KINETICS OF THE CORROSION OF IRON IN AQUEOUS-ELECTROLYTES AT TEMPERATURES UP TO 300-DEGREES-C

The corrosion behaviour of iron was investigated in aqueous borate buffer solutions with and without additions of sodium chloride and in quaternary salt brine at temperatures between 150-degrees-C and 300-degrees-C. A magnetite film grows according to the parabolic rate law. The parabolic rate constant is independent of the pH-value, increases with the electrode potential and with the chloride concentration, and attains a maximum at about 250-degrees-C. The parabolic rate constants are compared to literature data and theoretical calculations. - After some time the corrosion rate drops to a steady state value, if the chloride concentration is not too high and the electrode potential not too positive. The steady state corrosion rate increases with the temperature according to an activation enthalpy of 41 kJ/mol and remains less than 10 mu-A/cm2 up to 300-degrees-C. Above some critical electrode potential becoming more negative with the concentration of chloride, a local corrosion is observed producing flat pits covered by thick oxide layers. At the very high chloride concentration of the quaternary salt brine the whole surface becomes activated and corrosion proceeds rather uniform.