77-17: Silica Species as Stabilizers of Stagnant Water Films

Stagnant water films have 8.8 and 2.2 times the resistance to 
heat transfer as equal thicknesses of anhydrite and gypsum, respectively. The resistance increases by a factor up to 25 if stagnant film temperature and pressure allows gas evolution to be trapped in the film. Reactions 
between the tetrahedral soluble silica, Si(OH) 4 , and the octahedral metal hydroxides are acid-base reactions which produce complex salts and water 
in floe form. These floculant precipitates stabilize the water films on 
metal surfaces. The order of solubility of metal hydroxide-silic acid 
products is similar to the order of solubility of the metal hydroxides. 
Wohlberg showed that freshly corroding metals are effective in removing 
soluble silica. 
This participation of soluble silica in the development of heat-resistant films occurred at circulating water silica contents below 75 ppm. An explanation is offered for some of the silica specie reactions that are