THE EFFECT OF ALLOYING ELEMENTS ON THE ELECTROCHEMICAL CORROSION OF LOW RESIDUAL CARBON-STEELS IN STAGNANT CO2-SATURATED BRINE

Low residual carbon steels are those containing very low concentrations of impurities, thus the amounts of Si, Mn, P, S, Mo, Ni, Co and Cu are kept below 0.03% each. The carbon content is in range of 0.1-0.4%. This paper reports on an experimental study of the corrosion performance of low residual carbon steels in a CO2 environment, an environment frequently encountered in oil fields. The program is based on potentiodynamic testing. The effects of variations of: carbon content in the range from 0.1 to 0.4%, small additions of chromium (1, 2 and 4%), cobalt (1, 2 and 3%), silicon (1, 2 and 4%) and molybdenum (0.3, 0.6 and 1%) were examined. It is found that the corrosion rate of the carbon steels is much higher in CO2-saturated brines as compared to de-aerated base brine and the corrosion rate increases with increasing the carbon content. The corrosion rate of low residual steels alloyed with 4 Cr in CO2-saturated brine is higher than those in de-aerated or aerated base brines. In CO2-saturated brines the corrosion rate of the steels decreases with increasing chromium content. Similar results are obtained for steels alloyed with molybdenum. It is concluded that the corrosion rate is controlled by diffusion of the cathodiC reagent, implying that the effect of the different alloy additions is to alter the permeability of the surface film.