PH13-8Mo不锈钢在半乡村大气环境中长周期腐蚀行为

Corrosion behavior of PH13-8Mo stainless steel after long-term exposure to semi-rural atmosphere

在北京半乡村大气环境中对未经预钝化及硝酸预钝化后的PH13-8Mo不锈钢进行5 a的长周期暴晒试验,通过表面形貌观察、质量损失分析、表面钝化膜及腐蚀产物膜层分析、力学性能检测及断口分析等方法,研究了硝酸预钝化处理对PH13-8Mo长周期腐蚀行为的影响规律及机理.结果表明,经5 a大气暴晒试验,硝酸预钝化处理减轻了PH13-8Mo不锈钢的点蚀、降低其均匀腐蚀速率,通过降低PH13-8Mo不锈钢钝化膜中的氢氧化物含量、提高Cr/Fe原子比并提高大气暴晒后表面的Kelvin电位,延迟了Cl−对钝化膜的破坏及点蚀的形核,进而提高了表面膜层对基体的保护作用.硝酸预钝化处理能减少在半乡村大气环境中PH13-8Mo不锈钢力学性能的下降,但对试样的断裂方式几乎没有产生影响,二者均为韧性断裂,断口均呈现典型的“杯锥状”.

PH13-8Mo is a precipitation-strengthened,martensitic stainless steel with ultra-high strength,and satisfactory toughness and plasticity.It is generally utilized in the fields of aviation and traditional energy because of its remarkable mechanical properties and corrosion resistance,as well as its stable performance in harsh service environments.Because of the wide applications of PH13-8Mo stainless steel and the complex corrosive environments it faces,its corrosion resistance is of great significance for deciding the lifetime and safety of aircrafts and ships.However,limited by factors,including a long outdoor exposure test cycle and a large professional experimental site required,only a few reports exist on the atmospheric corrosion behavior and mechanism of PH13-8Mo stainless steel,especially the influence of chemical pre-passivation on the steel still remains relatively uninvestigated.Therefore,the outdoor exposure tests of two samples of PH13-8Mo stainless steel,with and without nitric-acid-passivated film,respectively,were performed in a semirural atmospheric environment in Beijing for five years.The effect of the pre-passivation treatment on the corrosion behavior and mechanism of PH13-8Mo stainless steel was investigated by observing the surface morphology,using the mass loss method,analyzing the passivated film and corrosion products,testing the mechanical properties,and conducting fracture analyses.The results show that the pre-passivation treatment with nitric acid reduces the pitting corrosion and decreases the corrosion rate.The pre-passivation treatment with nitric acid delays the destruction of Cl−on the passivated film and also delays the nucleation of the pitting by increasing the hydroxide content and the atomic ratio of Cr/Fe of the passivated film,and it increases the surface Kelvin potential as well,further enhancing the protectiveness of the surface film.Additionally,the pre-passivation treatment with nitric acid reduces the loss in the mechanical properties after long-term exposure to the semi-rural atmospheric environment,although it has little effect on the fracture mode,and both the steel samples exhibit the typical morphologies of a ductile fracture.