To study the effect of surface finish is important to predict corrosion rate that happen in the as-delivered internal pipe surface. In between the very few systematic studies of this effect on carbon steel under carbonic acid attack, it was concluded CO₂ corrosion increased with increasing surface roughness. Nevertheless, previous studies has probe onto mild corrosion conditions (room temperature, one atmosphere of pressure). Given that the formation of a tenacious iron carbonate scale can protect the surface, it is important to perform these studies at harder conditions. In this paper, we present evidence that corrosion rate can do the opposite, it can increase as the surface roughness decrease. This, however, occurs for the same reason. A rougher surface can react faster and therefore, produce a protective iron carbonate layer.