Corrosion behavior of American Petroleum Institute (API) 5L grade B and X52 steels in 3% sodium chloride in distilled water was studied under CO2 and H2S gas mixtures (varying from 7 (483) to 72 (496) psi (Pa), and from 0.0017 (12) to 0.0060 (41) psi (Pa), respectively) at 100 °F (38 °C). Corrosion rates were determined through a coupon weight loss technique for both steels. In general, as CO2 partial pressure was augmented, corrosion rates increased for a given H2S partial pressure regardless of the type of steel. Nonetheless, as H2S partial pressure was increased (for a given CO2 partial pressure), corrosion rates varied differently according to the type of steel tested. For API 5L grade B, with incremental H2S levels, corrosion rates reached a maximum to then decrease, whereas for API 5L grade X52, corrosions rates kept increasing with H2S partial pressure.