PREVENTION OF COT BOTTOM PITTING CORROSION BY ZINC-PRIMER Y Inohara, - - PowerPoint PPT Presentation

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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

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Corrosion Problem in COT

・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)

20mm 10cm

3 Sludge, Salt water H2S H2S Inert Gas Crude Oil Oil coat

Oil-coat Salt-water about 8%NaCl, Salt-water Sludge Solids in Oil Drops from ceiling (Sulfur, Rust) Cause of Pitting Full Load Condition

Crude Oil Salt-water Oil-coat Steel Sludge, S

Environment on Inner Bottom of COT

Macro-cell corrosion Initiation and growth

• f pitting corrosion

Pitting Growth

Oil-coat Steel Pitting (Anode) Sludge, S (Cathode) Defect of oil-coat

《Mechanism of Pitting》

Salt-water

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Field Examination of Crude Oil Tankers

Table Examination items. Shop- primer applied No paint applied

Inner Bottom ○，★ 12.5Y-dock ○ ○ 10Y-dock － ○ ○ 7.5Y-dock ○，★ ○ VLCC E － － Suez/M F ○，★ ○ ○ ○ 5Y-dock ○，★ VLCC D ○ VLCC C ○ VLCC B ○ VLCC A 2.5Y-dock Tanker

○ : Investigation of the Amount of Pitting Corrosion ★ : Analyzing the Rust on the Inner Bottom Plate Tanker Type : Double Hull Tanker (VLCC, Suez-Max) Number : 6 Age : 2.5 - 12.5 years

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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)

684

12.5Y-dock

47 1138

10Y-dock

• 1756

1082

7.5Y-dock

39 88

VLCC E

• Suez/M F

61 213 2919 2356

5Y-dock

44

VLCC D

49

VLCC C

Shop- primer applied

1246

VLCC B

1323

VLCC A

No paint applied

2.5Y-dock Tanker Inner Bottom

Better result for when a shop-primer was applied!

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500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 5 10 15 20 25 Ship Age (year) Number of Pits / Vessel Shop-primer No-paint

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)

Figure Transition of the Amount of Pitting Corrosion

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Pitting Corrosion on Inner Bottom of COT

• Photo. VLCC B applied No-paint

(10 years after delivery, Total 1138 Pits, Max. 412 Pits/tank)

• Photo. VLCC D applied Shop-primer

(5 years after delivery, Total 61 Pits, Max. 9 Pits/tank)

A lot of pitting corrosion! Very little pitting corrosion!

10 cm 20 cm

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Cutting out the COT Bottom Plate

• Photo. Cutting out the Inner Bottom Plate of COT

(VLCC D applied shop-primer, 5 years after delivery)

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Quantitative Analysis of Zn in the Rust

3.3 351

B3

5.5 278

B2

4.0 443

A1 Suez/M F (12.5y)

3.4 352

A2

5.5 256

B1

2.6 5.9 1.8 7.0 0.3 1.7 4.1 8.9 Zn 316 271 242 406 235 235 244 205 Fe

Elements (g/m2) K8 J7 I9 E3 D3 C4 B0 A0 Sample No. VLCC E (5y) VLCC D (5y) VLCC D (2.5y) Tanker

Table Content of Fe and Zn in the Rust.

• Fig. Content of Fe and Zn in the Rust.

ICP Analysis Method (1) Specimen : About 30 x 30 mm (2) Dissolved all rust by HCl (3) ICP analysis

After 12.5 years, Zn is existing in the rust.

2.5 years 5 years 12.5 years

5 10 15 20 25 VLCC D VLCC D VLCC E Suez/M F Zn (g/m

2)

100 200 300 400 500 Fe (g/m

2)

Zn Fe

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Distribution of Zn in the Rust Layer (1)

Low← →High

• Fig. Results of EPMA Analysis.

(a) SEM Image (cross section) EPMA Method (1) Cutting out the cross section of sample with the rust layer (2) Polish (3) EPMA Mapping (b) Zn (c) S (d) Fe (e) O VLCC D (applied shop-primer, 2.5 years) Thickness of rust layer : 100～200μm

Zn coexists with O Zn coexists with S

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Distribution of Zn in the Rust Layer (2)

Zn coexists with S Zn coexists with O

Zn : Sulfide or Oxide

VLCC E (applied shop-primer, 5 years) Thickness of rust layer : 200～400μm (a) COMP Image (cross section)

Low← →High

• Fig. Results of EPMA Analysis.

(b) Zn (c) S (d) Fe (e) O

Zn coexists with S

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Identify the Chemical Composition in the Rust

• Fig. Results of XRD Analysis (VLCC D applied shop-primer, 5 years)

：α-FeOOH ：β-FeOOH ：γ-FeOOH ：Fe3O4 ：FeCO3 ：Fe0.85-xZnxO ：ZnS ：ZnSO4 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

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Chemical Composition in Rust

Table Results of XRD Analysis

(Certainty) α-FeOOH, β-FeOOH, γ-FeOOH, Fe3O4, (Uncertainty) Fe0.85-xZnxO, ZnS, ZnSO4 Chemical Composition VLCC D

(applied shop- primer, 5 years)

Tanker (1) Metal Zinc Not exist. (2) Chemical Composition of Zinc ZnS, ZnSO4, Fe0.85-xZnxO, etc.

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Influence of Zn (1)

Fe-oxide and Zn-oxide Zn-primer Oil-coat Steel Rust, S (Cathode) Restraint on the growth

• f pitting corrosion

<Reference> S. Fujita, H. Kajiyama, M. Yamashita, ‘Corrosion mechanism of zinc coated steel sheet inside the lapped portion’, CAMP-ISIJ, Vol. 9, P. 1283 (1996)

By Zn-oxide (ZnO, ZnCl24Zn(OH)2) → Decrease of Corrosion of Steel By Zn-rust (ZnO, ZnCl24Zn(OH)2) → Fe-rust Keeps the Amorphous State

(1) Zn-oxide, Zn-rust : Keep Fe-rust the amorphous state, and lift the ability to protect of steel.

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Zn sulfide & oxide Oil-coat Steel Pitting Restraint on the growth of pitting corrosion Acidity : ZnS → Zn2+

Influence of Zn (2)

<Reference> T. Hirasaki, A. Nishikawa, T. Tsuru : “Influence of Dissolved Zinc Ions on The Anodic Dissolution of Iron”, NIKGAV, Vol.66, No.6, p.643-648 (2002)

In the acidity chloride solution (pH=3.5), the anodic dissolution of Fe is decreasing with the increasing of the concentration of Zn2+ in the solution.

Neutral : ZnS

(2) Zn2+ : Decreasing the dissolution

• f Fe under acidity

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Conclusions

After the field examinations and analysis of pitting corrosion

• n 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.

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