耐候钢表面氧化皮的结构特征及其对大气腐蚀行为的影响

Structural Features of Oxide Scales on Weathering Steel and Their Influence on Atmospheric Corrosion

采用XRD、电化学测量和扫描电子显微术研究了耐候钢在400~700℃不同时间、不同氧流量条件下形成的氧化皮的结构及其对随后的大气腐蚀行为的影响,发现氧化皮的主要组成为Fe3O4和Fe2O3。氧化皮的电阻远高于裸钢表面的氧化膜,并使样品的自腐蚀电位显著上升。氧化皮分为内外两层,外层疏松,内层致密,氧化皮的保护性主要来自内层。致密的氧化皮形成于500~600℃。延长等温时间有利于提高氧化皮的致密度。限氧条件下进行氧化处理不利于氧化皮致密化。致密的氧化皮在腐蚀初期明显延缓了大气腐蚀进程,但在长期腐蚀过程中,反而使腐蚀进程有所加快。这些结果表明,致密的氧化皮在腐蚀过程中难以转化为腐蚀产物,保留于锈层中成为杂质和缺陷,促进了腐蚀。

Oxide scale on hot rolled strip steel has been successfully applied to decrease the corrosion loss of the steel during its transport and storage. In recent years, some efforts have been made to improve atmospheric corrosion resistance of weathering steel by oxide scale on its surfaces. However,the structure and electrochemical properties of oxide scale and their evolution during atmospheric corrosion still need to be characterized. In this work, XRD, electrochemical test and scanning electron microscopy have been carried out to investigate structures of oxide scales on surfaces of weathering steel samples and their influence on subsequent atmospheric corrosion of the samples. To produce oxide scales,the samples had been held isothermally at 400~700 ℃ in open or close spaces for different times. It has been found that oxide scales consist of Fe3O4 and Fe2O3. The electrical resistance of oxide scales is far higher than that of oxide film on sample which has not been subjected to oxidation treatment. Meanwhile,oxidation results in obviously raised free corrosion potential. Oxide scales are composed of loose outer layers and compact inner layers from which protective action is derived. The relatively compact oxide scales form at 500~600 ℃ while prolonged holding time promotes oxide scales to become compact. The limited oxygen providing inhibits oxide scales to become compact. The compact oxide scales slow down atmospheric corrosion in initial stage while they accelerate atmospheric corrosion after long time. These results indicate that compact oxide scales are difficult to transform into corrosion products and remain as inclusions and defects in the rust layers which accelerate corrosion.