High-temperature oxidation corrosion outward diffusion

In order to select the alloy best suited to this application, an extensive program of corrosion tests was carried out by ORNL (USA) on the available commercial nickel-base alloys and austenitic stainless steels [22,30—34]. These tests were performed in a temperature gradient system with various fluoride media and different temperatures (maximum temperature and temperature gradient). Chromium, which is added to most alloys for high-temperature oxidation resistance, is quite soluble in molten fluoride salts. Metallurgical examination of the surveillance specimens showed the corrosion to be associated with outward diffusion of Cr through the alloy. It was concluded that the chromium content shall be maintained as low as reasonably possible to keep appropriate air oxidation properties. The corrosion rate is marked by initial rapid attack associated with dissolution of Cr and is largely driven by the impurities in the salt [22,30—34]. This is followed by a period of slower linear corrosion rate... 

Mrowec et examined the resistance to high-temperature corrosion of Fe alloys with Cr contents between 0.35 and 74 at% Cr in 101 kPa S vapour. They found that the corrosion was parabolic, irrespective of the temperature or alloy composition, and noted that sulphidation takes place at a rate five orders of magnitude greater than oxidation at equivalent temperatures. At less than 2% Cr, the alloys formed Fe, j.,S growing by outward diffusion of Fe ions, with traces of FeCr2S4 near the metal core. 

Position of inert markers (e.g., Pt or AljOj partides) after oxidation when the scale growth is diffusion controlled (a) growth by outward metal cation diffusion, (b) growth by inward oxygen anion diffusion, and (c) growth by counterditfusion of both spedes. (From Kofstad, R, High Temperature Corrosion, Elsevier Applied Science, London, U.K., 1988.)... 

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