Advanced surface analytical methods are outlined, and they are used to obtain new information on the surface phenomena of materials surfaces. For instance, angle-resolved X-ray photoelectron spectroscopy is used for analyzing the surface segregation of chromium and nitrogen on stainless steels and the native oxidation of their surfaces. X-ray absorption spectroscopy using synchrotron radiation is utilized for analyzing the chemical state and local structure of Fe in green rust.
In situ analyzing techniques for investigating a surface and interface change during corrosion and oxidation of metals by using Raman scattering spectroscopy (Raman), X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) are present. The Raman spectra revealed that a crystal structure and distribution of corrosion products varied during corrosion progress at elevated temperature and high pressure electrolyte. Time dependent XRD measurments made clear the behavior of the electrochemical reduction of a rust and the iso thermal transformation of a scale on a steel. It was demonstrated that XPS was capable of the in-situ measurments for initial stage of high temperature oxidation.
Cyclic corrosion tests, consisting of NaCl solution spraying, drying, and wetting process, were conducted using a crevice-structured spot-weld specimen of SUS436L in order to clarify the relation between corrosion behavior and amount of chloride accumulated into the crevice. The main component of chlorides accumulated into the crevice was sodium chloride coming from sprayed NaCl solution, although far less amount of metallic chlorides was also contained. The chloride content in the crevice depended on the concentration of spraying NaCl solution. When the concentration of spraying NaCl solution was 0.86 kmol･m－3, the amount of chloride reached about 20 mmol･m－2 and the maximum corrosion depth in the crevice increased as cycle number increased. When the amount of chloride reached about 20 mmol･m－2, the total amount of iron and chromium ions reached about 0.5 mmol･m－2 in the crevice. It is considered that pH in the crevice reached under depassivation pH of SUS436L, when the total amount of iron and chromium ions were 0.5 mmol･m－2. It is necessary that chloride which contains specific amount of metallic chlorides accumulate into the crevice for corrosion to proceed in the depth direction.
The objective of this study is to clarify dissolution behavior of duplex stainless steels in crevice corrosion growth step. Crevice corrosion grew on the duplex stainless steels by holding potential at various values. After a certain period, dissolution behavior of the crevice corrosion was analyzed based on observation of corroded area. Change in crevice corrosion dissolution behavior with time was also investigated by means of in-situ observation. Four types of dissolution regions could be observed in the corrosion area from outside toward inside direction, i.e., tip of the corroded area toward initiation point, in a crevice; they were passive region, the preferential dissolution region of γ phase, α phase and γ phase dissolution region, and the preferential dissolution region of α phase. The crevice corrosion which maintained that four dissolution regions grew toward the edge of the crevice. This dissolution behavior was given at all the potential where crevice corrosion occurred.