JFE 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
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. While in the single-hull tankers 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.
These well known facts prompted three Japanese steel manufacturers JFE Steel Corporation, Nippon Steel Corporation and Kobe Steel Ltd to submit a proposal to IMO to accept the use of corrosion-resistant steels as an alternative method of preventing corrosion in crude oil tanks. Following thorough discussion, IMO issued its new Performance Standard for Alternative Means of Corrosion Protection for Cargo Oil Tanks of Crude Oil Tanker' as an extension of the Performance Standards for Protective Coatings in 2010.
Research performed over a ten year period by JFE Steel Corporation in a crude oil tanker delivered in 2008 confirmed that the corrosion-resistant steel dramatically reduced both pitting corrosion and general surface corrosion compared to uncoated conventional steel. Various other vessels featuring cargo tanks fitted with these advanced steels have been in operation since the new IMO PSPC standard came into effect and have demonstrated vastly improved corrosion resistance. The technology can thus be considered as validated.
DNV GL has since revised its relevant ship construction rules to incorporate corrosion-resistant steels for cargo tanks, and recently added the new classifier to its existing corrosion protection class notation. Announced in July 2020, the new notation COAT-PSPC(CA) confirms a ship’s compliance with the corrosion protection requirements for cargo oil tanks of crude oil tankers, by application of approved corrosion-resistant steel grades in one of the following areas of a cargo tank:
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)
Besides using the approved steel grades, the notation also implies the use of appropriate, approved welding consumables to join the plates. Compliance with both requirements must be substantiated by submitting specific documentation. The class notation will enter into force six months after publication of the rules, i.e. in January 2021.