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 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 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
Actual Corroded Hold Frame of 13-Year-Old Bulk Carrier (Tar Epoxy Paints at Construction) � Heavy blisters and pitting corrosion were observed. � Unevenness of the corroded surfaces of the web plates is extremely large.
Shape of Corrosion Pit (Tar Epoxy Paints at Construction) Ratio of diameter to depth 40 10 to 1 Pit Diameter (mm) 8 to 1 30 20 BC-A(14years) BC-B(12years) 10 BC-C(20years) BC-D(13years) 0 1 2 3 4 Pit Depth (mm) Relation between pit diameter and its depth � Pit shape : Circular Cone � Ratio of diameter to depth : between 8 to 1 and 10 to 1
Generation Mechanism of Corrosion Pit (Tar Epoxy Paints at Construction) Protective Coating The outline of the generation and Steel Plate progress of pitting corrosion can be explained as follows: Mechanical Damage to the coating 1. Mechanical damage of the protective coating occurs due to the scratch of cargo. Rust Blister 2. Corrosion starts at the small point where the coating is broken by the scratch of cargo. Corrosion Pit 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 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 Measurement results Measurement results Max.depth (web) Ave. (web) Max.depth (web) Max.depth (face) Ave. (face) Max.depth (face) Std. dev. (web) Min. cross section ave. (web) Std. dev. (web) Min. cross section ave. (web) Std.dev (face) Min. cross section ave. (face) Std.dev (face) Min. cross section ave. (face) 2 2 Thickness Diminution (One Side) (mm) Thickness Diminution (One Side) (mm) 0 0 -2 -2 -4 -4 -6 -6 -8 -8 -5 -4 -3 -2 -1 0 0 20 40 60 80 100 Average Diminution (mm) Degree of Pitting Intensity DOP (%) Statistics of corroded surface Statistics of corroded surface conditions and DOP (One Side) conditions and average diminution (Tar Epoxy Paints at Construction) DOP: D egree O f P itting intensity defined as a ratio of the pitted surface area to the entire surface area.
Statistics of Corroded Surface Conditions Measurement results Measurement results Max.depth (web) Ave. (web) Max.depth (web) Max.depth (face) Ave. (face) Max.depth (face) Std. dev. (web) Min. cross section ave. (web) Std. dev. (web) Min. cross section ave. (web) Std.dev (face) Min. cross section ave. (face) Std.dev (face) Min. cross section ave. (face) 2 2 Thickness Diminution (One Side) (mm) Thickness Diminution (One Side) (mm) 0 0 -2 -2 -4 -4 -6 -6 -8 -8 -5 -4 -3 -2 -1 0 0 20 40 60 80 100 Average Diminution (mm) Degree of Pitting Intensity DOP (%) Statistics of corroded surface Statistics of corroded surface � Average and standard deviation vary with small scatter bands. conditions and DOP (One Side) conditions and DOP (One Side) � 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
Progress Rate of Corrosion Probabilistic corrosion model Period before the generation of active pitting points T 0 ⎧ ⎫ 2 ⎪ (ln t ) ⎪ 1 − µ 0 f T ( t ) exp ⎨ ⎬ = − 2 ⎪ ⎪ 0 2 t 2 ⎩ ⎭ π σ σ 0 0 Transition time from active pitting points to progressive pitting points T r g ( t ) exp( t ) = α ⋅ − α T r Depth of pitting points b z ( τ ) a = ⋅ τ ⎧ ⎫ 2 ⎪ ⎪ 1 (ln x ) − µ a h ( x ) exp ⎨ ⎬ = − a 2 ⎪ ⎪ 2 x 2 ⎩ ⎭ π σ σ a a
Progress Rate of Corrosion 5 Average Thickness Diminution (mm) Probabilistic Corrosion Model Structural Members in Cargo Hold 4 Oil Coatings At present, there are no bulk carriers 3 whose hold frames have oil coatings 2 1 Tar Epoxy Coatings 0 5 10 15 20 25 Ship Age (years) Thickness Diminution of Structural Members with Different Coating Types (Bulk Carriers, DWT > 50,000 ton)
Progress Rate of Corrosion 5 Average Thickness Diminution (mm) Probabilistic Corrosion Model Structural Members in Cargo Hold 4 Oil Coatings At present, there are no bulk carriers 3 whose hold frames have oil coatings 2 1 Tar Epoxy Coatings 0 5 10 15 20 25 Ship Age (years) � 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.
Progress Rate of Corrosion 5 Average Thickness Diminution (mm) Probabilistic Corrosion Model Structural Members in Cargo Hold 4 Oil Coatings At present, there are no bulk carriers 3 whose hold frames have oil coatings 2 1 Tar Epoxy Coatings 0 5 10 15 20 25 Ship Age (years) � 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.
Progress Rate of Corrosion 5 Average Thickness Diminution (mm) Probabilistic Corrosion Model Structural Members in Cargo Hold 4 Oil Coatings At present, there are no bulk carriers 3 whose hold frames have oil coatings 2 1 Tar Epoxy Coatings 0 5 10 15 20 25 Ship Age (years) � 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.
Example of Experiments Tensile test with actual corroded members Original Thickness t 0 : 10mm σ u = P max / A 0 1 σ u0 ： Tensile strength of Material σ u / σ u0 0.5 Small specimen Wide specimen 0 1 2 3 4 Average Thickness Loss (mm) Nominal strength vs. average thickness loss Example of specimens � Nominal tensile strength decreases with the progress of pitting corrosion.
Example of Experiments Test with artificially pitted structural members An example of tests with structural models with pitting (3PB test). Shell water pressure Web Face HF (Web Face) Shell Shell HF Drilled pits Web Pit shape : Circular Cone Ratio of diameter to depth : Face 8 to 1
Concluding Remarks - 1 � 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.
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