Landmark anticorrosion measure

Newly developed anticorrosive steel plates offer environmental friendly and cost effective solution to pitting corrosion.

The damage caused to steel plates by pitting corrosion can result in holes several millimetres wide. The bottom plates of VLCCs’ cargo tanks have a tendency to be damaged by such corrosion, which in turn presents a significant maintenance problem.

Moreover, this corrosion can progress as fast as three to four millimetres per year at its maximum and, while high-grade alloy steel can resist such corrosion, its use has so far been considered economically unfeasible. That is why the next best method of treating the surface of plates, i.e. coating with paint, is often the preferred option and is generally adopted.

Nippon Yusen Kaisha (NYK) Line is one company that has made the coating of the entire surfaces of its fleet of VLCC?s cargo tank bottoms standard practice in recent years in order to prevent oil spillage due to corrosion and to ultimately ensure the safe navigation of the vessels. However, because of growing concerns about the application of paints adversely affecting the environment, the company has now turned toward another method of protection against damage from pitting corrosion.

NYK Line has chosen a new line of anticorrosive steel plates, recently developed by Nippon Steel Corporation, for the cargo tank bottom of its new VLCC tanker to be delivered by the Nagasaki Shipyard of Mitsubishi Heavy Industries (MHI) in the second quarter of 2004. Regarded by the ship operator to be a ?landmark product?, the new steel is claimed to slow the progress of pitting corrosion to about one fifth of that which occurs in regular steel. Additionally, Nippon Steel Corporation?s new steel plates for the cargo tank bottoms of VLCCs are not only resistant to pitting corrosion, but are also said to be available and usable at acceptable cost.

This means that this solution effectively meets NYK Line?s safety needs, with its enhanced anticorrosive property eliminating the need to use paint and thus making the company?s VLCCs even more environment-friendly. Reduced maintenance costs are a further bonus.

Shipbuilder MHI has conducted extensive tests to verify welding standards for the steel and its designed strength, to ensure its suitability for shipbuilding. The test results are said to confirm that the new steel is superior to conventional steel.

As for being superior to coatings that can be applied to cargo tank bottom plates, the steel plates obviously do not require anywhere near the same level of maintenance. More importantly, however, inferior coatings can even contribute to accelerating pitting corrosion. By contrast, the new steel is epochal in that it resists corrosion irrespective of the surrounding environment or the quality of maintenance work.

Safety and environmental protection have been the subject of much attention in recent years, particularly when it comes to the possible effects on the marine environment of volatile substances and other chemicals contained in paints for coating ships. On this matter NYK Line says “Our company has actively pursued the development of new techniques and the protection of the environment. We are hopeful, therefore, that, if the newly developed steel plates to be used for our new VLCC are confirmed to be effective against pitting corrosion in the actual navigational environment, the new steel will eliminate the need to coat cargo tank bottoms with paint and thereby go a long way toward lessening damage to the marine environment.”