COLLOIDS A AND Colloids and Surfaces SURFACES A: Physicochemical and Engineering Aspects 132 (1998) 315-319 ELSEVIER Brief communication On the mechanism of hematite deposition on a metal surface under nucleate boiling conditions 1 L. Wen, C.A. Melendres * Argonne National Laboratory, Chemical Technology Division, 9700 South Cass Avenue, Argonne, IL 60439, USA Received 27 January 1997; accepted 19 April 1997 Abstract The deposition of colloidal hematite particles on a stainless steel surface under nucleate boiling conditions at 100°C was studied using video and scanning electron microscopy techniques. Deposition was found to occur primarily at the interface boundary of the gas-bubble-liquid-solid (heated surface). The amount of deposit depended mainly on the size of the bubble and its residence time on the surface. Particle-bubble collisions apparently result in attachment of the particles to the bubble because of their hydrophobic nature and subsequent accumulation at the triple interface. These results suggest a mechanism of deposition which is different from the generally employed microlayer evaporation model. A "Partiubble Theory" which emphasizes the central role of particle-bubble interaction in the deposition of hydrophobic hematite suspensions appears more appropriate and is suggested. © 1998 Elsevier Science B.V. Keywords: Hematite; Deposition; Metal surface; Scanning electron microscopy; Microlayer evaporation model I. Introduction The phenomenon of scaling, or unwanted depos- ition of oxide particles on heat-transfer surfaces, such as boiler tubes, is of great interest because it reduces heat transfer efficiency and can exacerbate localized corrosion of piping and tank walls [1,2]. The oxide particles, generally considered to be hematite (~-Fe203), are corrosion products of col- * Corresponding author. Also with the Materials Science Division, ANL. t The submitted manuscript has been authored by a con- tractor of the US Government under contract No. W-31-109-ENG-38. Accordingly, the US Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for US Government purposes. 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0927-7757 (97)00189-1 loidal dimensions generated in various parts of the boiling water loop and redeposited on heated areas under nucleate boiling conditions. A number of studies have been carried out in order to elucidate the mechanism of deposition, and various theories have been developed to explain the scaling process. Most prominent among them is the microlayer evaporation model (MEM), originally suggested by Moore and Mesler [3] and later developed by other workers [4,5]. While the theory adequately predicts the relationship between the amount of deposit and the heat flux, it does not provide a satisfactory mechanism for the deposition process. Recently, Iwahori et al. [6] suggested a possible mechanism of deposition based on the interaction of the suspended particles with the bubbles because of the hydrophobic character of the hematite col-