Pitting Corrosion on Epoxy-Coated Surface of Ship Structures › main › cooperation › imo_iso › ISST2007 › CD › ... · 2. Corrosion process starts at the damaged parts of

Pitting Corrosion on Epoxy-CoatedSurface of Ship Structures

Tatsuro Nakai, Hisao Matsushita, Norio YamamotoResearch Institute, Nippon Kaiji Kyokai (ClassNK)

BackgroundThere were bulk carrier losses in the late 80’s and early 90’s with considerable loss of human life.

One of the main causes for the losses was severe corrosion of the hold frames.

It was made mandatory in 1992 to apply epoxy coating or equivalent to hold frames.

Introducing the coating system, the Enhanced Survey Program (ESP) and retroactive requirements for existing bulk carriers (Bulk Carrier Safety) have helped to improve the safety of bulk carriers.

Example of Damaged Hold Frames of Bulk Carriers

Purpose

To investigate the corrosion patterns observed in structural members of cargo holds of bulk carriers with tar epoxy paint and those with no protective coating

To check the effectiveness of applying tar epoxy paint

Scope

1. Actual state of corrosion (No protective coatings)

2. Actual state of corrosion (Tar epoxy paints)

3. Progress rate of corrosion

4. Concluding remarks

Scope





Actual Corroded Hold Frame of 13-Year-Old Bulk Carrier

(No Protective Coatings at Construction)

Unevenness of the corroded surfaces of the web plates is small.This type of corrosion is categorized as general (uniform) corrosion.

Scope





Heavy blisters and pitting corrosion were observed.Unevenness of the corroded surfaces of the web plates is extremely large.

Actual Corroded Hold Frame of 13-Year-Old Bulk Carrier

(Tar Epoxy Paints at Construction)

Shape of Corrosion Pit(Tar Epoxy Paints at Construction)

1 2 3 4

10

20

30

40

0Pit Depth (mm)

Pit

Dia

met

er (m

m) Ratio of diameter to depth

10 to 1 8 to 1

BC-A(14years)BC-B(12years)BC-C(20years)BC-D(13years)

Pit shape : Circular ConeRatio of diameter to depth : between 8 to 1 and 10 to 1

Relation between pit diameter and its depth

Mechanical Damage to the coating

Generation Mechanism of Corrosion Pit(Tar Epoxy Paints at Construction)

Protective Coating

Steel Plate

Rust Blister

Corrosion Pit

The outline of the generation and progress of pitting corrosion can be explained as follows:

1. Mechanical damage of the protective coating occurs due to the scratch of cargo.

2. Corrosion starts at the small point where the coating is broken by the scratch of cargo.

3. This leads to generation of pitting corrosion and rust blisters.

Surface of members with pitting corrosion(Tar Epoxy Paints at Construction)

Progress of Pitting Corrosion 800mm

Surface of members with pitting corrosion(Tar Epoxy Paints at Construction)

Progress of Pitting CorrosionGeneration and progress of pitting corrosion could be

explained as follows:1. Mechanical damage to the protective coating occurs

due to the scratch of cargo.2. Corrosion process starts at the damaged parts of the

protective coating.3. This leads to pitting corrosion.4. In the early stage of corrosion, each corrosion pit

exists independently.5. Then, the number of corrosion pits increases and each

corrosion pits develops, and some of them start to overlap.

6. Some parts of the plate surface remain uncorroded in this stage.

7. When the number of corrosion pits increases further and each corrosion pit develops further, they form a very uneven surface all over the plate.

8. In the later stages of corrosion, unevenness of the plate surface due to pitting corrosion becomes smaller with the progress of corrosion.

Statistics of Corroded Surface Conditions

0 20 40 60 80 100-8

-6

-4

-2

0

2

Degree of Pitting Intensity DOP (%)

Thic

knes

s D

imin

utio

n (O

ne S

ide)

(mm

)

Measurement results Ave. (web) Ave. (face) Std. dev. (web) Std.dev (face)

Max.depth (web) Max.depth (face) Min. cross section ave. (web) Min. cross section ave. (face)

-5 -4 -3 -2 -1 0-8

-6

-4

-2

0

2

Average Diminution (mm)

Thic

knes

s D

imin

utio

n (O

ne S

ide)

(mm

)

Measurement results

Std. dev. (web) Std.dev (face)


Statistics of corroded surface conditions and DOP (One Side)

Statistics of corroded surface conditions and average diminution

(Tar Epoxy Paints at Construction)DOP: Degree Of Pitting intensity defined as a ratio of the pitted surface area to the entire surface area.

Statistics of Corroded Surface Conditions

0 20 40 60 80 100-8

-6

-4

-2

0

2

Degree of Pitting Intensity DOP (%)

Thic

knes

s D

imin

utio

n (O

ne S

ide)

(mm

)

Measurement results Ave. (web) Ave. (face) Std. dev. (web) Std.dev (face)


-5 -4 -3 -2 -1 0-8

-6

-4

-2

0

2

Average Diminution (mm)

Thic

knes

s D

imin

utio

n (O

ne S

ide)

(mm

)

Measurement results

Std. dev. (web) Std.dev (face)




Average and standard deviation vary with small scatter bands.Maximum pit depth and average thickness loss at the minimum cross section are scattering.DOP reaches 100% when the average thickness diminution on one side exceeds approximately 2mm.A form of corrosion changes from pitting corrosion to general (uniform) corrosion with further progress of corrosion.

Scope





Probabilistic corrosion model

Period before the generation of active pitting points T0

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

σμ−−

σπ= 2

0

20

0 2)(lnexp

21)(

0

tt

tfT

Transition time from active pitting points to progressive pitting points Tr

)exp()( ttgrT α−⋅α=

Depth of pitting pointsbaz τ⋅=τ)(

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

σμ−−

σπ= 2

2

2)(lnexp

21)(

a

a

aa

xx

xh

Progress Rate of Corrosion


Thickness Diminution of Structural Members with Different Coating Types

(Bulk Carriers, DWT > 50,000 ton)

5 10 15 20 25

1

2

3

4

5

0Ship Age (years)

Aver

age

Thic

knes

s D

imin

utio

n (m

m)

Probabilistic Corrosion ModelStructural Members in Cargo Hold

Oil CoatingsAt present, there are no bulk carrierswhose hold frames have oil coatings

Tar Epoxy Coatings


Thickness diminution of structural members with tar epoxy coatings is smaller than those with oil coatings.

The average amount of corrosion is significantly reduced by applying tar epoxy coatings.

5 10 15 20 25

1

2

3

4

5

0Ship Age (years)

Aver

age

Thic

knes

s D

imin

utio

n (m

m)



Tar Epoxy Coatings


Applying epoxy coatings is a very effective measure to protect structural members from deterioration due to corrosion.

Applying epoxy coatings or equivalent to the hold frames is mandatory at present.

5 10 15 20 25

1

2

3

4

5

0Ship Age (years)

Aver

age

Thic

knes

s D

imin

utio

n (m

m)



Tar Epoxy Coatings


However, in the case of members with tar epoxy paints, pitting corrosion occurs and this makes it difficult to evaluate the residual thickness and/or residual strength.

5 10 15 20 25

1

2

3

4

5

0Ship Age (years)

Aver

age

Thic

knes

s D

imin

utio

n (m

m)



Tar Epoxy Coatings

Nominal strength vs. average thickness loss Example of specimens

Nominal tensile strength decreases with the progress of pitting corrosion.

Tensile test with actual corroded members

1 2 3 4

0.5

1

0

σ u/σ

u0

Original Thickness t0: 10mm

Average Thickness Loss (mm)

Small specimenWide specimen

σu=Pmax/A0

σu0：Tensile strength of Material

Example of Experiments

water pressure

Shell

Web

Face

Example of ExperimentsTest with artificially pitted structural members

An example of tests with structural models with pitting (3PB test).

Drilled pits Pit shape : Circular Cone Ratio of diameter to depth : 8 to 1

HF(Web Face)Shell

Shell

Web Face

HF

The present study describes the corrosion pattern observed in tar epoxy coated structural members of cargo holds of bulk carriers.

Tar epoxy paint is a very effective measure to protect structural members from deterioration due to corrosion.

However, it has been revealed that the typical corrosion pattern for the tar-epoxy coated structural members of cargo holds of bulk carriers carrying coal and iron ore is pitting corrosion.

Concluding Remarks - 1

Such a situation was not expected at the time the coating systems were applied.

It should be noted that corrosion patterns may change when new coating systems are introduced.

It can be said from this lesson that monitoring the relation between the corrosive environment and actual state of corrosion is important when the new coating systems are introduced.

Concluding Remarks - 2

Documents

Pitting Corrosion on Epoxy-Coated Surface of Ship Structures › main › cooperation › imo_iso › ISST2007 › CD › ... · 2. Corrosion process starts at the damaged parts of