3% Cr steel has been developed as a tubing material for CO₂ oil well considering CO₂ corrosion resistance. However, an oil leakage problem occurred 6 months after the start of oil production in a CO₂ oil well which was completed by tubing of 3% Cr steel. 3% Cr steel was exposed to an alkaline NaCl completion fluid environment before exposed to the oil production environment. Corrosion under the black deposit, which developed locally on 3% Cr steel in the NaCl completion fluid, was identified to be a cause of oil leakage. The conditions of the black deposit formation on 1% Cr and 3% Cr steels were investigated by immersion test in a simulated NaCl completion fluid varying dissolved oxygen (DO) concentration, pH and temperature. 1% Cr steel is a representative Cr bearing steel and used as a comparable material. If the fluid was alkaline and DO concentration was above 77 ppb, the black deposit formed locally on 1% Cr and 3% Cr steels. Localized corrosion was observed just under the black deposit. If the DO concentration was equal to or less than 33 ppb, the black deposit did not form. Corrosion behavior of the specimens with the black deposit was also investigated in a simulated oil production environment. It is concluded that corrosion progress under the black deposit on 3% Cr steel in the production environment resulted in the oil leakage 6 months after the start of production. The black deposit was identified as Fe₃O₄ mainly by X-ray diffraction analysis. In order to prevent oil leakage on 3% Cr steels in the oil production environment, it is necessary to control the DO concentration in the alkaline NaCl completion fluid to or below 33 ppb that prevents formation of the black deposit.