NSGP-1&2: Suppressing the Development of Corrosion in Two Different Corrosive Environments—Tank Bottom Plate Sections and Tank Rear Sections

Highly corrosion-resistant NSGP features both paint-free application and high performance as a steel plate for crude oil tankers. It is available in two grades: NSGP-1 for bottom plate sections of tanks and NSGP-2 for rear sections of tanks, both of which demonstrate the highest level of corrosion resistance due to the addition of trace amounts of alloying elements.

NSGP-1&2 have already been applied in the record number of tankers in the world, and have proven world-class reliability through year-long application tests in working tankers over ten years. Their paint-free performance enables reductions in both maintenance costs and shipbuilding costs. Also, the welding and fabrication of NSGP steel plates can be conducted in the same manner as conventional steel plates, and thus no special process control is required.

Furthermore, NSGP-1&2 help to prevent environmental contamination caused by painting and offer diverse benefits in shipbuilding and maintenance.

*NSGP: Nippon Steel Green Protect

Highly Corrosion-resistant Steel Plate for Bottom Plate Sections of Crude Oil Tankers NSGP-1

The pitting (pitting corrosion) that occurs in the bottom plate sections of crude oil tankers has conventionally been a critical problem. NSGP-1 can suppress the occurrence of this corrosion. Nippon Yusen Kaisha and NIPPON STEEL have developped NSGP-1 and in 2004, this product was experimentally adopted in a tanker and, in a periodic inspection 2.5 years later, demonstrated that it remained free of pitting corrosion, even under paint-free conditions. NSGP-1 has since been applied in many tankers.

Highly Corrosion-resistant Steel Plate for Tank Rear Sections of Crude Oil Tankers NSGP-2

Every surface of upper decks (ceiling parts) is subject to corrosion. NSGP-2 can suppress such corrosion thanks to corrosion-protection performance derived from original research. This plate formally came to market in 2014 after diverse application tests conducted on working tankers. Particularly the shape of the ceiling(deck) sections of most tanks is complex, and thus the time required for repairs can be significantly reduced due to the paint-free properties peculiar to NSGP-2.

Reducing Corrosion in Tank Bottom Plate Sections to 1/15 NSGP-2

There are cases in which the depth of pitting (pitting corrosion) that occurs in the bottom plate sections of tanks reaches 4 mm or more in a year. If left untended for five years, there is a danger of such pitting corrosion to penetrate entirely through the plate thickness.

Even in a comparison of the number of occurrences of pitting corrosion of 4 mm or more in depth (per tank) after five years of service, tanks using conventional steel registered about 200 instances, while tanks using NSGP-1 registered about 13 instances－reduction to less than 1/15. This proves that NSGP-1 possesses high corrosion resistance.

Comparison of Number of Occurrence of Pitting Corrosion (4 mm or more in depth) after Five-year Application

While pitting corrosion to a depth of 4 mm or more occurs at as many as 1,300 locations in tankers using conventional steel over a five-year service period, the adoption of NSGP-1 can suppress the maximum occurrence of such corrosion to about 1/50 that of conventional steel, or 1/15 in average.

Comparison of Corrosion Test Results for Welding Material

Conventional welding materials show a large degree of corrosion in corrosion tests, and welds made with these materials are sometimes rejected in welding inspections due to corrosion in weld boundaries. In order to secure reliable welding, highly corrosion-resistant welding materials for exclusive use with NSGP have been prepared.

Reducing Corrosion in Tank Rear Sections by About 60%

The ceiling (or deck) plate section of a tank presents two factors attributable to the progress of corrosion.

One of these is the corrosive environment caused by the waste gas and the effect of H2S that escapes from crude oil as mentioned above. Another factor is the adverse conditions caused by repeated drying and condensation due to daytime and nighttime temperature differences resulting from the fact that the ceiling plate sections of the tanks also collectively serve as the deck of a tanker. It is reported that the average corrosion rate of these ceiling sections using conventional steel reaches nearly 0.1 mm/year, but there is the possibility of this level being exceeded depending on the change of the application environment.

　It has been proven from the results of 10-year application of NSGP-2 in working tankers that the estimated loss of plate thickness of NSGP-2 after 25 years can be suppressed to about 60% that of conventional steel.

Comparison of Change in Corrosion Loss

When corrosion loss over a 25-year span is estimated from 10-year application of conventional steel and NSGP-2 in working tankers, it is predicted that, while corrosion loss in conventional steel will surpass the target value of 2 mm, the corrosion loss in NSGP-2 will be held to about 60% that of conventional steel.