TMCP Steel is not to be blamed for higher corrosion rate

This is the conclusion of a 3-year project carried out by the Japanese Ship Research Panel 242.

The SR242 Panel included participants from universities, tanker operators, shipbuilders, steel mills and Class NK. The Study was triggered by reports on excessive corrosion in uncoated plating of cargo oil tanks in newly built crude oil tankers. Most tanker owners and operators have been concerned about this accelerated corrosion.

TMCP (thermo mechanical controlled processed) steel, used in shipbuilding by the Far East shipyards, was suspected of being less resistant to corrosion. The findings of this study found that TMCP steel has the same maximum corrosion rate as MS (mild steel).

Extensive laboratory tests combined with measurements in the cargo tanks of crude carriers concluded that an oil coat applied to the tank top and tank bottom is as protective against corrosion as paint is. Pitting corrosion occurs when the oil coat is defective. The Study concluded that the high pressure of COW machines might damage the oil coat layer. This would explain a higher pitting density in double hull tank bottoms and upper decks. A double-hulled tanker has fewer bottom transverses and longitudinals in its cargo tanks and thus its tank top is therefore subject to more direct bombardment during the COW process and thus more pitting corrosion than in a single-hulled tanker. However, the maximum corrosion rate during a dock-to-dock period is the same for both single- and double-hulled tankers.

One recommendation to prevent excessive pitting corrosion in double-hulled tankers is to minimize the defect frequency of the oil coat by reducing the COW pressure. INTERTANKO has identified the general need to reconsider the COW application for double-hulled tankers. This is addressed in a special study called CRUCLEAN, which is likely to be concluded in 2003.

Sulphur, especially hydrogen sulphide (H2S), accelerates corrosion to a great extent. The Study confirmed that reducing the sulphur content in crude oil would markedly reduce corrosion in oil cargo tanks. INTERTANKO has data which show that unfortunately the H2S content in some cargo oil has increased over recent years.

The Study Report was presented during the Shanghai seminar and workshop (8 & 9 November 2002) but an official publication is not expected before the end of 2003. From the presentation given, it was not clear whether the Study did or did not address microbial induced corrosion, which might also play an important role in the initiation of pitting corrosion.

Contact: Dragos Rauta