The Influence of Small Amounts of KCl(s) on the Initial Stages of the Corrosion of Alloy Sanicro 28 at 600 deg C

摘要

The influence of KCl(s) on the oxidation of Sanicro 28 (35Fe27Cr31Ni) austenitic stainless steel at 600 deg C in 5 percent O_2 + 40 percent H_2O and in 5 percent O_2 was investigated. The samples were coated with 0.1 mg/cm~2 KCl(s) prior to exposure. The exposure time was 1, 24, 72 and 168 h. Uncoated samples were exposed for reference. The oxidized samples were analyzed by SEM/energy-dispersive X-ray (EDX), X-ray diffraction (XRD), and AES. The amount of chloride and chromate on the samples was analyzed quantitatively by ion chromatography after exposure. KCl(s) is very corrosive towards Sanicro 28. Corrosion is initiated by the formation of potassium chromate through the reaction of KCl(s) with the protective oxide, chloride leaving the sample in the form of HCl. Chromate formation is a sink for chromium in the oxide and leads to a deterioration of its protective properties. In the 5 percent O_2 + 40 percent H_2O environment, there is a stoichiometric relationship between the chromate formed and the chloride consumed. In dry O_2 chromate formation is relatively slow, leaving more unreacted KCl(s) on the surface than in 5 percent O_2 + 40 percent H_2O. Once the protective, chromium-rich oxide has been depleted in chromium by chromate formation, the alloy becomes susceptible to direct attack by the remaining KCl(s).