PITTING CORROSION ON EPOXY-COATED SURFACE OF SHIP …

Shipbuilding Technology ISST 2007, Osaka, 2007

© 2007: JASNAOE-RINA 59

PITTING CORROSION ON EPOXY-COATED SURFACE OF SHIP STRUCTURES T Nakai, H Matsushita and N Yamamoto, Nippon Kaiji Kyokai (ClassNK), Japan SUMMARY It should be noted that corrosion patterns may change when new coating systems are introduced because corrosion patterns highly depends on the coating types of structural members (e.g., no protective coatings, oil coatings, tar epoxy coatings, etc.). As for the hold frames of bulk carriers, it was made mandatory in 1992 to apply epoxy coating or equivalent. When there were oil coatings or no protective coatings on hold frames, general corrosion was observed. After hold frames of bulk carriers carrying coal and iron ore came to have tar epoxy paints, the typical corrosion pattern changes to pitting corrosion. The present paper deals with corrosion observed in structural members of cargo holds of bulk carriers carrying coal and iron ore. Firstly, the corrosion pattern observed in non-coated structural members is briefly explained. Secondly, the corrosion pattern observed in structural members with tar epoxy coating is presented in detail. Thirdly, progress rate of corrosion in structural members with different coating systems is briefly described. It can be said that applying tar epoxy coating is a very effective measure to protect the structural members from deterioration due to corrosion. However, the evaluation of residual thickness and/or residual strength became difficult because the corroded surfaces of the structural members with tar epoxy coatings have large unevenness due to pitting corrosion. Such a corrosion pattern was not expected when the coating system was made mandatory. 1. INTRODUCTION It is well known that corrosion is one of the dominant life-limiting factors of ships because hull structural members are exposed to corrosive environment after commissioning and ageing effect such as thickness diminution due to corrosion may be unavoidable. It is recognized that one of the main measures to protect hull structural members from deterioration due to corrosion is to apply protective coatings. There were bulk carrier losses in the late eighties and early nineties with considerable loss of human life. One of the main causes for the losses was severe corrosion of the hold frames of cargo holds. For the purpose of protecting the hold frames from deterioration due to corrosion, it was made mandatory in 1992 to apply epoxy coating or equivalent to hold frames in way of cargo holds of bulk carriers. Introducing the coating system, the Enhanced Survey Program (ESP) and retroactive requirements for existing bulk carriers (so called ‘Bulk

Carrier Safety’) have helped to improve the safety of bulk carriers. To ensure the structural integrity of ships, it is of crucial importance to understand the corrosion process, estimate the corrosion rate and evaluate the effect of corrosion wastage not only on overall strength but also on local strength accurately. It should be noted that corrosion patterns may change when new coating systems are introduced because corrosion patterns highly depends on the coating types of structural members (e.g., no protective coatings, oil coatings, tar epoxy coating, etc.). Therefore, the corrosion pattern observed in the target structural members should be accurately evaluated.

Figure 2: Actual Corroded Hold Frame of 13-Year-Old Bulk Carrier (No Protective Coatings at Construction) Figure 1: Schematic View of Cargo Hold of Bulk Carrier

hold frame loading conditionof coal

loading conditionof iron ore



The corrosion pattern observed in hold frames of cargo holds of bulk carriers changed completely after it was made mandatory to apply epoxy coating or equivalent. The present paper focuses on the corrosion patterns observed in structural members of cargo holds of bulk carriers which carries exclusively carry coal and iron ore. Firstly, the corrosion pattern observed in non-coated structural members is briefly explained. Secondly, the corrosion pattern observed in structural members with tar epoxy coating is presented in detail. Thirdly, progress rate of corrosion in structural members with different coating systems is briefly described.

2. CORROSION PATTERN IN NON-COATED STRUCTURAL MEMBERS Figure 1 shows a schematic view of cargo hold of bulk carrier and Figure 2 gives the example of actually corroded hold frames in way of cargo holds of a 13-year-old bulk carrier. The hold frames were not coated at construction. In this case, unevenness of the corroded surfaces of the web plates is small and this type of corrosion is categorized as general (uniform) corrosion. 3. CORROSION PATTERN IN EPOXY COATED STRUCTURAL MEMBERS An example of the actual corroded web plates (approximately 500mm×700mm) of hold frames taken from a 13-year-old bulk carrier is shown in Figure 3. The bulk carrier is different from the one mentioned in the previous chapter and its hold frames were coated with tar epoxy paints at construction. Figure 3(a) shows the web plate before sand-blasting and many heavy rust blisters can be seen on the plate surface. Figure 3(b) represents the same web plate after sand-blasting to remove the heavy rust blisters covering pitting corrosion. It can be seen that progressive pitting corrosion is prevailing on the plate surface. Pitted regions of the web plate are shown in Figure 3(c) in black shade. Figure 4 shows a cross-sectional view of corrosion pit with rust blister. The rust blisters are very hard and it is difficult to remove these blisters even with hammers. Figure 5 gives the relationship between pit diameter and depth. It can be seen that the ratio of diameter to depth is in the range between 8-1 and 10-1, and the diameter of corrosion pit might become up to 50mm [1]. Figure 6 gives examples of corroded webs after sand-blasting taken from the 13-

(a) before Sand-Blasting

(b) after Sand-Blasting

(c) Corroded Part Shown in Black Shade

Figure 3: Actual Corroded Hold Frame of 13-Year-Old Bulk Carrier (Tar Epoxy Coatings at Construction)

Figure 4: Cross-Sectional View of Corrosion Pit with Rust Blister (Tar Epoxy Coatings at Construction)

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

Figure 5: Relationship between Pit Diameter and Depth



year-old bulk carrier with a wide variety of DOP (Degree Of Pitting intensity) defined as ratio of the pitted surface area to the entire surface area. Although the webs in Figure 6 were taken from the same bulk carrier, this figure demonstrates well the progress of pitting corrosion. Generation and progress of pitting corrosion could be explained as follows (see Fig.7)

(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 pit 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.

In order to investigate the geometrical characteristics of corroded surface conditions of pitted hold frames in more detail, surface unevenness of these actual corroded web and face plates has been investigated using laser displacement sensors. Figure 8 show the relationship between statistics of corroded condition and DOP and Figure 9 depicts the relationship between statistics of corroded condition and average thickness diminution for the webs and face plates taken from the 13-year-old bulk carrier, where measured area of the webs is 80mm×200mm (see Figure 6) and that of the face plates in 50mm×200mm. It can be seen that average thickness diminution and standard deviation of thickness diminution vary within small scatter bands, and DOP reaches 100% when the average thickness diminution on

(a) DOP = 19%

(b) DOP = 35%

(c) DOP = 60%

(d) DOP = 87%

(e) DOP = 100%

Figure 6: Examples of Corroded Surface Conditions of Webs of Hold Frames (80mm×200mm, 13-Year-Old Bulk Carrier, Tar Epoxy Coatings at Construction)

Protective Coating Steel Plate

Mechanical Damage to the coating

Rust Blister

Corrosion Pit

Figure 7: Mechanism of Generation of Pitting Corrosion



one side exceeds approximately 2mm. A trend is observed that a form of corrosion changes from pitting corrosion to general corrosion with further progress of corrosion after DOP reaches 100%. The probability distribution of diminution varies exponential to normal one depending on DOP and thickness diminution [2] and it is considered that the probability distribution of diminution follows normal distribution when a form of corrosion changes almost completely to general corrosion with the progress of corrosion. 4. PROGRESS RATE OF CORROSION Figure 10 shows an example of average thickness diminution behavior of structural members in cargo holds of bulk carriers with different coating types. The average thickness diminution behavior shown in this figure is obtained by applying the probabilistic corrosion model

[4] to the thickness measurement records. It can be seen from Fig. 10 that thickness diminution of structural members with tar epoxy coatings is smaller than those with oil coatings. It is clear from this figure that the average amount of corrosion is significantly reduced by applying tar epoxy coatings and applying tar 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 and there are no bulk carriers whose hold frames have oil coatings or no coating. In the case of structural members with oil coatings or no protective coatings, the typical corrosion pattern is considered to be general corrosion which uniformly reduces the thickness and the evaluation of residual thickness and/or residual strength is relatively easy, because the thickness diminution can be directly compared with the allowable diminution level and residual strength calculations can be performed by excluding the uniform thickness diminution. On the other hand in the case of structural members with tar epoxy paints, pitting corrosion occurs as previously mentioned and this makes it difficult to evaluate the residual thickness and/or residual strength. From such a point of view, visual assessment of pitting corrosion observed in structural members of cargo holds has been developed [3]. 5. CONCLUSIONS The present paper describes the corrosion pattern observed in tar epoxy coated structural members of cargo holds of bulk carriers. It can be said that 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 and this makes it difficult to evaluate residual thickness and/or residual strength. Such a situation was not expected at the time the coating systems were applied. It

0 20 40 60 80 100-8

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Degree of Pitting Intensity DOP (%)

Thic

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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)

Figure 8: Relationship between Statistics of CorrodedCondition and DOP

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

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

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

Figure 9: Relationship between Statistics of CorrodedCondition and Average Diminution

5 10 15 20 25

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Probabilistic Corrosion ModelStructural Members in Cargo Hold

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

Tar Epoxy Coatings

Figure 10: Thickness Diminution of Structural Members with Different Coating Types (Bulk Carriers, DWT > 50,000 ton)



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 also important when the new coating systems are introduced. 6. REFERENCES 1. Nakai, T., Matsushita, H., Yamamoto, N. and Arai, H., ‘Effect of pitting corrosion on local strength of hold frames of bulk carriers (1st report)’, Marine Structures, 17(5), 403-432., 2004. 2. Nakai, T. and Yamamoto, N., ‘Pitting Corrosion - Probabilistic Modeling and Its Effect on the Ultimate Strength of Steel Plates Subjected to Uni-axial Compression’, Proceedings of the 10th International Conference on Applications of Statistics and Probability in Civil Engineering (ICASP10), 2007. 3. Nakai, T., Matsushita, H. and Yamamoto, N. ‘Visual Assessment of Corroded Condition of Plates with Pitting Corrosion Taking into Account Residual Strength - In the Case of Webs of Hold Frames of Bulk Carriers’, Proceedings of the 26th International Conference on Offshore Mechanics and Arctic Engineering (OMAE2007), 2007. 4. Yamamoto, N. and Ikegami, K. ‘A Study on the Degradation of Coating and Corrosion of Ship’s Hull Based on the Probabilistic Approach’, Transactions of the ASME, Journal of Offshore Mechanics and Arctic Engineering, 120, 121-128., 1998 7. AUTHORS’ BIOGRAPHIES Tatsuro Nakai holds the current position of researcher at Research Institute of Nippon Kaiji Kyokai (ClassNK). He is a member of material and equipment section of the institute. Hisao Matsushita holds the current position of senior researcher at Research Institute of Nippon Kaiji Kyokai (ClassNK). He is a member of material and equipment section of the institute Norio Yamamoto holds the current position of principal researcher at Research Institute of Nippon Kaiji Kyokai (ClassNK). He is the head of material and equipment section of the institute.



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PITTING CORROSION ON EPOXY-COATED SURFACE OF SHIP …