Commercial alloys for sulfuric acid vaporization in thermochemical hydrogen cycles

Most thermochemical cycles being considered for producing hydrogen include a processing stream in which dilute sulfuric acid is concentrated, vaporized, and then decomposed over a catalyst. The sulfuric acid vaporizer is exposed to highly aggressive conditions. A screening study was undertaken to evaluate the suitability of various commercial alloys for concentration and vaporization of high-temperature sulfuric acid. The corrosion rates of three commercially-available alloys, Saramet #23, Saramet #35, and Zecor, approached those accepted by the chemical industry. These iron-based alloys have relatively high silicon contents and the protective surface film that develops is rich in silicon, probably SiO₂. Regardless of the long term corrosion rate, the initial corrosion rate is quite high. Therefore, any components made from these alloys will have to be seasoned in sulfuric acid at elevated temperatures prior to use to prevent unacceptable levels of corrosion products in the process stream. The static corrosion rates determined in this study might be much lower than in flowing systems. This is an abstract of a paper presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 10/30/2005-11/4/2005).