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 1
Contents 1. Simulation Test of Inner Bottom Environment 2. Development of Anti-Corrosion Steel for COT (1) Pitting Corrosion Resistance of Developed Steel (2) Corrosion Resistance in Acid Solution of Developed Steel (3) Mechanical Properties of Developed Steel 3. Conclusions 2
Contents 1. Simulation Test of Inner Bottom Environment 2. Development of Anti-Corrosion Steel for COT (1) Pitting Corrosion Resistance of Developed Steel (2) Corrosion Resistance in Acid Solution of Developed Steel (3) Mechanical Properties of Developed Steel 3. Conclusions 3
Corrosion Problem in COT 20mm 10cm ・ A form of corrosion on inner bottom : Pitting of bowl shape ・ On every dry docking (every 2.5 years) ⇒ Pitting corrosion, 4 mm and more depth, are repaired. ⇒ Many pitting corrosion = Much repair cost occur! Max. Pitting Rate ： 4mm/y (10mm/2.5y) 4
Environment on Inner Bottom of COT Ballast Condition Inert Gas Oil-coat High Humidity (Acid Dew) Crude Oil Inert-gas Condition H 2 S ・ SO 2 , O 2 contain Residue of Salt-water Sludge, Salt water H 2 S Sludge ・ Solid in Oil Oil coat ・ Sulfur, Rust Full Load Condition Ineat-gas (SO 2 , O 2 +H 2 S) Oil-coat Crude Oil Salt-water Sludge, S Oil-coat about 8%NaCl, Salt-water Salt-water Sludge, S Sludge Steel Oil-coat Solids in Oil Steel Drops from ceiling (Sulfur, Rust) Cause of Pitting 5
Mechanism of Pitting Corrosion (1) Protective Coating Defect of Oil-coat ・ Oil-coat Sludge, Rust, S Salt-water (Cathode) (2) Salt-water ・ Chloride Ion Pitting (3) Starting Point of Pitting (Anode) ・ Defect of Oil-coat Pitting Growth ・ Uneven Distribution of Water Factors of Pitting Growth (Cathode, Oxidizer, etc.) ・ Sludge Oil-coat ・ Rust, and S Steel ・ Inert-gas (SO 2 , O 2 , etc.) ・ H 2 S Macro-cell corrosion Initiation and growth of pitting corrosion 6
Simulation Test Method on the Inner Bottom Factors of Pitting Corrosion Simulation Test Condition of COT Environment (1) Protective Coating (Oil-coat) ・ Oil-coat (gathered from actual VLCC) applied (2) Salt-water (Chloride Ion) ・ Simulated Gas (include 5%O 2 , H 2 S, etc.) (3) Starting Point of Pitting ・ 10%NaCl Solution and Accelerator (Sludge, S) ・ No oil-coat section with Sulfur Gas (include H 2 S) < Laboratory Test Method > Specimen (1) Shot-Blast Finish (with ‘Oil-coat’) (2) Applying the Oil-coat (3) Making the Starting Point of Pitting 10%NaCl Corrosion (4) Test (5) After 36 days, The maximum pitting 313K corrosion depth is measured. Fig. Pitting Corrosion Test Method. 7
Comparison between Lab. Test with Actual Ship Actual Pits 40 Lab. Test Diameter of Pits (mm) 35 30 25 20 15 10 10 mm 5 Photo. Pits on the laboratory Test. 0 0 1 2 3 4 5 6 7 8 Depth of Pits (mm) Fig. The Shape of Pitting Corrosion. The pitting corrosion shape of the lab. test is similar to actual pitting 20 mm corrosion shape. Photo. Pits on the Inner Bottom. 8
Contents 1. Simulation Test of Inner Bottom Environment 2. Development of Anti-Corrosion Steel for COT (1) Pitting Corrosion Resistance of Developed Steel (2) Corrosion Resistance in Acid Solution of Developed Steel (3) Mechanical Properties of Developed Steel 3. Conclusions 9
Concept of Anti-Corrosion Steel Reduction of Life Cycle Cost of Tanker Target (= Reduction of Maintenance Cost of Tanker) Point “Anti-Corrosion Steel” : Resistance to Pitting Corrosion and Acid “Anti-Corrosion Steel + Zinc-primer” : Resistance to Pitting for Long Time <add some elements> Improvement of Resistance to Pitting and Acid (1) Forming of the surface film with resistance to acid Prolongation of Effect of Zinc-primer (2) Forming of thick and protective rust layer (3) Forming of un-resolvable corrosion products One of the Protection Methods of COT 10
Resistance to Pitting Corrosion <Simulation Test of Inner Bottom> 1) apply ‘Oil-coat’ 1.5 Max. Pitting Depth (mm) 2) pitting starting point : center, 2 mm Test period : 36 days 3) 313K 4) Evaluation : Max. Pitting Depth 1.0 decrease to about 65% Gas (include H 2 S) 0.5 Specimen (with ‘Oil-coat’) 0.0 10%NaCl conventional steel anti-corrosion steel with zinc-primer with zinc-primer Fig. Pitting Corrosion Test Method. Fig. Resistance to Pitting Corrosion. The max. pitting depth of the anti-corrosion steel decreases to about 65% compared with conventional steel. 10mm Photo. Pitting Corrosion by lab.-Test. 11
Presumption of the Amount of Pitting Corrosion When the anti-corrosion steel is adopted, “Pitting depth decreases to about 65%” compared with conventional steel. 4mm 50 50 needless needless pitting in need pitting in need Table Test Calculation of the Number of Pits. of repair : 31 of repair : 31 Number of pits in need of repair 40 40 (4 mm depth or more) 0 Frequency Frequency 30 30 Anti-Corrosion Steel Tanker Conventional Steel + 20 20 “Pitting depth decreases measured measured + Zinc-primer calculated to about 65%” Zinc-primer (Estimation) 10 10 0 31 Suez/M F 0 0 0 -1.9 -1.9 2.0- 2.0- 2.5- 2.5- 3.0- 3.0- 3.5- 3.5- 4.0- 4.0- 4.5- 4.5- 5.0- 5.0- 5.5- 5.5- 42 VLCC G 2.4 2.4 2.9 2.9 3.4 3.4 3.9 3.9 4.4 4.4 4.9 4.9 5.4 5.4 Pitting Depth (mm) Pitting Depth (mm) Fig. Decrease of the Number of Pits in Need Extremely decrease! of Repair by Decrease of Pitting Depth. There is a possibility that the number of pits in need of repair becomes “0” by applying the anti-corrosion steel. 12
Contents 1. Simulation Test of Inner Bottom Environment 2. Development of Anti-Corrosion Steel for COT (1) Pitting Corrosion Resistance of Developed Steel (2) Corrosion Resistance in Acid Solution of Developed Steel (3) Mechanical Properties of Developed Steel 3. Conclusions 13
Resistance to Hydrochloric Acid <Corrosion Factor Test of Inner Bottom> 1) 10%NaCl, + HCl 1.2 2) pH=0.9 pH=0.9 3) 313K (conventional Steel = 1.0) 1.0 Corrosion Rate Ratio 4) Evaluation : Corrosion Rate 0.8 0.6 decrease to about 14% Specimen 0.4 0.2 10%NaCl 0.0 (pH=0.9) Conventional Steel Anti-Corrosion Steel Fig. Test Method. Fig. Resistance to Hydrochloric Acid. The corrosion rate of the anti-corrosion steel in the hydrochloric acid decreases to about 14% compared with conventional steel. 14
Contents 1. Simulation Test of Inner Bottom Environment 2. Development of Anti-Corrosion Steel for COT (1) Pitting Corrosion Resistance of Developed Steel (2) Corrosion Resistance in Acid Solution of Developed Steel (3) Mechanical Properties of Developed Steel 3. Conclusions 15
Mechanical Properties Table 1 Characteristics of Base Metal. YS TS EL CVN at 253K Grade (N/mm 2 ) (N/mm 2 ) (%) Energy (J) Anti-corrosion Steel 399 485 31 326 Thickness : 20 (mm) IACS, 32D ≧ 315 440 ～ 590 ≧ 18 ≧ 31 CVN ： Charpy V notch test Table 2 Characteristics of Weld Joint. TS Charpy V notch test at 273K Grade Notch position Energy (J) (N/mm 2 ) WM 106 Anti-corrosion Steel FL 149 Thickness : 20 (mm) 515 HAZ 1 (mm) 247 Welding method : FCB HAZ 3 (mm) 273 Heat input : 108 (kJ/cm) HAZ 5 (mm) 317 IACS, 32D ≧ 440 - ≧ 34 16
Contents 1. Simulation Test of Inner Bottom Environment 2. Development of Anti-Corrosion Steel for COT (1) Pitting Corrosion Resistance of Developed Steel (2) Corrosion Resistance in Acid Solution of Developed Steel (3) Mechanical Properties of Developed Steel 3. Conclusions 17
Conclusions · 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 on the decrease of pitting corrosion was developed. The maximum pitting corrosion depth has decreased to about 65% compared with conventional steel. · The developed steel has mechanical properties and construction performance equal with those of conventional steel as steel plate for shipbuilding. 18
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