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(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 CoFe₂O₄ on the heated surface. The reaction of Ni deposit with α-Fe₂O₃ 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.

KEYWORDS:

• deposition
• precipitation
• nucleate boiling
• reaction rate
• nickel ions
• cobalt ions
• nickel ferrite
• cobalt ferrite
• nickel oxides
• cobalt oxides
• hematite
• iron crud
• amorphous iron hydroxides
• concentration dependency
• BWR type reactors
• nuclear fuels