Abstract
Molybdenum-bearing alloys are widely used in industry because of their excellent corrosion resistance. However, the role of molybdenum in passivation is a subject which has been a matter for discussion and controversy for many years. In the previous work carried out in this laboratory, Professor Castle and Dr Qiu suggested that molybdenum oxide might provide the nuclei for formation of the passive film. This hypothesis is the basis of the present work. In order to find out the evidence for the existence of the molybdenum oxide nuclei, furthermore, to establish a model of passivation for the molybdenum-bearing alloys, the passivation of molybdenum-bearing alloys are investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), especially in-situ AFM, in conjunction with SEM and TEM. During the passivation study, it was found that the chemical composition on anodically polarised surfaces varied with potential. For 516 alloy, the peak value of molybdenum in the surface is at the low potential of the passive region close to the Flade potential. Therefore, molybdenum exerts its greatest function in this potential region. A further AFM study at this potential revealed, surprisingly, that a platelet layer formed on the passivation surface. The dendritic structure on the surface of the platelets presented under the high resolution of STM, which shows the crystallization property of the platelets. These platelets are mainly composed of chromium and molybdenum oxides and they are only found in the passivation of the molybdenum-bearing alloy, so the formation of the platelets may be associated with molybdenum nucleation in passivation. The in-situ AFM studies provide the evidence for the formation and disappearance of the platelets during the passivation of molybdenum-bearing alloys, i.e. the platelets form at the early stage of passivation and they gradually merge into the passive film if they are not disturbed by the environment. Based on the above finding, the role of molybdenum in passivation is proposed as following: molybdenum oxide precipitated on the surface seeds chromium oxide to form platelets at the early stage of passivation and the formed platelet layer prohibits the dissolution of the passive species from the metal. By this way, molybdenum facilitates the formation of the passive film on alloys. Using Fe-Cr-Mo duplex stainless steels, the corrosion of molybdenum-bearing alloys are studied in HCl and the kinetics of the corrosion are traced by in-situ AFM. During corrosion, it was observed that the ferritic phase in duplex stainless steel dissolves more rapidly than the austenitic phase and the dissolution occurs on the austenite preferentially along grain boundaries, sub-grain boundaries and the planes with high energy. By switching the electrochemical condition from active dissolution to passivation and then changing it back, it is found that under the passivation condition after the active dissolution, the corrosion changes from selective dissolution of the crystallographic feature to general corrosion. This situation persists even though the electrochemical condition changes back to the active condition from the passivation condition. Since AFM is a new technique and this is one of the first attempts at applying it to a corrosion study, an in-depth explanation of the images obtained from a corroded rough surface is an important topic on which so far little work has been reported. In this study, therefore, the artifacts and the reality of the structure in AFM images obtained in the corrosion study are discussed.