X80钢在干湿交替与水饱和哈密土壤环境下的腐蚀行为

目的揭示X80钢在干湿交替与水饱和哈密土壤环境下的腐蚀行为与规律。方法采用失重实验和电化学测试分析腐蚀速率与阴阳极电化学过程的变化规律,利用金相分析观察母材和焊缝的组织特征,通过SEM、EDS、XRD等微观分析手段观察腐蚀产物形貌、元素含量与物相组成,从而研究干湿交替与水饱和土壤环境对X80钢腐蚀行为的影响。结果X80钢在干湿交替环境下的腐蚀速率是水饱和下的2~3倍,其在干湿交替与水饱和哈密土壤环境下的腐蚀产物物相均由Fe3O4、FeOOH、FeS所构成。X80钢在干湿交替环境下,表面的腐蚀产物膜出现大量凹坑与裂隙,使O2在试样表面分布不均,形成氧浓差电池,并且该凹坑与裂隙有利于腐蚀性离子进入,加剧局部腐蚀。在同一环境下,由于焊接接头各区域组织差异引起的微电偶腐蚀,X80钢焊缝的腐蚀速率明显高于母材。结论干湿交替环境与土壤中大量存在的Cl−显著加速了X80钢母材及焊缝的局部腐蚀,且X80焊缝耐蚀性明显低于母材,其腐蚀机理均为氧浓差电池和局部腐蚀自催化效应共同作用,腐蚀形态也由以全面腐蚀为主(水饱和环境)转变为以点蚀+溃疡状腐蚀为主(干湿交替环境)。

Corrosion Behavior of X80 Pipeline Steel in Wetting-Drying Alternating and Water Saturated Hami Soil

The work aims to reveal the corrosion behavior and regularity of X80 pipeline steel in wetting-drying alternating and water saturated Hami soil. The weight loss experiment and electrochemistry test were used to analyze the change law of the corrosion rate and electrochemical process of cathode/anode. The metallographic analysis was used to observe the structural characteristics of base metal and welding sample. The microstructure, element content and phase composition of corrosion products were observed by SEM, EDS and XRD to explore the effect of wetting-drying alternating and water saturated soil on the corrosion behavior of X80 pipeline steel. The corrosion rate of X80 pipeline steel in wetting-drying alternating soil was 2~3 times higher than that in water saturated soil. The phase composition of the corrosion products in water saturated and wetting-drying alternating Hami soil consisted of Fe3O4, FeOOH and FeS. For the X80 pipeline steel in the wetting-drying alternating soil, a large number of pits and cracks appeared on the corrosion product film of the sample surface, resulting in the uneven distribution of O2 on the sample surface and the formation of oxygen concentration cell and the pits and cracks were conducive to the entry of corrosion ions, aggravating the local corrosion. In the same circumstances, the corrosion rate of X80 steel welding sample was obviously higher than that of X80 steel base metal, which was mainly caused by the galvanic corrosion induced by the difference of the microstructure of welding joint. The presence of a large amount of Cl- in wetting-drying alternating soil circumstance accelerates the local corrosion of X80 steel base metal and welding sample and the corrosion resistance of X80 steel welding sample is obviously lower than that of X80 steel base metal. The corrosion mechanism of X80 steel is the combination of oxygen concentration cell and local corrosion autocatalytic effect. The corrosion morphology is changed from general corrosion (water saturated circumstance) to pitting corrosion + ulcerated corrosion (wetting-drying alternating circumstance).