The influence of chloride ion concentration on the passivation film and corrosion product film of martensitic stainless steels (SS) (13% Cr, modified 13% Cr-1% Mo, modified 13% Cr-2% Mo, and 15% Cr SS) have been investigated using immersion tests and electrochemical measurements in sweet environments at temperatures of 150°C and 180°C in the presence of carbon dioxide (CO 2 ). The corrosion rate of conventional 13% Cr SS was found to increase with increasing chloride ion concentration at 150°C. However, the corrosion rates of modified 13% Cr SS and 15% Cr SS were independent of the chloride ion concentration at 150°C. The corrosion rates of modified 13% Cr SS increased with increasing chloride ion concentration above 1,000 ppm at 180°C. The corrosion rate of 15% Cr SS slightly increased at high chloride ion concentration at 180°C. Chromium-enriched corrosion product films were produced on all alloys considered, except for 15% Cr SS, at 180°C. A passivation film was maintained for 15% Cr SS at 180°C. In addition, the pitting corrosion behaviors were investigated by potentiodynamic polarization curves and open-circuit potential measurements. The behavior of the current density of anodic curves corresponded to the immersion test results. Pitting corrosion on modified 13% Cr-2% Mo SS is considered to repassivate because of low potential compared with pitting potential. KEY WORDS: CO 2 corrosion, high temperature, immersion test, martensitic stainless steel, modified martensitic stainless steel, pitting potential ISSN 0010-9312 (print), 1938-159X (online)

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An electrochemical monitoring of corrosion of a high pressure stainless steel decomposes in a urea plant, one of corrosion hazards often taking place in various synthesis processes of urea, was made with a newly developed electrochemical test apparatus. The process liquid in the decomposer was composed of NH3, H2O, CO2, urea, ammonium carbamate and some other minor elements. The decomposer was operated at a temperature of 120-160C and a pressure of 15-100kg/cm2. Test of corrosion of the stainless steels for the decomposer were conducted for both the process liquid in the high pressure decomposer and simulated solutions prepared from pure chemicals. Anodic polarizations showed that 316L stainless steel in the process liquid had two current

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