Corrosion Behavior of Low-Residual Carbon Steels in a Sour Environment

Potentiodynamic testing was used to study the corrosion performance of low-residual carbon steels in a sour brine, as typically encountered in Kuwaiti oil fields. Twenty-eight tests were performed to determine the effects of variations of carbon content from 0.1 to 0.4%; 1, 2, and 3% additions of cobalt; and 1, 2, and 4% additions of chromium. Results indicated carbon content decreased the corrosion rate in the absence of impurities and alloying elements because of the enhanced formation of corrosion products in the sour brine. The corrosion rate was about 254 μm/y (10 mpy). The addition of small amounts of Cr tended to increase the corrosion rate, regardless of the C content. For the 4% Cr samples, the corrosion rate averaged 2032 μm/y (80 mpy). From the values of β_c for C steels and Cr-alloyed steels in the deaerated base brine, aerated base brine, and in the sour brine, the study concluded that Cr always depolarized the cathodic reaction independent of the environment. The Co-alloyed steels exhibited higher corrosion rates when compared to C steels, and the corrosion rate increased in the Co-alloyed steels with higher C content. Hydrogen sulfide acted as a cathodic depolarizer in Co-alloyed low-residual carbon steels. Co caused the corrosion reaction to change from a kinetic control reaction to a diffusion control reaction. Therefore, in the presence of Co, the corrosion reaction depended highly on hydrodynamic conditions.