Pitting Corrosion on Epoxy-Coated Surface of Ship Structures - - PowerPoint PPT Presentation

Pitting Corrosion on Epoxy-Coated Surface of Ship Structures

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

Background

There 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

• f 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

• 1. Actual state of corrosion (No protective coatings)
• 2. Actual state of corrosion (Tar epoxy paints)
• 3. Progress rate of corrosion
• 4. Concluding remarks

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

• 1. Actual state of corrosion (No protective coatings)
• 2. Actual state of corrosion (Tar epoxy paints)
• 3. Progress rate of corrosion
• 4. Concluding remarks

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

Pit Depth (mm) Pit Diameter (mm)

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 Cone Ratio 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 Corrosion

Generation 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

• verlap.
• 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

20 40 60 80 100

• 8
• 6
• 4
• 2

2

Degree of Pitting Intensity DOP (%) Thickness Diminution (One Side) (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
• 8
• 6
• 4
• 2

2

Average Diminution (mm) Thickness Diminution (One Side) (mm)

Measurement results

• Std. dev. (web)

Std.dev (face) Max.depth (web) Max.depth (face)

• Min. cross section ave. (web)
• Min. cross section ave. (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

20 40 60 80 100

• 8
• 6
• 4
• 2

2

Degree of Pitting Intensity DOP (%) Thickness Diminution (One Side) (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
• 8
• 6
• 4
• 2

2

Average Diminution (mm) Thickness Diminution (One Side) (mm)

Measurement results

• Std. dev. (web)

Std.dev (face) Max.depth (web) Max.depth (face)

• Min. cross section ave. (web)
• Min. cross section ave. (face)

Statistics of corroded surface conditions and DOP (One Side) Statistics of corroded surface conditions and DOP (One Side)

• 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

• 1. Actual state of corrosion (No protective coatings)
• 2. Actual state of corrosion (Tar epoxy paints)
• 3. Progress rate of corrosion
• 4. Concluding remarks

Probabilistic corrosion model

Period before the generation

• f active pitting points T0

⎪ ⎭ ⎪ ⎬ ⎫ ⎪ ⎩ ⎪ ⎨ ⎧ σ µ − − σ π =

2 2

2 ) (ln exp 2 1 ) ( t t t fT

Transition time from active pitting points to progressive pitting points Tr

) exp( ) ( t t g

r

T

α − ⋅ α =

Depth of pitting points

b

a z τ ⋅ = τ) ( ⎪ ⎭ ⎪ ⎬ ⎫ ⎪ ⎩ ⎪ ⎨ ⎧ σ µ − − σ π =

2 2

2 ) (ln exp 2 1 ) (

a a a a

x x x h

Progress Rate of Corrosion

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

Ship Age (years) Average Thickness Diminution (mm) Probabilistic Corrosion Model Structural Members in Cargo Hold Oil Coatings At present, there are no bulk carriers whose hold frames have oil coatings Tar Epoxy Coatings

Progress Rate of Corrosion

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

Ship Age (years) Average Thickness Diminution (mm) Probabilistic Corrosion Model Structural Members in Cargo Hold Oil Coatings At present, there are no bulk carriers whose hold frames have oil coatings Tar Epoxy Coatings

Progress Rate of Corrosion

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

Ship Age (years) Average Thickness Diminution (mm) Probabilistic Corrosion Model Structural Members in Cargo Hold Oil Coatings At present, there are no bulk carriers whose hold frames have oil coatings Tar Epoxy Coatings

Progress Rate of Corrosion

However, in the case of members with tar epoxy paints, pitting corrosion

• ccurs and this makes it difficult to evaluate the residual thickness and/or

residual strength.

5 10 15 20 25 1 2 3 4 5

Ship Age (years) Average Thickness Diminution (mm) Probabilistic Corrosion Model Structural Members in Cargo Hold Oil Coatings At present, there are no bulk carriers whose hold frames have oil coatings 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 σu/σu0

Original Thickness t0: 10mm

Average Thickness Loss (mm)

Small specimen Wide specimen

σu=Pmax/A0 σu0：Tensile strength of Material

Example of Experiments

water pressure

Shell Web Face

Example of Experiments

Test 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

• bserved 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