Effects of heat flux on corrosion of high pressure boilers

Abstract

The results of experiments on high pressure boiler rigs at Harwell and elsewhere are used to illustrate the processes which can give rise to increased corrosion rates under heat flux. The presence of a heat flux increases the metal surface temperature, and, where boiling occurs, can lead to the concentration of solutes present in the feedwater. In the absence of boiling, the increase in corrosion rate owing to the presence of a heat flux should be small and related to the known activation energy of the process. In the nucleate boiling region, the increased surface temperature and mass transport to the wall can lead to the production of thicker coherent oxide films in the presence of soluble corrosion products with reverse solubility curves. Where defects are present in the surface of the boiler tube, or porous structures develop, enhanced boiling can occur, and corrosive salt concentrations many orders of magnitude greater than in the feedwater can be developed within them. Similar concentrations of salt may form, where the tube surface becomes dry, at low steam qualities, as a result of the effects of tube geometry on flow, or where the annular film dries out in a once-through boiler. Impingement of droplets on the boiler tube wall beyond the dryout zone can also increase corrosion. Salts dissolved in superheated steam interact with the boiler wall and may become irreversibly incorporated in the growing oxide. The effects of different water chemistry regimes on these processes are outlined, and the value of radiotracers shown in studying salt concentration and metal corrosion under heat flux conditions at high temperatures and pressures.