Electrochemical Impedance Spectroscopy was utilized to investigate the corrosion behavior of carbon steel in a salt spray test (SST) chamber. The salt spray was applied using 5% NaCl solution at 35 °C. A two-electrode cell comprising a pair of identical carbon steel electrodes embedded in epoxy resin were placed at six different angles (0°, 30°, 45°, 60°, 75°, and 90°) to the horizontal in the chamber. The corrosion rate (CR) of the carbon steel samples and the thickness of the solution film formed on the sample surface were evaluated from the impedances at low and high frequencies, respectively. The CR of carbon steel fixed horizontally (0°) exhibited a low value compared with that fixed at other angles. When the angle changed from 30 to 75°, the solution film thickness decreased greatly, but there was no significant difference in the CR. The CR of carbon steel under the employed SST conditions was more than five times higher than that in a bulk 5% NaCl solution under natural convection. The corrosion mechanism of carbon steel in the SST chamber is also discussed.

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This study investigated the effects of Ni addition on the corrosion resistance of steel in subtropical seashore environments. Carbon steel and 3, 5, and 7% Ni steels were exposed in such an environment for a year. Addition of Ni depressed the corrosion rate of steels and number of cracks in the rust layer. Quantitative and three-dimensional measurement of the cracks with a wide range of widths and volumes in the rust layer was carried out for the exposed steel specimens using the mercury intrusion method. The total crack volume in the rust layers on 5% Ni steel was 60% lower than that for the carbon steel. It is considered that rust layers with less crack volume suppressed Clmigration through the rust layer. The Cl concentration near the metal interface was relatively lower in the 5% Ni steel by EPMA analysis. And the rust layer on 5% Ni steel also showed a higher permeation resistance than that formed on carbon steel. Considering the formation of rust layers with less volume crack on Ni-added steel based on Morcillo's model, it is concluded that the Ni addition promoted the formation of a-FeOOH and suppressed the reduction of γ -and β-FeOOH, thus resulting in a more intact rust layer.

Pitting corrosion on the bottom plates of cargo oil tanks (COT) in very large crude carriers (VLCC) is very serious problem. Each tank may suffer up to 1,000 pits, with some reaching a depth of as much as 10 mm. As a result, the workload of repairing such pitting corrosions in periodical dock inspection is extremely heavy. Many studies have already been conducted to clarify the mechanism of pitting corrosion, but it has yet to be fully understood. We have clarified the pitting corrosion mechanism through onboard research on some VLCCs and various corrosion tests, in addition to the findings obtained by previous studies. Based on our understanding of the mechanism, we developed a corrosion test method to simulate the corrosive environment within the pits. Furthermore, we developed a new corrosion resistant steel (CRS) with trace amounts of alloying elements. The corrosion rate for CRS is less than one-fifth of conventional steels. Due to its very low alloy content, the weldability and mechanical properties of CRS remain similar to conventional steels. This CRS has already been applied to several VLCCs and we have examined its corrosion resistance through onboard investigations of the two VLCCs. One was for all its uncoated COT bottom plates, which were built of CRS, at the first docking (after 2.5 years). No pits deeper than 4 mm were found in the bottom plates of any COTs. Also, only about twenty pits of 2∼4-mm depth were found. The other was for the bottom plates of six unpainted COTs built of CRS at her first and second dockings (after 2.3 and 5 years). At five years, tens of pits deeper than 4 mm were found in all, but the pit count was much lower than that of VLCCs constructed of conventional steel. Thus, the good corrosion resistance of CRS was confirmed. In addition, it was also revealed through onboard research during a dock inspection that pit growth halted on VLCCs with more than five years service. SOLAS II-1 Cargo Oil Tank Corrosion Protection, which adopts the test method developed by us as the qualification test for bottom plates, comes into effect in 2013. In addition, CRS has already been certified by Class NK as corrosion resistant steel for COT bottom plates. CRS is set to play its part in the safe navigation of oil tankers. Furthermore, CRS does not require a protective coating of paint, which also benefits the global environment by reducing the use of volatile organic compounds.

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