Cation distribution in ferrites and its effects on the chemical dissolution behaviour

Ferrites are formed on the steel surfaces as a protective corrosion oxide film on the heat transport surfaces in the water cooled nuclear reactors. These oxides film acts as a host to many neutron activated corrosion products (ACPs) leading to man-rem problem during the service maintenance. Understanding of chemical dissolution kinetics of these ferrites is important aspect in the development of decontamination process with aim of good decontamination factors. Ferrite shows a cation distribution as a function of parameter like metal ion substitution, crystallite size and temperature. Change in the cation distribution in ferrite can effect its dissolution process. The following three ferrites namely CoFe₂O₄/ZnFe₂O₄/MgFe₂O₄ were studied for its chemical dissolution behaviour as a function of the cation distribution. CoFe₂O₄, MgFe₂O₄ and ZnFe₂O₄ shows an inversion parameters of 0.95, 0.46 and 0.06 respectively. The above ferrites with different cation distribution were achieved by the thermal treatment. The variation of cation distribution in ferrite was monitored/characterised by the Raman spectroscopy. Chemical dissolution of these ferrites were carried out in NAC formulation. Dissolution process was monitored by the metal ion dissolution in the solution. Dissolution data was fitted to the following two models 'Shrinking sphere model' and 'Factual chain mechanism model' to elucidate the kinetic parameter. We tried to establish correlation between the cation distribution in the ferrite and the dissolution kinetics of ferrites. ZnFe₂O₄ ('δ'= ∼ 0.06) showed k⁸⁰_obs(Fe) = 1.250 x 10^-3 min^-1and ZnFe₂O₄ ('δ' = ∼ 0.30) showed k⁸⁰_obs(Fe) =2.295 x 10^-3min^-1, indicating ZnFe₂O₄ with high inversion parameter showed higher dissolution rate. Activation energy for the ZnFe₂O₄ ('δ'= ∼ 0.30) and ZnFe₂O₄ ('δ'= ∼ 0.06) in NAC formulation was 58.4 and 61.5 kJ mol^-1 respectively. CoFe₂O₄ and MgFe₂O₄ also showed the dependence of its chemical dissolution behaviour on the cation distribution. From the above study we could establish effect of cation distribution on the dissolution kinetic of the ferrites. (author)