Abstract
The specific influence of calcium and sodium cations on the rate of deposition of alpha -Fe sub 2 O sub 3 (a main corrosion product in boilers and heat exchangers) has been experimentally studied. A deposition model based on the microlayer evaporation and dryout phenomena that occur in the nucleate boiling bubble is put forward for an interpretation of the deposited layer. It has been found that the rate of deposition of Fe sub 2 O sub 3 increases with an increase in valency of the soluble cations. With calcium, the deposition rate increases linearly with an increase in its ionic concentration, whereby the rate is multiplied by 5.9, 6.8 and 7.6 with 200, 400 and 600 ppm calcium respectively. Development of the depositon layer takes place in the hollows in the surface contour according to a microlayer evaporation mechanism. Successive deposition occurs at the periphery of the first deposit. A reduction in cation content in the crude solution and the selection of smooth heated surfaces are recommended to reduce the alpha -Fe sub 2 O sub 3 deposition on heated surfaces in boiling water. 18 ref.--AA