Shipbuilding Technology ISST 2007, Osaka, 2007 DEVELOPMENT OF CORROSION RESISTANT STEEL FOR BOTTOM PLATE OF COT Y Inohara, JFE steel corporation, Japan T Komori, JFE steel corporation, Japan K Kyono, JFE steel corporation, Japan K Ueda, JFE steel corporation, Japan S Suzuki, JFE steel corporation, Japan H Shiomi, JFE steel corporation, Japan SUMMARY Recently, the corrosion problem in the ship is paid to attention Especially, the pitting corrosion occurred on the inner bottom plate of COT (Cargo Oil Tank) of crude oil tanker to need the inspection and the repair of every dock is one of the big problems The inner bottom plate is being covered with the ‘oil-coat’ that is the crude oil element and protected from corrosion But, the pitting corrosion occurs, and grows up by the ‘oil-coat’ defect part become a local anode site This phenomenon was reproduced in the laboratory, and the laboratory pitting corrosion test method was established And the pitting corrosion decrease effect of the zinc-primer on which it reported before is used, the low alloy corrosion resistant steel to which the number of pitting corrosion was greatly decreased by using it together with the zinc-primer was developed When this corrosion resistant steel is applied to a tanker, the number of pitting corrosion for which the repair at the dock is necessary can be decreased sharply at a low level ¶ from crude oil In addition, it is presumed that the elements of ‘inert gas’ and H2S volatilized in crude oil merge in this salt water This environment is very severe for corrosion of conventional steel [1] INTRODUCTION Recently, the corrosion problem in the ship is paid to attention Especially, the pitting corrosion occurred on the inner bottom plate of COT (Cargo Oil Tank) of crude oil tanker to need the inspection and the repair of every dock is one of the big problems In Japan, for three years from 1999, field examination was carried out to make the corrosion phenomenon of COT of cargo oil tanker clear As a result, the actual corrosion environments in COT were clarified Because of the investigation result, it was presumed that the pitting corrosion on the inner bottom plate occurred and grew up under the environment not uniform, such as a defect of ‘oil coat’, localized salt water or etc., and existence of oxidizer, such as iron oxide, iron sulfide, elemental sulfur or etc [1] This phenomenon was reproduced in the laboratory, and the laboratory pitting corrosion test method was established And the pitting corrosion decrease effect of the zinc-primer on which it reported before is used, the low alloy corrosion resistant steel to which the number of pitting corrosion was greatly decreased by using it together with the zinc-primer was developed PITTING CORROSION 2.1 ACTUAL ENVIRONMENT Ex : 13%CO2, 5%O2, 5%H2O, 0.2%H2S, 0.01%SOx, Bal N2 Vapor space Crude oil Salt water Sludge Oil coat Figure 1: Cross section of crude oil tanker 2.2 MECHANISM OF PITTING CORROSION GENERATION AND GROWTH Usually, the inner bottom plate covered with ‘oil coat’ has corrosion protection However, piling up of the ‘sludge’, COW (Crude Oil Washing) and etc cause the defect of ‘oil coat’ If the low protective point such as the defect of ‘oil coat’ occurs on the inner bottom plate, the pitting corrosion is generated in this point In the salt water, the low protective point becomes anode site, the ‘oil coat’ and the ‘sludge’ become cathode site In this area, macro-cell is formed and pitting corrosion grows up [1] As a result of field examination, it was presumed that the ‘sludge’ is piling up various solid such as solid in the crude oil, iron rust and sulfur generated in COT and etc Figure shows the image of pitting corrosion growth Figure shows the cross section of the crude oil tanker In operation, the gas part of COT is always filled with ‘inert gas’ for the explosion-proof ‘Inert gas’ is the exhaust gas of the low oxygen concentration It is composed of CO2, O2, SO2, N2 and etc The inner bottom plate surface of COT was covered with ‘oil coat’ composed of heavy ingredients of crude oil, piled up ‘sludge’ composed of rust and solid in crude oil, and collected the high concentration salt water separated © 2007: JASNAOE-RINA H2S 33 Shipbuilding Technology ISST 2007, Osaka, 2007 Gas (Simulated inert gas + H2S) Pitting (anode) Salt water (10%NaCl) Sludge (cathode) Specimen Oil coat Seawater Oil coat (313K) Steel plate Figure 3: Simulated pitting corrosion test for COT Figure 2: Image of pitting growth CHARACTTERISTICS OF DEVELOPED STEEL PITTING CORROSION TEST METHOD 4.1 EFFECT OF DECREASE OF PITTING CORROSION OF ZINC-PRIMER Figure shows the laboratory pitting corrosion test method Test solution was 10% NaCl solution, and it was saturated with 13%CO2-5%O2-0.01%SO2-0.2%H2Sbal.N2 gas The temperature of the solution was maintained 313K with a double cell Specimen size was 75 x 50 x millimeters Surface of specimen was covered with the seal tape expect one test surface Test surface of specimen was coated with the crude oil residue gathered from COT, and at the center of test surface, no coated area (diameter: 5mm) imitated the defect of oil coat’ was made The specimens were soaked upward in the solution After these specimens were soaked from 28 for 36 days, the shape of the pitting corrosion that occurred on the surface was measured, and each specimen was evaluated by each maximum pitting corrosion depth The shape (ratio of average diameter and depth) of the pitting corrosion that occurred by this test was corresponding to the shape of the pitting corrosion observed on the inner bottom plate of COT well © 2007: JASNAOE-RINA As a result of field examination, the number of pitting corrosion occurred on the inner bottom painted zincprimer was clearly lower than the number of pitting corrosion occurred on the non-painted inner bottom plate In the first dock, the number of pitting corrosion occurred on the inner bottom painted zinc-primer was from one-fifteenth to one-thirtieth as compared with the number of pitting corrosion occurred on the non-painted inner bottom plate Various addition elements that strengthened the effect of the zinc-primer of the pitting corrosion decrease were examined, and the corrosion resistant steel was developed 4.2 PITTING CORROSION RESISTANCE The zinc-primer painted specimen of conventional steel and that of developed steel were prepared, and they were evaluated by the above-mentioned pitting corrosion test method Figure shows the result of pitting corrosion test The maximum pitting corrosion depth of the developed steel decreased by about 35% compared with that of conventional steel When this result is applied to the distribution of the depth of the pitting corrosion measured by field examination of the zinc-primer specification tanker, the number of pitting corrosion in need of repair of developed steel that painted the zinc-primer is provisionally calculated that it is possible to decrease to one third or less of the conventional steel painted the zinc-primer 34 Shipbuilding Technology ISST 2007, Osaka, 2007 Table 2: Mechanical properties of welded joint Charpy Impact Test TS at 273K Grade (N/mm2) Notch Energy position (J) WM 106 Developed steel FL 149 515 HAZ 1mm 247 (FCB welding method, HAZ 3mm 273 Heat input: 108(kJ/cm)) HAZ 5mm 317 IACS, 32D > 440 > 34 1.5 Max Pitting Depth (mm) with Zinc-primer, Test period: 36days 1.0 a decrease of about 35% 0.5 0.0 Conventional Steel Developed Steel Figure 4: Maximum pitting depth of conventional steel and developed steel 4.3 • The pitting corrosion test method that was able to simulate the pitting corrosion that occurred on the inner bottom plate of COT was established • The corrosion resistant steel that strengthened the effect of the zinc-primer of the pitting corrosion decrease was developed The maximum pitting corrosion depth has decreased by about 35% compared with conventional steel • The developed steel has mechanical properties and construction performance equal with conventional steel as steel plate for shipbuilding MECHANICAL PROPERTIES Table shows an example of the typical mechanical properties of base metal of the developed steel The developed steel was satisfied with the specification of 32D grade of IACS Table shows the results of tensile test and Charpy V notch impact test of the welded joint of the developed steel The welded joint of the developed steel was satisfied with the specification of 32D grade of IACS, too The mechanical properties of developed steel are equal to them of conventional steel, and in building of the tanker, the welding and construction performance similar to conventional steel are usually possible REFERENCES Ship Research Panel 242, ‘Study on Cargo Oil Tank Corrosion of Oil Tanker’, Ship Research Summary Report No.431, Tokyo, JSRA, 2002 Table 1: Mechanical properties of developed steel Charpy Impact YS TS EL Test Grade (N/mm2) (N/mm2) (%) at 253K Energy (J) Developed 399 485 31 326 steel IACS, 32D > 315 440/590 > 18 > 31 © 2007: JASNAOE-RINA CONCLUSIONS AUTHORS’ BIOGRAPHIES Yasuto Inohara holds the current position of senior researcher at Corrosion Protection Research Department, Steel Research Laboratory, JFE Steel Corporation He is responsible for development of corrosion resistant steel 35 Shipbuilding Technology ISST 2007, Osaka, 2007 © 2007: JASNAOE-RINA 36