Corrosion-resistant steels for cargo oil tanks
Crude oil is a complex mixture of substances at varying ratios and in most cases contains at least some salt water. Even after desalination some of this brine remains dissolved in the crude and gradually sinks to the bottom of the cargo oil tank during transport. The corrosive nature of salt, along with microorganisms and other aggressive substances contained in the cargo oil, causes rust, most notably in the form of pitting, which are cavities in the steel that deepen over time.
Double-hull tankers are more prone to pitting
“Pitting corrosion is typically found in the bottom area of cargo oil tanks,” explains Dr Eva Junghans, Senior Principal Engineer and Lead of Practice Materials & Welding and Additive Manufacturing at DNV GL. “Above the cargo surface, especially on the underside of the upper deck, corrosion tends to be more evenly spread,” the expert continues. “It is primarily caused by aggressive chemicals contained in the inert gas, which is flue gas from auxiliary engines pumped into the cargo tanks to prevent an explosion of fumes rising up from the cargo.”