Zr-4合金氧化膜显微组织与疖状腐蚀机制研究

经过不同热处理后的几种Zr-4合金样品,在550 ℃/25 MPa超临界水中腐蚀时都不同程度地发生了疖状腐蚀.用扫描电镜研究了氧化膜的显微组织.提出Zr-4合金发生疖状腐蚀的机制:Zr-4合金腐蚀生成的部分氧化膜具有微孔和微裂纹少、比较致密的特性,生长到一定程度后,在应力作用下,局部薄弱区发生平行于O/M界面的开裂并不断扩大,造成表层氧化膜破裂,腐蚀介质水进入裂纹中,形成有效的供氧源,使局部腐蚀加速,发生不均匀腐蚀,这种不均匀腐蚀在适当条件下发展成疖状腐蚀.氧化膜局部产生了可向O/M界面提供充足氧的直接供氧源,是引发锆合金产生疖状腐蚀的最密切因素.所有与发生疖状腐蚀有关的其它因素,如合金元素、第二相的大小和分布、氧化膜生长各向异性等,都是通过对氧化膜相关性质的影响而发生作用.

Microstructure of Oxide Film and Nodular Corrosion Mechanism of Zircaloy-4 Alloy

The corrosion behavior of Zircaloy-4 alloy after different heat treatments in 550 oC/25 MPa supercritical water was investigated. Microstructure of the oxide film was observed by SEM. Nodular corrosion was observed in all the alloys regardless of heat treatment. Based on the microstructure observation, the nodular corrosion mechanism of Zircaloy-4 was discussed. The initial oxide film formed on Zircaloy-4 after corrosion is dense with very few micro-pores and micro-cracks. However, densities of oxides and Zircaloy-4 substrate are different and it leads to expansion tendency of brittle oxide film which imposes complex stress on the film. Therefore, crack formation and propagation occur at oxides/metal interface and result in the rupture of the film in the weakest regions under the effect of the stress which increases with the growth of the film. Subsequently, corrosive water penetrates into the cracks and forms effective oxygen sources, which accelerate the localized corrosion. The non-uniform corrosion in the film finally evolves to nodular corrosion. The formation of the effective oxygen sources at oxides/metal interface is the most important factor for nodular corrosion. Other factors, such as alloying elements, heat treatments, size and distribution of secondary phases and anisotropy growth of oxides, could also affect the nodular corrosion behavior via changing the microstructure of oxide film.