变化规律，不仅可以评价大气腐蚀性的严酷程度并对其进行分类分级，还可以研究大气腐蚀的演化特征与机制。.鉴于此，本项目使用梳齿型阻抗探头，对材料大气腐蚀过程进行原位连续监测，研究钢铁材料表面液膜随大气环境的变化规律。高频阻抗值可以实时反映样品表面液膜状态，即液膜存在时间长短及液膜的厚薄，以及液膜的化学成分。低频阻抗值与腐蚀速度相对应，可以实现腐蚀速度的原位实时监测，评价其耐蚀性。结果表明：阻抗谱测试方法可用于材料的室外腐蚀监测研究，可以反映不同测试地点的腐蚀特征。青岛和文昌地区的大气腐蚀过程主要分为3个阶段，在有腐蚀产物覆盖，锈层不厚的第2阶段腐蚀最为严重。同时，在1天之内，阻抗谱随着温湿度的变化可分为5个阶段，且阻抗模值大小与湿度呈反向变化，即湿度越大，阻抗模值越小。根据高频相角的变化规律提出了表面润湿性判据，沈阳为65°、青岛为75°、文昌为40°。与ISO 9223润湿判据相比，除了在湿度较低的沈阳，青岛和文昌由阻抗判据所得的润湿时间远小于ISO 9223判据所得润湿时间。在沈阳和青岛两地，通过阻抗连续监测所计算的腐蚀速率与实际挂片失重结果相近，而在海南文昌地区，由阻抗计算的腐蚀速率远小于实际挂片失重结果。这说明阻抗监测虽然适用于户外，但在应用过程中仍需对所得数据进行进一步优化处理。室内干湿交替加速腐蚀试验结果进一步证实了阻抗监测的合理性，实现了对材料耐蚀性的快速准确评价，预估材料腐蚀寿命。