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This study investigates the oxidation behavior of several stainless steels and nickel-based superalloys exposed to supercritical water at 800 °C for 12 h. Characterization of the resulting oxide layers were conducted using weight change measurements, X-ray diffraction, scanning/transmission electron microscopy, and energy dispersive spectroscopy. Although the exposure time is only 12 h, the thickness of the oxide layers formed was as high as 1 μm, comprising different spinel structures. The influence of alloying elements such as Al, Nb, Mo, Mn and Ti on the corrosion behavior is investigated and possible corrosion mechanisms for each candidate alloy are discussed.

To evaluate the residual life and safe operation of metallic materials, it is crucial to accurately identify the intensity and depth of corrosion pits. Scanning acoustic microscopy (SAM) using tomographic acoustic microimaging (TAMI) under C-mode was employed to determine corrosion pit morphology and depth on aluminum alloy 7050, and the reults were cross-checked by optical microscopy. Maximum and average pitting depths of 204 μm and 80 μm, respectively, were determined by multilayer scannning, and a pitted area of 30 ∼ 40 % was calculated using binary image processing.

Passive films formed on Alloy 800 in sulphate, thiosulphate, and chloride solutions were investigated using electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS), and Mott–Schottky analysis. Experimental results revealed that the p-type or n-type semiconductivity of the passive film depended significantly on the solution chemistries. In a chloride–sulphate system, competitive adsorption of these two ions was believed to determine the type of semiconductivity. However, in a thiosulphate–chloride solution, the type of semiconductivity was attributed to the electrochemical reduction of the sulphur in thiosulphate and the subsequent adsorption of reduced sulphur species in the passive film.