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A countermeasure to preventCOT bottom pitting corrosion by shop primer

１５　ＳＥＰ 2006Ｔ echnical DivisionMitsui O.S.K.Lines

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Crude oil

Sludge, Seawater H2S

Oil Coat Salt Water 　・ abt.8%NaCl, Salt Water 　　　　　　　　　　　　　　Sludge 　・ Solids in Oil 　・ Drops from ceiling 　　（ Sulfur ， Rust ）　⇒　 Cause of Pits

Crude Oil

Concentrated Salt Water

Oil Coat

Steel

Sludge

Oil Coat 　High Humidity （ Acid Dew ） Inert Gas condition　・ SOX ， O2 containResidue of Salt Water Sludge 　・ Solids in Oil 　 　・ Sulfur ， Rust

Ballast Condition

Inert Gas

Oil Coat

Steel

Sludge

Full Load Condition Cargo Tank Bottom

Pits

Cargo Tank Environment

3（ the worst case 1320 pits per tank ）

Photo of pits at No paint applied VLCC 　（ VLCC B ２９１９ pits per Vessel ）

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No paint applied 　 2.5y dock 5 y dockVLCC A 1323 pits 　 2356 pitsVLCC B 1246 pits 2919 pits 　

S/P applied 2.5y dock(TMCP-YP32) VLCC C 50 pits VLCC D 44 pits (per vessel)

⇒ Better result for S/P ！

Record of Pitting Corrosion　 NO PAINT vs SHOP PRIMER

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S/P reduced the number of bottompits dramatically (1000 to 50 !)

Shop primer seems to be remaining（ very minute and hard layer ！）

Measured thickness of S/P layer was about 50μm（ Why did the thickness of S/P increase from 15μm to 50μm ？）

Results of onboard investigation at G. dock

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S/P did really remain even at G.dock?(about 2.5 year after delivery)

S/P seems to be effective in reducing the number of bottom pits 　

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VLCC C JUL 2000 DeliveryVLCC D SEP 2000 Delivery

JAPAN ～ P/G trade

PAINT SPECCARGO TANK 　 S/P 15μmSLOP TANK 　　　 T/E 250μm

S/P applied D/H VLCCs

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Cutting C.O.T. bottom plate for analysis by JFE Steel

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SEM Cl S

Fe O Zn200μm

High

low

⇒ Minute layer including iron and zinc oxide has been formed on the steel plate

EPMA analysis of shop-primer layer at the VLCC COT bottom

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Not at all the simplest effect of zinc-anode

But the layer of iron-and-zinc-oxide, hard, minute and more anti-corrosive in nature than pure iron-oxide, has been considered formed

Conclusion

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New Record of Pitting CorrosionTable No. of Pits （ Depth 4mm over 、 per vessel ）

Bottom Spec.

TANKER2.5Y-dock

5Y-dock7.5Y-dock

10Y-dock

12.5Y-dock

No Paint

VLCC A1323 pits

2356pits

1082pits

- -

VLCC B1246 pits

2919pits

- - -

ShopPrimer

VLCC C 50 pits213 pits

- - -

VLCC D 44 pits 61 pits - - -

VLCC E 88 pits 39 pits - - -

Suez Max. Tank - - -

47 pits

684 pitsNo. of Pitting Corrosions are still very few

At 5years after deliveryShop Primer is still effective in reducing pits on bottom

※Pits of Suez Max. Tank. are depth 3mm over

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S/P still reduced the number of bottompits dramatically (1000-2000 to 50 !)

Shop primer seems to be remaining

Results of onboard investigation at 5 years dry docking

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Cutting C.O.T. bottom plate for analysis

Photo VLCC D Sample （ No.4 COT ( P ) ） Photo VLCC E Sample （ No.2COT ( P) ）

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EPMA Analysis - Zn in Rust Layer

Low← 　　　　　　　　　→ High

Fig. 　 EPMA Analysis （ VLCC D ， C4-1 ）

図　 EPMA Analysisi （ VLCC E ， J7-1 ） （ａ） COMP 　　　　 　　　　　（ｂ） Zn 　

（ａ） COMP 　　　　　　　　 　（ｂ） Zn 　

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Quantitative analysis – Zn in Rust

TANKERVLCC D

（ 2.5years Dock ）

VLCC D（ 5years Dock ）

VLCC E（ 5years Dock ）

Sample code

A0 B0 C4 D3 E3 I9 J7 K8

Element

(g/m2)

Fe205.

2243.

8234.

9235.

1406.

0242.

1270.

6316.

0

Zn 8.9 4.1 1.7 0.3 7.0 1.8 5.9 2.6

Table 　 Fe, Zn Content

Fig. 　 Zn Content in Rust

ICP Analysis・ Sample plate ： 30×30mm ・ Dissolution of Rust

・ After 5 yearsZinc is existing in Rust0.0

2.0

4.0

6.0

8.0

10.0

12.0

VLCC D (2.5y) VLCC D (5y) VLCC E (5y)

Zn 　

g/m

2 )

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Summary

RESULT From Investigation into COT bottom plate「 VLCC D 」 and 「 VLCC E 」　　　 （１） KEEP few pitting repair

Zinc primer effect is continued 5 years

　（２） Zinc is still existing in the RustIi was supposed that Zinc was used up in less than 1 year.　　　　 But Zinc is still existing after 5 years

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Reinforcement of Zn primer’s effect by developed steel

Decrease in 35% in pitting corrosion depth was confirmed by applying the developed steel compared with conventional steel.

Temperature controlled bath （ 50℃ ）　

Artificialseawater 　

Sample（ surface: Oil-coat or Sludge ）　

Gas （ 5%O2+10%H

2S+N2 ）　

Developed steel with Zn primer⇒ Pitting depth reduction

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Conventional Steel with Zn primer

Developed steel with Zn primer

Maxim

um

pit

tin

g

dep

th (

mm

)

Reductionabout 35%

Laboratory corrosion test simulated COT bottom

Laboratory corrosion test simulated COT bottom

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Presumption of number of repair necessary pitting corrosion

The number of necessary repair pits decreases greatly.

0

10

20

30

40

50

-1.9 2.0-

2.4

2.5-

2.9

3.0-

3.4

3.5-

3.9

4.0-

4.4

4.5-

4.9

5.0-

Pitting corrosion depth (mm)

Fre

qu

en

cy

(pit

s)

Real Pits in Tanker F Adopted developed steel

Range of repair necessary pits

　 Each pitting depth reduce

35%

Adopted steel

Pits depth

Conventional

Over 3.0mm 89 pits

Over 4.0mm 31 pits

DevelopedDeveloped

13 pits13 pits

0 pits0 pits

Investigation of 111 pits among 684 pits in practical tanker 12.5 years old

<111 pits>

Adopted steel

Pits depth

Conventional

Over 3.0mm 548 pits

Over 4.0mm 191 pits

DevelopedDeveloped

80 pits80 pits

0 pits0 pits

<per tanker>

684 pits/111 pits

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Effect of developed steel application in practical tanker

Fig. Number of repair necessary pitting corrosion in practical tanker bottom

0

1,000

2,000

3,000

4,000

0 5 10 15 20 25Ship age (year)

Nu

mbe

r of

nec

essa

ry r

epai

r pi

ttin

g co

rros

ion

(Pit

s)

Tanker (Zn primer)Tanker (bare)

Conventional Steel (bare)

Conventional steel with Zn primer

Developed steelWith Zn primer

The number of necessary repair pitting corrosion decreases greatly by the Zn primer application.

In addition

The number of necessary repair pitting corrosion decreases more

greatly by applying the developed steel.

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《 Acid salt water dip corrosion test 》　 Sample : Conventional steel, Developed steel　 Solution : Sulfuric acid (20%) and NaCl ( 3%)　 Temp. : 40℃　 Test time ： 6 hours　 Evaluation: Corrosion speed

In very severe acid environmentCorrosion speed : Reduction to about 10%

Temperature controlled bath （ 40℃ ）

Sulfuric acid + NaCl solution 　

Sample

　　　 It is thought that the application of the developed steel is effective to high acidity part in ship.

(Reference) Acid examination result of development steel

0

20

40

60

80

100

120

Conventional SteelC

orro

sion

spe

ed (

g/m

2 hr)

Acid salt water dip corrosion test

About 90% decrease

Developed Steel

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Shop Primer is an effective pitting corrosion prevention system.

New steels are developed, They are also an effective pitting corrosion prevention system.

Combinations of Shop primer and Anti-Corrosion steel is more effective

Conclusion

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Thank you for your attention