Carbon steel, being made up of mainly iron and carbon is by far the most important alloy that have been extensively used in various petroleum refinery applications. Working at different ocean depth, carbon steel used in the petroleum refinery applications are generally exposed to five different zones, namely atmospheric zone, splash zone, tidal zone, submerged zone and seabed-embedded zone which are subjected to the pH level between 7.5 to 8.3 as well as variation in temperature with ocean depth. The corrosion rate of carbon steel tends to be more vigorous at condition with lower pH level and higher temperature. In this book, the effect of corrosion agents on the microstructure and relationship between the pH conditions and the corrison rate, surface hardness, profilometry of carbon steel, corrosion mechanism in petroleum refinery applications were discussed.