Characterization of the Films on Alloys in High Temperature and High Pressure Water

The role of film on surface is critical for the corrosion rate estimation and for the stress corrosion crack initiation.  There exists many results by ex-situ characterization, such as the film thickness measurement, composition and microstructure by XPS, XRD, Raman, SIMS, RBS, TEM, the electric resistance measurement, hardness detection.  In order to understand the film formation and changes in high temperature and pressurized water, the in-situ Raman technique were developed by T. Devine (1999), J.E. Maslar, NIST (2001), T. Shoji (2002), I. S. Huang (2003). 

In many cases, there exist two layers film on the surface of nickel based alloys and stainless steels.  The inner layer is Cr₂O₃, and the outer layer is spinel.  The film thickness related with the oxygen concentration, species, and exposure time.  There exist the Ni rich matrix beneath the Cr₂O₃ layer.  Electrochemical reaction is the film formation mechanism.  However, there still exist some unclear questions such as how many layers exist and whether they relate with environmental conditions?  What are the composition and microstructure of the film? What are the film properties (including the mechanical, physical and chemical properties)?  Do the film properties change with time?  Is the electrochemical reaction the only film formation mechanism?  And how does the film relate with the crack initiation?

The morphologies, microstructure and chemical compositions of oxide films grown on 316L stainless steel, 625 and 690 nickel based alloys exposed to simulated Pd contaminated pressurized water reactor (PWR) secondary side water and primary water were investigated using a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDX), an X-ray photoelectron spectroscopy (XPS) analyzer, an X-ray diffraction (XRD) analyzer and transmission electron microscope (TEM).  The main problems existing in current studies were also discussed.  The relation between the film properties and crack initiation has been discussed.