Deposition of Nickel and Cobalt Ions with Iron Crud on Heated Stainless Steel Surface under Nucleate Boiling Conditions

Abstract

Deposition behaviors of Ni(II) and Co(II) ions on a heated surface using simulated Fe crud which was mainly composed of amorphous Fe(ill) hydroxide have been studied under nucleate boiling conditions at 553K and 70atm. The deposition process of Ni(II) and Co(II) ions is divided into two stages. The first stage is the deposition of hydroxide precipitate on the heated surface by microlayer evaporation and drying out in the nucleate boiling bubble. The second is settlement by conversion of hydroxide into oxides such as NiO, NiFe,o., CoO and CoFe 2 0 4 • The effective deposition coefficients of Ni (II) and Co (II) ions, without supplying Fe crud, are smaller than that of a-Fe,O, because of the solubility of those hydroxides at a low concentration condition. Their effective deposition coefficients increase with the simulated Fe crud concentration in feedwater and saturate at a value of 0.3 which is calculated theoretically, because the Ni and Co hydroxides react with the simulated Fe crud to produce insoluble NiFe 2 0 4 and CoFe,O. on the heated surface. The reaction of Ni deposit with a-Fe,0 3 does not proceed, but NiO is produced. The reaction rate of Ni deposit with the simulated Fe crud on the heated surface is higher than that of Co deposit.