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

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Deposition behaviors of Ni(II) and Co(II) ions on a heated surface using simulated Fe crud which was mainly composed of amorphous Fe (III) hydroxide have been studied under nucleate boiling conditions at 553 K and 70 atm. 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, NiFe2O4, CoO and CoFe2O4. The effective deposition coefficients of Ni(II) and Co(II) ions, without supplying Fe crud, are smaller than that of α-Fe2O3 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 NiFe2O4 and CoFeO4 on the heated surface. The reaction of Ni deposit with α-Fe2O3 does not proceed, but Ni0 is produced. The reaction rate of Ni deposit with the simulated Fe crud on the heated surface is higher than that of Co deposit.