Development of New Anti-Corrosion Steel for COTs of crude

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Development of New Anti-Corrosion Steel for COTs of crude oil carrier

The NSGP-1Nippon Steel’s Green Protect-1

2

The Studies on COT Corrosion in Japan2000 2002 2004 20061998 2000 2002 2004 20061998

Shift to Double Hull Tanker1990~

1999~2002 Panel SR242 Investigation(The Shipbuilding Research Association of Japan)

2002~

2004~NSGP-1 Field Test on actual Vessels

Anti- corrosion steelLaboratory Test

Scientific Research & understandingof the corrosion phenomena

(Over 10 VLCC s)Basis for Anti-corrosion SteelCorrosion test method

NSGP-1

3

↑Inner Bottom

Inert Gas H2O H2S CO2

Crude Oil

Drain Water

Upper Deck PlateUpper Deck PlateUniform CorrosionUniform Corrosion

Inner Bottom PlateInner Bottom PlatePitting CorrosionPitting Corrosion

Upper Deck↓

Corrosion problems on COT of crude oil carriers

COT corrosion ・Upp.DK uniform corrosion・Pitting on bottom plate

4

Upp.Dk corrosion=Uniform CorrosionLarge amount of Flake is observed and drops off

Flake≠Corrosion Product

60wt%60wt%60wt%

・Thickness loss : Controllable by design・Much Flake → Large Corrosion → Misunderstanding

SRSR--242 Result242 Result

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 10 20 30Age at inspection year

Aver

age L

oss m

m

60wt%

53 actual vessels(Class NK inspection data)

0.1mm/year

Corrosion rates are relatively small

of S!of S!of S!of S!

5

Observed Pits in COT

・Very high frequency・Very Deep : Max. abt.10mm/2.5years・Periodical Repair is Essential

0

200

400

600

800

1000

1200

1400

1P 2P 3P 4P 5P 1C 2C 3C 4C 5C 1S 2S 3S 4S 5S

COT PositionPi

ts Co

unt o

ver 4

mm D

epth

DH1 2.5yDH1 5.0yDH2 2.5y

6

How does the pitting corrosion start?

7

Observed electric resistance of oil coat and tar epoxy painting

Oil coat exists inside of COTOil coat shows high insulating resistance equal to T/E

0

500

1000

1500

2000

2500

3000

Oil Coat Tar Epoxy

Resis

tance

kΩ･ c

Oil coat T/E paint

SensorSensorRSTRST®®

Measurement of resistance


8

Cause of Oil coat defect-1 Change of oil coat resistance by wetting.

Wetting decreases Insulating resistance of oil coat

0

100

200

300

400

500

600

Befor AfterWetting effect

Resis

tance

kΩ･ c

m2

SensorSensor

Distilled WaterDistilled Water


9

COT No Na T-Fe Fe3+ Cl- SO42- Mg2+ pH H2S

DH-1 13600 2 2 42500 14 ND 7.0 NDDH-3 1S 40000 42 11 48000 1470 ND 7.2DH-3 2P 40000 2.5 1 54000 1350 ND 7.5

Chemical analysis of water sampled from actual COT bottom

Significant quantity of water exists on COT bottomHigh concentration of NaCl solutionNot seawater but Brine from Oil Well

↓

Brine damages oil coat resulting pitting CorrosionBrine damages oil coat resulting pitting Corrosion(Existence of Brine : Not depends on crude oil a/o route)



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Laser Pointer

Pits on Longi. facePits between drain holes

Pits under cargo pipe Pits under side Longi. face

←S/H

← D/H

←←Oil coatingOil coating

←←Oil coatingOil coating

←←Oil coatingOil coating↑↑

Drain holeDrain hole↑↑

Drain holeDrain hole

Pits are observed at the path where Brine flows through



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Effect of COW is remarkable!!On insulating performance of oil coat,

masking structure to COW has remarkable effect

0

500

1000

1500

2000

2500

3000

No COW 1COW 2COW

Number of COW

Res

ista

nce

kΩ･ c

!!COW#1

COW#2

Apparent corrosion!

Cause of Oil coat defect-2 Change of oil coat resistance by removal.


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Detrimental Factors to Oil Coat defectDetrimental Factors to Oil Coat defect①① BrineBrine : by wetting: by wetting②② COWCOW : by removal: by removal

↓↓Both decreases insulating resistanceBoth decreases insulating resistance

↓↓Localized defect of oil coat results pitting corrosionLocalized defect of oil coat results pitting corrosion

Oil coat is not stable especially on D/H flat cargo tank topOil coat is not stable especially on D/H flat cargo tank top(compared to S/H bottom (compared to S/H bottom structure) structure)

Inner Bottom

Outer Bottom Outer Bottom

D/H S/HBottom Longi.Inner Bottom

Hidden ribNo shield for bottom Ribs : Shield for bottom

Inner Bottom

Outer Bottom Outer Bottom

D/H S/HBottom Longi.Inner Bottom

Hidden ribNo shield for bottom Ribs : Shield for bottom

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Short Summary - 1Oil Coat Layer exists inside of COT

Oil Coat Layer provides protective effect to corrosion

Brine a/o COW damages Oil Coat protective effect

Pit starts at the damaged oil coat area

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How much is the pitting corrosion rate?

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: Pits over 4mm at 1st inspection (repaired) : Pits less than 2mmless than 2mm at 1st1st inspection (NOT repairedNOT repaired)

: Pits over 4mm at 2nd inspection (repaired)Old pits() did not grow!

New pits() were observed at different points.↓

Pitting growth terminated at the 1st Dock inspection→ Corrosion rate = Pit Depth/Dock Interval

1C

Change of pitting location in plural inspectionsSRSR--242 Result242 Result

16

1P

Change of pitting location in plural inspection-2

102 103 104 105 106 107 108109

110

111112

113

L.BHD



New pits() appear at different points.

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1S


102 103 104 105 106 107 108

109

110111

112113

L.BHD

FWD




18

3C


82 83 84 85 86 87 88 89 90 91 92

PORT

STBDFWD




19

3S


82 83 84 85 86 87 88 89 90 91 92STBD

L.BHD

FWD




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･Maximum pitting rate follows statistical distribution･Ship age based corrosion rate : Distribution varies widely by ship →Irrational･Dock Interval Corrosion rate : Same & identical between ships

→ Rational (Corrosion environment cannot differ from ship to ship largely)

1.011.051.111.251.431.6722.53.335

10

202533.350

100

0 1 2 3 4Corrosion Rate mm/year

Return Period T

VLD-12.4Years

VLS-12.5Years

VLS-22.5Years

F

99%

98%97%96%95%

90%

80%70%60%50%40%30%20%10%5%1%


Cumulative Frequency F

VLS-17Years

VLD-12.4years

VLS-25years

C.R=Depth/Dock IntervalC.R=Depth/Ship Age

Statistical Distribution of max pitting rate (by Statistical extreme value analysis)

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Brine : NaCl ~10wt.%

Corrosion condition would be reset after Dock inspection→Pitting growth stops at dock cleaning

(1) Under servicing condition

(2) Dock cleaning for inspection

(3) Re-Start of service

(4) Nuclear of new pit

Sludge & Corrosion products

Oil coating

Cleaned Steel Surface = No oil coat

Oil coating ,Sludge & Corrosion productsare cleaned and dried for inspection

Cleaned and dried pits are re-coated by new crude oil

New oil coat = Resetting insulating condition

defect

New defect in oil coat =Nucleation of pitting

New defect : by COW, Brine …..

Discussion on pit growth

Brine : NaCl ~10wt.%


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Pit Growth Stops at a dock(tank cleaning)

Pitting Stops

Pit D

epth

↑Dock Inspection Ship Age

New Pitting starts at new location

Important Basis for Anti-Corrosion SteelSRSR--242 Result242 Result

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Short Summary - 2Pitting corrosion rate follows statistical distribution

Pitting stops at a dock (COT cleaning)

Pitting growth duration is dock interval

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What is the pitting environment?

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Pitting progresses isotropically

keeping constant depth/diameter ratio

If pitting progresses by Galvanic Action…

Irrational to the fact

Shape of Pits

Depth is in linear relation with diameter

0

5

10

15

20

25

30

35

40

45

0 2 4 6 8 10 12

Depth mm

Dia

mete

r m

m

VLS-B

VLD

VLD-D

VLS-F

・Pitting progresses isotropically with Large corrosion rate : abt. 10mm/2.5year・This process hard to think by Galvanic corrosion・Environment inside of pit is supposed to be highly corrosive


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Observed pH inside of pitspH : Lower than 1.5 → Strong Acid environment

Actual Vessel

pH of PIT inside: <1.5

→→Unfavorable for MIC ((SRB SRB active pH : 6~9active pH : 6~9))


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Max. pitting rate : Very high abt. 3mm/y in actual vessel→ can be explained under strong acid corrosion environment → meets the observed fact : pH inside of pit lower than 1.5

H2 evolution ← →Oxygen consumption

Pitting can be explained as corrosion in strong acid environment

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12pH

Corr

osi

on R

ate m

m/ye

ar

NaCl 10wt.%30

Conventional Steel

Observed pH and Max. corrosion rate

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Corrosion growth process inside of Pit

【1st Step】Corrosion starts

pH~70.2mm/y

【2nd Step】4< pH <7

pH goes down ~40.2mm/y

【3rd Step】Main corrosion process

pH goes down lower than1.52~4mm/y

Crude Oil

Bottom Plate

Oil Coat DefectBrine (NaCl 10%)

Oil CoatFe2+

↑Fe

Crude Oil

Oil Coat

H+HH++

pH<1.5pH<1.5pH<1.5

Cl-Crude Oil

Oil CoatCl-

Fe2+ + 2H2O → 2H+ + Fe(OH)2

pH>4pH>4pH>4

0

1

2

3

4

5

6

7

024681012pH

Corro

sion R

ate m

m/ye

ar

【2nd Step】

【3rd Step】

【1st Step】

Conventional Steel

Fe2+ + 2H2O → 2H+ + Fe(OH)2

High conc. [Cl-] in Brine

↑Fe (Corrosion)

Cl- ← Balance → H+

in High conc.

Low pH inside of pit↓

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Short Summary - 3

Corrosion environment inside of pit is Strong Acid

pH lower than 1.5 inside of pit

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Development target and result

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Established Corrosion test condition simulating environment inside of pit

Beaker

SpecimenTest solution

NoticeNaCl 10 mass%

pH Lower than 1.5 Adjust by HCl

Amount 20cc/.cm2 or more

Change Every 24 to 48 hoursAir open

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Size 40mm x 50mm x t t: less than 4mml

Surface #600 emery paper Follow JIS G0591

repeat n=3 or more

144hours

Dipping specimen Follow JIS G0591Immersion

Gas

Temperature

Condition

Solution

Specimen

Duration

32

0.010.05

0.10.20.30.40.50.60.7

0.8

0.9

0.950.960.97

0.98

0.99

1.01

1.051.111.251.431.6722.53.33

5

10

202533.350

100


Cumu

lative

Fre

quen

cy F

Retur

n Per

iod TConventional

Steel

Average

15 COT

← Corresponds toLaboratorytest result

← Corresponds toOnboardMax. Depth

Development TargetRepair Free at every 2.5year Dock inspection

Repair Essential

Max Corrosion rate for4mm depth for 2.5years in 15COT

Target less than 1.0mm/year in laboratory test

No Repair ← TypicalConventional

Steel(Actual Vessel)

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0

1

2

3

4

5

6

7

8

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5pH

Corro

sion R

ate m

m/ye

ar

← Conventional Steel

NaCl 10wt.%30

Air OpenStagnant

n=3

NSGP-1 provides superior corrosion resistance (NaCl 10wt.% pH less than 1.5)

Conventional pH=1

NSGP-1 pH=1NSGP-1NSGPNSGP--11

↓ Target

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Outline of the NSGP-1Newly Developed Anti-corrosion Steel for VLCC COT

A typical example of chemical analysis （mass%)

NSGP-1 established high corrosion resistancewith Least Micro-alloy Elements & ControlNegligible difference to conventional steel

↓No galvanic action with conventional steels & welds

No effect to Physical Properties

C Si Mn P S Al Ti CeqNSGP-1 0.124 0.331

AH32 0.140 0.20 1.09 0.018 0.006 0.031 0.014 0.322IACS Standard ≦0.18 ≦0.5 0.9～1.6 ≦0.035 ≦0.035 ≧0.02 ≦0.02 ≦0.36

meets IACS Standard (including all ally elements)

The Chemical composition meets the IACS rules

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Short Summary - 4Developed corrosion test NaCl 10wt.% pH lower than 1.5

Target for Anti-corrosion steelCorrosion Rate : Less than 1.0mm/yearfor repair-free at 2.5 dock intervals

Successful development of NSGP-1 has completed bearing high corrosion resistance with Least Micro-Alloy Elements

(Negligible difference to conventional steel)

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ConclusionOil Coat Layer in COT provides protective effect to corrosionPit starts at the damaged oil coat area

Pitting stops at a dock (COT cleaning)Pitting growth duration is dock interval

Corrosion environment of pit inside is Strong Acid

Developed corrosion test : NaCl:10wt.% pH:lower than 1.5Development Target : Corrosion Rate Less than 1.0mm/year

(for repair-free at 2.5 dock intervals)

Developed NSGP-1 shows high corrosion resistance with Least Micro-Alloy Elements (Negligible difference to conventional steel)

Documents

Development of New Anti-Corrosion Steel for COTs of crude