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Shinji Sakashita Akihiko Tatsumi Hiroki Imamura Hideji Ikeda KOBE STEEL., LTD.
DEVELOPMENT OF ANTI-CORROSION STEEL FOR THE BOTTOM PLATES OF CARGO OIL TANKS
International Symposium ON SHIPBUILDING TECHNOLOGY (ISST 2007)
- Fabrication and Coatings –
6-7 September 2007, Osaka University, Japan
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Table of Contents
Background
Corrosion problem and countermeasure
Experimental
Corrosion mechanism and laboratory test method
Highlight data
Performance of anti-corrosion steel
Summary and future plan
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Corrosion problem
Crude oil
★Inner bottom Pitting corrosion Repair by painting and/or welding
・Number: several thousands ・Depth: ～10mm (in the dock inspection)
Figure: Corrosion problem in COT of oil tanker (SR242*)
Environment ・Water (+Cl-) ・Oil coating with defect ・Elemental S ・Low pH solution (in pit) * The Shipbuilding Research Association of Japan Panel #242
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Countermeasure for Corrosion
Inspection and maintenance after service Construction period and process of ship Environmental load minimization
★Anti-corrosion steel
Conventional steel A n t i
r
i
s t e e l Age of ship Corrosion depth
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・Anodic reaction： Fe → Fe2+ ＋ 2e- ・Cathodic reaction： S ＋ 2H+ ＋ 2e- → H2S
Corrosion Mechanism (1)
Figure: Corrosion mechanism at the inner bottom of COT (SR242*)
Bottom plate Brine (Defect) Pitting Oil coat Sludge & corrosion products Elemental S (several %) Crude oil
* The Shipbuilding Research Association of Japan Panel #242
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Figure: Schematics of simulated corrosion test method for inner bottom plate of COT
Test method (1)
Specimen 303K Water bath NaCl solution including Elemental S
*Assuming that specimen corresponds to pit inside.
Bottom plate Brine (Defect)
Pitting
Oil coat Sludge & corrosion products
Elemental S (several %)
Crude oil
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Corrosion Mechanism (2) Corrosion Mechanism (2)
(Defect)
Oil coat
Pitting Oil coat Bottom plate Brine Crude oil
Low pH solution (pH<1.5)
・Anodic reaction: Fe → Fe2+ ＋ 2e- ・Cathodic reaction: 1) 2H+ ＋ 2e- → H2 2) 2Fe3+ ＋ 2e- → 2Fe2+ Figure: Corrosion mechanism at the inner bottom of COT (SR242)
Fe3+
* The Shipbuilding Research Association of Japan Panel #242
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Test method (2)
Specimen
NaCl + FeCl3 solution (pH:1)
333K, 95%RH
Fixed temperature and humidity chamber *Assuming that specimen corresponds to pit inside.
(Defect)
Oil coat
Pitting Oil coat Bottom plate Brine Crude oil
Low pH solution (pH<1)
Figure: Schematics of simulated corrosion test method for inner bottom plate of COT
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Promoting effect of corrosion factors
Figure: Immersion corrosion test results (Specimen: conventional steel)
NaCl + elemental S NaCl + FeCl3 (pH1) NaCl
Temp:303K
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Corrosion resistance
(a)Elemental S (b)Low pH solution
1/4 1/5 Figure: Results of simulated corrosion test (Spacemen: DH32, 19mmt )
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Figure : Distribution of maximum pit growth rate
- btained by simulated tests.
Corrosion Resistance of Base Metal
1/3 1/3
(a)Elemental S (b)Low pH solution
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Corrosion Resistance of Welded Joint
(a)Elemental S (b)Low pH solution WM Weld metal HAZ Heat affected zone (Fusion line ～ 5mm) BM Base metal
Figure: Results of simulated corrosion test
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≧31 ≧22 440 - 590 ≧315 AH32 specification 254 297 vE0(J) 26 481 349 Conventional 28 476 368 Developed El(%) TS(MPa) YP(MPa) YP :Yield point TS : Tensile strength El : Elongation vE0 : Absorbed energy at 0 ℃ Table Mechanical properties of the developed steel
Mechanical properties (1)
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Mechanical properties (2)
≧34 (WM) // //
Figure: V-notch charpy impact test results of FCB welded joints (Heat input: 130kJ/cm). WM: Weld metal
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Estimation of Effect
Conventional steel
Repair
// //
Repair Repair
Developed steel
Repair by welding Repair by painting
Figure : Estimation of maximum pit depth
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Summary and future plan
An anti-corrosion steel for the bottom plates of cargo oil tanks has been developed. The maximum pit growth rate of the developed steel is suppressed to under 1/3 that of conventional steel. The application of the developed steel to the inner bottom plate of COT will contribute to improvement in safety and reduction in the life cycle cost of oil tankers. The developed steel has been applied to the inner bottom plates of COTs of a oil tanker without coating. Corrosion condition of the COTs which are being built with the developed steel will be evaluated quantitatively.
