Reactions of iron oxides at elevated pressures and temperatures

There is major concern in the nuclear industry that accumulation of deposits in steam generators may cause severe under-deposit corrosion of the heat exchange tubes. Surprisingly, based on studies of corrosion products removed from the steam generators at one power plant, it was discovered that under certain secondary side conditions hard deposits are formed and they retard the corrosion of the underlying Alloy 600 tube surfaces. The main components of these deposits are iron oxides. According to the results of SEM/EDX measurements, radiotracer studies and leaching studies, corrosive species such as sulfate ions present in consolidated samples are trapped as inclusions. Consequently, low migration rates {dollar}rm({lcub}<{rcub}1.5times10sp{lcub}-17{rcub} msp2ssp{lcub}-1{rcub}){dollar} and high resistance to leaching of sulfates from these materials are observed. This prevents corrosive species from reaching the tubes surfaces.; During the present study a model of tube deposit and sludge formation was developed. The reason for the formation of consolidated deposits and hard, hematite-based sludge, at least in part, was early generation of ferrihydrite in the low-temperature part of the secondary system in the process of corrosion of iron-based components. The ferrihydrite was stabilized by the presence of dissolved copper and zinc. These species, formed as a result of the corrosion of brass components, were initially complexed by ammonia and hydrazine and later converted to oxides at steam generator temperatures {dollar}({lcub}sim{rcub}300spcirc{dollar}C). The stabilized ferrihydrite particles were then deposited on Alloy 600 tube surfaces, modified by phosphates used during early operation. This promoted strong adhesion of the accumulating layers. Drying of the deposits under high pressure resulted in the formation of a dense coating containing 30 wt % of hydrated ferric oxides with disordered structure. Additional hard material was formed as long as conditions were not strongly reducing. It was concluded that formation of this material involved transformation of ferrihydrite into hydrohematite. The study conducted on this transformation showed that ferrihydrite contains both tetrahedrally and octahedrally coordinated anion layers. During the transformation of ferrihydrite into hard hydrohematite, maghemite and protohematite are formed.; These studies succeeded in establishing the sequence of transformation processes as a function of prevailing conditions and in developing a model which correlates these transformations with the structure of the oxide species involved.