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A rheological study is performed for a new, aqueous magnetorheological (MR) fluid based on dual oxide-coated carbonyl iron (CI), which has the potential of increasing the MR fluid lifetime due to its strong corrosion resistance to water. The paper describes the preparation and characterization of a pumpable MR fluid based on this corrosion-resistant CI with a high solids concentration for polishing of optical materials. The viscosity is studied as a function of pH of initial liquids, dispersant type, combination of two dispersants and concentration. The dual oxide-coated CI particles exhibit hydrophilic properties, resulting in a high MR fluid viscosity. The addition of glycerol significantly reduces the viscosity. The combination of diammonium citrate (DAC) polyelectrolyte with glycerol produces the lowest viscosity, which can be explained by the electrosteric effect of the surfactants and the short-chain molecular structure of DAC. There is an optimum concentration of surfactants where a maximum concentration of CI particles can be reached while maintaining a low fluid viscosity.