PREVENTION OF COT BOTTOM PITTING CORROSION BY ZINC-PRIMER Y Inohara, JFE steel corporation, Japan T Komori, JFE steel corporation, Japan K Kyono, JFE steel corporation, Japan H Shiomi, JFE steel corporation, Japan T Kashiwagi, Mitsui O. S. K. Lines, Ltd., Japan 1
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. ⇒ A lot of pitting corrosion = A lot in repair cost! Max. Pitting Rate ： 4mm/y (10mm/2.5y) 2
Environment on Inner Bottom of COT 《 Mechanism of Pitting 》 Inert Gas Defect of oil-coat Sludge, S Salt-water (Cathode) Crude Oil H 2 S Pitting Sludge, Salt water H 2 S (Anode) Pitting Growth Oil coat Full Load Condition Oil-coat Oil-coat Crude Oil Salt-water Steel about 8%NaCl, Salt-water Salt-water Sludge, S Sludge Oil-coat Macro-cell corrosion Solids in Oil Steel Initiation and growth Drops from ceiling (Sulfur, Rust) Cause of Pitting of pitting corrosion 3
Field Examination of Crude Oil Tankers Tanker Type : Double Hull Tanker (VLCC, Suez-Max) Number : 6 Age : 2.5 - 12.5 years Table Examination items. Inner Tanker 2.5Y-dock 5Y-dock 7.5Y-dock 10Y-dock 12.5Y-dock Bottom VLCC A No paint ○ ○ ○ applied VLCC B ○ ○ ○ ○ VLCC C ○ ○ Shop- VLCC D ○，★ ○，★ primer VLCC E ○ ○，★ applied Suez/M F － － － ○ ○，★ ○ : Investigation of the Amount of Pitting Corrosion ★ : Analyzing the Rust on the Inner Bottom Plate 4
Results of Field Examination Table The Amount of Pitting Corrosion Need to Repair (per vessel, VLCC : 4 mm and more depth, Suez Max. : 3 mm and more depth) Inner Tanker 2.5Y-dock 5Y-dock 7.5Y-dock 10Y-dock 12.5Y-dock Bottom 1323 2356 1082 VLCC A No paint applied 1246 2919 1756 1138 VLCC B 49 213 VLCC C Shop- 44 61 VLCC D primer 88 39 VLCC E applied - - - 47 684 Suez/M F Better result for when a shop-primer was applied! 5
Transition of the Amount of Pitting Corrosion 4,000 Shop-primer 3,500 No-paint Number of Pits / Vessel 3,000 2,500 2,000 1,500 1,000 500 0 0 5 10 15 20 25 Ship Age (year) Figure Transition of the Amount of Pitting Corrosion No-paint Applied : Always 1000 or More Shop-primer Applied : Keep Low Level (at 10 Years after Delivery) 6
Pitting Corrosion on Inner Bottom of COT A lot of pitting corrosion! Very little pitting corrosion! 20 cm 10 cm Photo. VLCC B applied No-paint Photo. VLCC D applied Shop-primer (10 years after delivery, (5 years after delivery, Total 1138 Pits, Max. 412 Pits/tank) Total 61 Pits, Max. 9 Pits/tank) 7
Cutting out the COT Bottom Plate Photo. Cutting out the Inner Bottom Plate of COT (VLCC D applied shop-primer, 5 years after delivery) 8
Quantitative Analysis of Zn in the Rust Table Content of Fe and Zn in the Rust. VLCC D VLCC D VLCC E Suez/M F Tanker (2.5y) (5y) (5y) (12.5y) Sample No. A0 B0 C4 D3 E3 I9 J7 K8 A1 A2 B1 B2 B3 Fe 205 244 235 235 406 242 271 316 443 352 256 278 351 Elements (g/m 2 ) Zn 8.9 4.1 1.7 0.3 7.0 1.8 5.9 2.6 4.0 3.4 5.5 5.5 3.3 ICP Analysis Method 2.5 years 5 years 12.5 years 25 500 (1) Specimen : About 30 x 30 mm Zn (2) Dissolved all rust by HCl Fe 20 400 (3) ICP analysis 2 ) 2 ) 15 300 Zn (g/m Fe (g/m 10 200 5 100 After 12.5 years, 0 0 Zn is existing in the rust. VLCC D VLCC D VLCC E Suez/M F Fig. Content of Fe and Zn in the Rust. 9
Distribution of Zn in the Rust Layer (1) VLCC D EPMA Method (applied shop-primer, (1) Cutting out the cross 2.5 years) section of sample with the rust layer Thickness of rust layer : (2) Polish 100 ～ 200 μ m (3) EPMA Mapping (a) SEM Image (cross section) Low ← → High Zn coexists with S (b) Zn (c) S (d) Fe (e) O Fig. Results of EPMA Analysis. Zn coexists with O 10
Distribution of Zn in the Rust Layer (2) VLCC E Zn coexists with S (applied shop-primer, Zn coexists with O 5 years) Thickness of rust layer : Zn : Sulfide or Oxide 200 ～ 400 μ m (a) COMP Image (cross section) Low ← → High Zn coexists with S (b) Zn (c) S (d) Fe (e) O Fig. Results of EPMA Analysis. 11
Identify the Chemical Composition in the Rust ：α -FeOOH ：β -FeOOH ：γ -FeOOH ： Fe 3 O 4 ： FeCO 3 ： Fe 0.85-x Zn x O ： ZnS ： ZnSO 4 XRD Analysis Method • Scraping and collecting the rust • Grinding • Deciding the existing elements by EDX : Fe, Zn, Si, S, Cl, C, O, (Ca, Na) • XRD analysis Fig. Results of XRD Analysis (VLCC D applied shop-primer, 5 years) 12
Chemical Composition in Rust Table Results of XRD Analysis Tanker Chemical Composition VLCC D (Certainty) α -FeOOH, β -FeOOH, γ -FeOOH, Fe 3 O 4 , (applied shop- (Uncertainty) Fe 0.85-x Zn x O, ZnS, ZnSO 4 primer, 5 years) (1) Metal Zinc Not exist. (2) Chemical Composition of Zinc ZnS, ZnSO 4 , Fe 0.85-x Zn x O, etc. 13
Influence of Zn (1) (1) Zn-oxide, Zn-rust : Keep Fe-rust the amorphous state, and lift the ability to protect of steel. By Zn-oxide (ZnO, ZnCl 2 4Zn(OH) 2 ) → Decrease of Corrosion of Steel By Zn-rust (ZnO, ZnCl 2 4Zn(OH) 2 ) → Fe-rust Keeps the Amorphous State Rust, S (Cathode) Oil-coat Zn-primer Fe-oxide and Zn-oxide Steel <Reference> S. Fujita, H. Kajiyama, M. Yamashita, ‘Corrosion Restraint on the growth mechanism of zinc coated steel sheet inside the lapped portion’, of pitting corrosion CAMP-ISIJ, Vol. 9, P. 1283 (1996) 14
Influence of Zn (2) (2) Zn 2+ : Decreasing the dissolution of Fe under acidity In the acidity chloride solution (pH=3.5), the anodic dissolution of Fe is decreasing with the increasing of the concentration of Zn 2+ in the solution. Neutral : ZnS Acidity : ZnS → Zn 2+ Oil-coat Zn sulfide & oxide <Reference> T. Hirasaki, A. Nishikawa, T. Tsuru : “Influence of Pitting Steel Dissolved Zinc Ions on The Anodic Dissolution of Iron”, NIKGAV, Vol.66, No.6, p.643-648 (2002) Restraint on the growth of pitting corrosion 15
Conclusions After the field examinations and analysis of pitting corrosion on the inner bottoms of COT, the following results were obtained. (1)Zinc-primer is effective to the decrease of the number of pitting corrosion in need of repair. (2)Zinc in the zinc-primer stays in the iron oxide on the inner bottom plate of COT after oxidation, and keeps giving the effect to corrosion resistance. This effect continues for several years at least. 16
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