The effects of steel microstructure and composition on the inhibition of CO2 corrosion were studied at 25 °C, pH 5, 1 bar CO2 in 3 % NaCl solutions. Generic inhibitor compounds cocoalkyl- dimethyl-benzyl ammonium chloride (QUAT) and sodium thiosulfate (THIO) were used. Sixteen different carbon steels were tested with a blend of 40 ppm QUAT and 0.5 ppm THIO. The test specimens were precorroded for six days in the corrosive medium prior to inhibitor addition. The effect of inhibitors was assessed by electrochemical polarization tests. The inhibitor efficiency ranged from 84 to 98 %. The variations in the inhibitor efficiency were attributed to differences in the amount of carbon (as pearlite) and copper in the steel. The inhibition of the cathodic reaction on cementite
particles exposed by corrosion was identified as an important factor in the inhibition of CO2 corrosion of carbon steel.
Key words: carbon dioxide, carbon steel, inhibitors, precorrosion,