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Sumitomo Metals Demonstrates Superior Corrosion Resistance for Tanker Applications

12/29/2008 - Sumitomo Metal Industries completes an extended corrosion-resistance trial of its SMICORE steel plate in an actual vessel, proving that SMICORE has higher corrosion resistance than conventional steel in the severe corrosion environments of both upper decks and bottoms.

Sumitomo Metal Industries, Ltd. recently completed an extended corrosion-resistance trial of its SMICORE (SMI Crude Oil Corrosion Resistant) steel plate, which is designed to offer enhanced corrosion resistance for the upper decks and bottoms of crude oil tanks in oil tankers.
 

In the case of upper deck plates, corrosion can lead to losses in plate thickness of more than 0.1 mm a year. Although the plates can be painted to protect them against the corrosive atmosphere, this solution leads to increased maintenance costs due to repainting.
 
Bottom plates typically suffer localized pitting due to the corrosive conditions. Pitting is often confirmed at the time of dry dock inspections, which are carried out approximately every two-and-a-half years.

Sumitomo Metals developed SMICORE in 2004 as part of its materials-based solution for these problems. SMICORE, a low-alloy steel plate, has mechanical properties, weldability and weld-joint properties that are equivalent to those of conventional steel plates, but offers superior corrosion resistance in both the upper decks and bottoms of crude oil tanks because it is made of a single steel material.
The trial, which was carried out on an actual vessel over a period of approximately two years and nine months, proved that SMICORE has higher corrosion resistance than conventional steel in both upper decks and bottoms, which are typically exposed to severe corrosive conditions. The severe corrosive environment, set up by the coexistence of crude oil, oxygen, hydrogen sulfide, chlorides, and water, has led to long-standing problems including general corrosion1 of the obverse surface of the tanks' upper deck plates, and pitting corrosion2 of the bottom plates.

SMICORE was applied on a trial basis in "SANKO BLOSSOM", a crude oil tanker that went into commission in August 2005. SANKO BLOSSOM was built by Sumitomo Heavy Industries Marine & Engineering Co., Ltd., and chartered by The Sanko Steamship Co., Ltd.

At the time of the ship's first dry dock inspection (approximately 33 months after it was commissioned), Sumitomo Metals conducted corrosion research on the parts to which SMICORE had been applied, including the ship’s No. 2 and No. 3 Tanks. Corrosion results confirmed that the reduction in thickness of the upper deck plates was around 60% less than for plates made of conventional steel.

Although conventional steel plates exhibited pitting corrosion to depths exceeding 2 mm at the time of the dry dock inspection, SMICORE steel plated exhibited no pitting corrosion at a depth of more than 2 mm. The SMICORE plates also exhibited an approximately 25% decrease in the depth of pitting corrosion as compared to conventional steel in the No. 2 Tank, and around a 60% decrease compared to conventional steel in the No. 3 Tank. Based on the pitting corrosion thickness of the parts studied, when Sumitomo Metals estimated the maximum pitting corrosion depth for all tanks by means of statistical processing, the company found that the maximum pitting corrosion depth for SMICORE would be about 40% that of conventional steel.

Although there has been a movement within the International Maritime Organization (IMO) to make the painting of crude oil tanks in oil tankers compulsory, Japan has proposed permitting the use of corrosion-resistant steel for such applications. Sumitomo Metals’ trial on an actual vessel demonstrating the effectiveness of corrosion-resistant steel to counter the corrosion of crude oil tanks could help to facilitate the future adoption of the use of corrosion-resistant steel.

1) General corrosion, which is typicallydistributed more or less uniformly over the surface of a steel plate, can proceed at rates of 0.1 to 0.3 mm per year in the case of large crude oil tankers, depending on maintenance quality and frequency.

2) Pitting corrosion is defined as localized corrosion in the form of cavities or pits several millimeters in diameter. In the case of large crude oil tankers, a tank surface may develop thousands of pits, and this pitting corrosion proceeds depth-wise at a rate of several millimeters a year.