Corrosion resistance and nature of surface film on Fe-Si alloy in boiling sulfuric acid

The thermochemical hydrogen generation process has been developed at Japan Atomic Energy Research Institute. This process has a severely corrosive env ironment, such as sulfuric and hydriodic acids. A silicon-iron alloy with f ractional gradient is one of the candidate materials in a boiling sulfuric environment. So, the corrosion resistance and the passive film of Fe-20 mas s%Si alloy in boiling sulfuric acids were studied in this work. The Fe-20 mass%Si alloy was prepared by are melting of iron (99.9 mass%) an d silicon (99.99 mass%). The specimens were ground to a 600 grit finish and rinsed with distilled water and acetone. The specimens were immersed in 50 vol% or 95 vol% sulfuric acids at the boiling temperature (140 degrees C o r 320 degrees C) and then weighed to yield a corrosion rate. The passive fi lms on the specimens were evaluated in terms of the appearance, Auger elect ron spectroscopy (AES) analysis and X-ray photoelectron spectroscopy (XPS) analysis. The alloy showed good resistance in boiling 50 vol% and 95 vol% sulfuric ac ids. By AES, the passive film of the specimen exposed 50 vol% sulfuric acid consisted of O and Si. The passive film of the specimen exposed to 95 vol% sulfuric acid consisted of O, Si and S. It seems that S in the passive fil m of the specimen exposed to 95 vol% sulfuric acid is attributable to the c athodic reduction of sulfuric acid. The growth rate of the passive film of the specimen exposed to 95 vol% sulfuric acid is about 30 times as fast as that of the specimen exposed to 50 vol% sulfuric acid. It is considered tha t the faster growth rate is attributable to the low density of the film. Fr om the result of XPS, the passive films on the specimens exposed to 50 vol% and 95 vol% sulfuric acids can be described as SiO2 and as a composite of SiO2 and SiO, respectively.