[en] The corrosion of carbon steel in CANDU reactor outlet feeders is controlled by the dissolution rate of the magnetite protective layer. Several models have previously been developed to predict the corrosion rate and the extent of corrosion product transport in the coolant circuit. These predictions strongly rely on the knowledge of accurate magnetite dissolution rate constants under normal operating conditions. Dissolution is studied for a magnetite pellet and a magnetite single crystal. The pellet is fabricated by compacting pure magnetite powder at 500 MPa using a uniaxial compaction technique, then sintering the compact under argon atmosphere at 1100°C for 5 hours with a heating rate of 12°C/min. The resulting product is a pure, dense and hard magnetite body, capable of withstanding erosion in a high-velocity and high shear stress stream. Dissolution experiments are carried out under the typical coolant conditions in CANDU outlet feeders (310°C, 10.3 MPa and pH_25°C of 10.5 adjusted with LiOH) using a high-velocity jet impinging on the magnetite surface. Precipitation of hematite took place instead of dissolution during the first two Runs for the sintered pellet and for the two single crystal Runs. The corrosion of brand new stainless steel was suspected to cause the release of dissolved iron that precipitated on the magnetite surface. Dissolution occurred in all other Runs. For the sintered pellet, close to the jet impact, localized dissolution takes place and a grain-specific terrace structure forms. Further away, the dissolution is more uniform and is similar to an etching attack proceeding evenly on the surface. For the single crystal, more experiments are required to understand the mode of dissolution. The magnetite dissolution rate ranges from 1.3 to 7.23 x 10^-11 g_Fe/cm².s. The dissolution rate constant, ranging from 0.0032 to 0.044 cm/s, suggests a surface reaction control under these conditions. However, the disagreement with the microscopy analysis raises doubts about the validity of the correlations used to determine the mass transfer coefficients. (author)