Abstract CO2 is known as an acid gas and it is commonly found in oil and gas production or in gas lift injection systems used to transport oil to the surface. During the gas injection operation, the CO2 gas in contact with completion fluid promotes the decreasing the pH value. This acidizing promotes a more corrosive environment for martensitic steels used in oil and gas wells, such as 1%Cr low alloy steel, 13%Cr martensitic stainless steel and Super 13%Cr martensitic stainless steel. Electrochemical tests were carried out in static conditions to verify the behavior of martensitic steels in completion fluid, saturated with CO2 and without CO2, at room temperature and at 60°C. According to anodic polarization curves, 1%Cr low alloy steel presented active dissolution in all tested conditions. 13%Cr and SCr13% stainless steels presented passive behavior nearby open circuit potential for both temperatures. Gravimetric tests were carried under static conditions in an autoclave at 50 psi partial pressure of CO2 and at 60° C for 1%Cr low alloy steel, 13%Cr martensitic stainless steel and SCr13% stainless steel, where SCr13% stainless steel presented lower corrosion rate, when compared to 13%Cr and 1%Cr steels. It has been observed that oxygen is an important experimental factor that affects the behavior of martensitic steels during anodic polarization.