Corrosion-resistant Steel for Tanker Bilges

Corrosion-resistant Steels for Cargo Oil Tanks

The use of corrosion-resistant steels in cargo oil tanks offers certain advantages compared to coatings. Tried and tested for about 12 years, they have now been included in DNV GL’s rules, and a new class notation for these steels is available.

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.”

While in the single-hull oil tanker of the past the cooling effect of the seawater slowed down bacterial growth by keeping the cargo relatively cool, the double hull of compliant modern tankers insulates the cargo from the low temperature of the seawater. As a result, the cargo stays relatively warm, providing ideal conditions for corrosion-causing microorganisms to thrive. Pitting therefore progresses rapidly on an unprotected tank bottom, weakening the metal and risking cargo loss, structural damage and environmental pollution.

“The traditional way to prevent pitting and general corrosion has been to apply specially formulated coatings to the affected surfaces,” says Dr Junghans. “This is an expensive, time-consuming process that needs to be supervised to ensure proper execution. What is more, protective coatings typically have to be renewed from time to time.”

Corrosion-resistant Steel — A Proven Alternative Approach

These well-known facts prompted Japan's three largest steelmakers - JFE Steel, Nippon Steel and Kobe Steel - to submit a proposal to the IMO accepting the use of corrosion-resistant steels as an alternative method of tank protection against crude oil corrosion. After in-depth discussions, a new "Performance Standard for Alternative Methods of Corrosion Protection for Cargo Tanks of Crude Oil Tankers" was issued as an extension of the performance standard for protective coatings.

JFE Steel conducted a study on a crude oil tanker that confirmed that corrosion-resistant steels significantly reduce pitting and general surface corrosion compared to uncoated conventional steels. Since the new IMO PSPC standard came into force, various other ships equipped with cargo tanks made of these advanced steels have entered into service and shown much improved corrosion resistance.

New Aotation COAT-PSPC(CA) Confirms Compliance

As a result, DNV GL has since revised the relevant ship construction rules to include corrosion-resistant steels for cargo tanks and added a new classification symbol "CA" ("Corrosion Protection by Alternative Methods") to its existing corrosion protection class notation. This confirms that the ship meets the corrosion protection requirements for oil tanker:

a) Lower surface of the strength deck and surrounding structures (RCU)

b) Upper surface of the inner bottom plating and surrounding structures (RCB)

c) Both the strength deck and the inner bottom plating (RCW)

Conclude

The selection of corrosion-resistant steels for the corrosion-prone areas of crude oil tanks can bring a variety of benefits to shipowners, charterers and shipyards. For example, the cost of coatings can be eliminated and the time to complete a newbuilding at the shipyard can be shortened. In addition, because International Maritime Organization (IMO) performance standards assume a 25-year service life for cargo oil tank (COT) steel until its thickness is reduced to an acceptable level, there is no need to reapply coatings to the relevant bulkhead areas throughout the ship's service life. Owners choosing this class notation can demonstrate to cargo owners that their crude oil tankers are at a lower risk of tank bottom or wall failure, cargo loss and environmental contamination, thereby increasing cargo owners' confidence in their ships.