07004 Effects of Surface Preparation Methods and Protective Coating Types on the Performance of Erection Joint Weld Seam (51300-07004-SG)

Effects of Surface Preparation Methods and Protective Coating Types on the Performance of

Erection Joint Weld Seams in Water Ballast Tanks

Chung Seo Park, Sung Mo Son, Chil Seok Shin, Mong Kyu Chung and Kwang Ki Baek Hyundai Industrial Research Institute

Hyundai Heavy Industries Co. Ltd., 1 Cheonha-Dong, Ulsan, Korea 682-792 E-mail : [email protected]

ABSTRACT

Ship's water ballast tanks are exposed to the extremely corrosive environments due to

immersion or non-immersion by cyclic loading/unloading of ballast water. For cargo oil tankers, the temperature of the ballast tanks walls adjacent to the cargo oil tanks can reach up to 60 and drop to 0 in the cold sea causing higher hostility of corrosive environment. In ageing ships, coating failures, such as cracking, peeling off, are often observed in the areas of erection jointed weld seams and other stress concentrated areas.

To improve coating performance of these vulnerable areas, the current surface preparation methods and coating materials for the erection jointed seams of water ballast tanks are evaluated. Three types of surface preparation tool were tested and evaluated in terms of surface profile and subsequent coating performances, such as long term corrosion resistance, crack resistance, rust creepage, cathodic disbondment test, etc. From these results, optimum surface preparation methods and proper coating materials for erection jointed seams of water ballast tanks are proposed.

Keywords: Protective Coatings, Erection jointed seams, Power tool cleaning, Crack, Delamination

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INTRODUCTION To achieve good coating adhesion, steel substrate must have proper roughness in order to

provide an increased effective surface area for mechanical bonding. This roughness, also known as anchor pattern or surface profile, forms micro pattern of peaks and valleys at the surface, which can be obtained via power-tooling methods when an abrasive blast cleaning is not practical due to limited accessibility [1]. The surface preparation of a ships erection weld joints is a typical example, where the complexity and heavy weight of the process equipment for abrasive blasting cause poor accessibility to complicated and huge marine structures. Therefore, as an alternative, power tooling has been increasingly used for the surface treatment of some areas of marine vessels and offshore structures such as welded joints after erection. It has been known, however, that the conventional power tooling method provides less favorable final coating quality than the abrasive blasting method, probably due to its inferior surface profile. However, C. S. Park et al.[2] showed that proper selection of coating material itself, such as surface tolerant type, will also provide better long-term coating performance upon employing power tooling surface treatment. These results suggest that a certain type of power tooling method could be a reasonable alternative to abrasive blasting in the surface treatment of marine structures in their erection stage such as erection welded joints.

Water ballast tanks(WBT) in ships are exposed to extremely corrosive environments due to cyclic loading/unloading of ballast water. Especially, for the cargo oil tankers, the temperature of the ballast tanks adjacent to the cargo oil tanks can reach up to 60 and drop to 0 in the cold sea. Therefore, the inner water ballast tank surface area is very hostile to a coating's performance. Premature failures of protective coating systems, such as cracking, peel off in the areas of erection jointed weld seams and stress concentrated areas, i.e. weld toes, fillet welds, transition between structural details, etc., are often found in ballast tanks of ships in service and in consequence will lead to rapid corrosion of unprotected steel as shown in Figure 1. Following coating breakdown, it is extremely difficult to repair or reinstate the coating to the new building standard, stressing that coatings should be correctly applied at the new building stage.

This study was carried out to evaluate the current surface preparation methods and select proper coating materials for the erection jointed seams of WBT. Three types of surface preparation tool were tested and evaluated in terms of surface profile, subsequent coatings qualities including long term corrosion resistance, and other pros and cons. Selected coating materials were also evaluated in terms of crack resistance, rust creepage, cathodic disbondment test, etc.. From these results, optimum surface preparation methods and proper coating materials for erection jointed seams of WBT. are proposed.

2

Figure 1. Coating failure of erection jointed seam

IMOs CURRENT REGULATIONS AND SURFACE PREPARATION

- IMOs current regulations about surface treatment after erection[3] The requirements of IMO (International Maritime Organization) for protective coating systems

applied at ship construction for all dedicated seawater ballast tanks of all types of ships and double-side skin spaces arranged in bulk carriers of 150 m in length and upwards are they shall be coated during construction in accordance with the Performance Standard for Protective Coatings (PSPC). The PSPC mandates a target useful coating life of 15 years, which is considered to be the time period, from initial application, over which the coating system is intended to remain in GOOD condition. Among basic coating requirements, surface treatment after erection in secondary surface preparation is set out as follows:

- Butts St 3 or better or Sa 2 where practicable. - Small damages up to 2% of total area: St 3. - Contiguous damages over 25 m2 or over 2% of the total area of the tank, Sa 2 should be applied. - Coating in overlap to be feathered. - Reference standard : ISO 8501-1

Therefore, the shipyard shall apply the protective coating in accordance with the verified Technical Data Sheet and its own verified application procedures. - Power Tool Cleaning

Various types of mechanical equipment are used to clean the surface and thus provide proper coating adhesion. Table 1 lists the surface preparation specifications by the SSPC, NACE, British, and ISO in a descending order of each methods effectiveness. Power tool cleaning is useful and sometimes necessary for surface preparation where abrasive blasting is limited due to accessibility. For spot

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maintenance works, it is effective in preparing small areas for painting, feathering edges into sound paint, and avoiding damage to the adjacent sound paint. Power tool cleaning (St 3) requires removal of all loosely adherent rust, paint, mil scale, etc. using pneumatic and/or electrically operated tools. Power tool cleaning to bare metal state (SSPC SP 11) utilizes both newer fibrous disks and wheels to achieve a much cleaner surface than that prepared with St 3 tools. It also requires a surface profile of at least 25m. The cleaned surface will result in a distinctly cleaner and better-profiled surface than those prepared by either St 2 or St 3 Power Tool Cleaning [4].

Table 1. Surface preparation standards

NACE SSPC Swedish British ISO 8501-1 #1 White metal SP 5 White metal Sa 3 First Quality Sa 3 #2 Near white SP 10 Near white Sa 2 Second Quality Sa 2

#3 Commercial SP 6 Commercial Sa 2 Third Quality Sa 2 - SP 8 Acid pickling - - -

- SP 11 Power tool to bare metal

- - -

#4 Brush blast SP 7 Brush Blast Sa 1 - Sa 1 - SP 3 Power tool St 3 - St 3 - SP 2 Hand tool St 2 - St 2 - SP 1 Solvent wipe - - -

EXPERIMENTAL METHODS

In this study, four coating materials and three types of surface preparation methods, which are

widely used in new ship building yards, are selected for evaluation. Brief information and designation for each coating system and surface preparation method are summarized in Table 2 and Table 3, respectively. Test specimens such as flat welded specimens, T-bar and elongation specimens were prepared as shown in Figure 2. Figure 2(a) shows specimens for cathodic disbondment test and water immersion, as prepared by employing three different surface preparation methods and Figure 2(b) shows specimens for crack resistance test, as prepared by cleaning to Sa 2, followed with the selected coating materials applied by airless spray and being cured for 4 weeks at 25C. The dry film thickness(D.F.T.) of specimens were (a) about 250m (125m 2 coats) and (b) ranged from 1,200m to 1,500m. After complete curing of test panels, various tests such as cathodic disbondment, hot sea water immersion and crack resistance(thermal cyclic) test were carried out. Figure 2(c) shows dumbbell shape specimens used for elongation measurement with a commercial elongation tester (Model: H25KS, HOUNSFIELD). Thermal expansion of coating was also measured by a Thermo-Mechanical Analyzer (TMA, TA

4

Instruments) at the temperature ramping rate of 10oC/min.

Table 2. Coating materials and previous coating failures

Coating material Curing agent* Previous history of crack & delamination failure

Remarks

A Polyamide adduct - B Phenalkamine - C Amine Adduct - D Amine add.+Amide 3 ships Erection jointed seam

* provided by coating manufacturer.

Table 3. Types of power tool methods

Designation H F* W

Abrasive material Bonded abrasive

(Aluminum Oxide)Coated abrasive

(Aluminum Oxide)Steel wire

* Type F is the most widely used type for erection jointed seams.

20cm

15cm

10cm

10mm

20cm

15cm

10cm

10mm20cm

15cm

25~30mm

8~12mm15mm

30mm 25~30mm

8~12mm15mm

30mm

10mm

20cm

15cm

25~30mm

8~12mm15mm

30mm 25~30mm

8~12mm15mm

30mm

10mm

(a) (b)

25mm

20mm10mm

40mm25mm

20mm10mm

40mm

(c)

Figure 2. Test specimens

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RESULTS AND DISCUSSION

Cathodic Disbondment Test Cathodic Disbondment(CD) is defined as adhesion failure between a coating and metallic substrate

due to cathodic protection conditions[5]. Therefore, the CD test is to be used to measure the undercutting resistance of a coating system. Experiences in the oil and gas pipeline industry have clearly shown that coating with better CD resistance have better corrosion resistance and longevity. The coating systems with good adhesion to the steel substrate tend to have a similar resistance to CD. If a coating is able to adhere to the steel substrate, it will therefore tend to resist the undercutting damage of corrosion, thereby offering a longer service life [6].

In this study, the basic experimental set-up specified in the ASTM G8, was used to evaluate the CD performance of various surface preparation and coating samples. A 6 mm diameter holiday was drilled through the coatings to the metal surface in the center of welded seam of each specimen and specimens were immersed in natural sea water tank in presence of an open circuit potential of -1.55V to -1.80V vs. using Magnesium electrodes as shown in Figure 3. The sea water temperature was maintained at 25oC, and CD tests were carried out for 30days. At the end of the test, the extent of loosened or disbonded coating at the hole in immersed area was peeled off by knife to measure the extent of disbondment. The disbonded average diameter was calculated from this equation as below:

Radial disbondment = (average disbondment diameter - holiday diameter) / 2 The performances of coatings applied on the steel surface treated by H, F and W power tooling methods are shown in Figure 4. In these test results, the CD resistance is shown as follows: coating materials are better in the order of A > C > B >> D and surface preparations are better in the order of H > F > W. In particular, the performance of each coating materials is remarkably different for each type, whereas those of the surface preparation methods is less different for each other. Coating A showed far better performance than others, on the other hand, coating D was found to have the most inferior CD resistance among them. Therefore, coating specifier have to take coating D into strict consideration when selecting the coatings. It was also found that quality of coating material itself is more critical to the long term performance than types of surface preparation method.

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: Cathodicdelamination

M=M2+ + 2e-

2e- + 2H2O H2 + 2OH-

(Mg, Al, Zn)

: Cathodicdelamination

M=M2+ + 2e-

2e- + 2H2O H2 + 2OH-

(Mg, Al, Zn)

Figure 3. Cathodic disbondment test theory and actual tests

W

F

H

DCBA

W

F

H

DCBA

(a) CD specimens after being peeled off by knife

18

27

15

35

0

10

20

30

40

50

A B C DCoating Material

Rad

ial d

isbo

ndm

ent(

mm

)

27.224.5

19.7

0.0

10.0

20.0

30.0

40.0

50.0

H F WSurface Preparation

Rad

ial d

isbo

ndm

ent(

mm

)

(b) Coating material (c) Surface preparation

Figure 4. Results of CD test

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Sea Water Immersion Test

Hot sea water immersion test with scribe is an indicator of adhesion loss. Failure in water immersion may be caused by a number of factors, including deficiency in the coating itself, contamination of the substrate, or inadequate surface preparation. The test is particularly relevant to service performance because adhesion is considered a fundamental property for corrosion protection [7].

In this study, hot sea water immersion test was carried out in accordance with modified ISO 2812 and NACE TG 263 & 264. A 6mm diameter holiday was drilled through the coating to the metal surface in the center of weld seam of each specimen and specimens were immersed in the natural sea water chamber at 40 oC for 12 weeks, followed by evaluation in terms of mean rust creepage(from 6mm hole) and blistering.

Figure 5 shows the results of relative rust creepage resistance(corrosion resistance) for corrosion test. In these test results, the rust creepage resistance is shown as follows: coating materials, A > C > B > D in order and surface preparations, H > F > W in order. This result is similar to that of CD test. In reality, the sea water immersion test with drilled holiday into steel will assay the adhesive strength of the coating and its ability to resist film undercutting.

(a) Specimens after being peeled off by knife

12.612.7

7.1

14.5

0

5

10

15

20

A B C DCoating Material

Dis

bon

d R

adiu

s(m

m)

13.011.6

10.5

0

5

10

15

20

H F WSurface Preparation

Dis

bond

Rad

ius(

mm

)

(b) Coating material (c) Surface preparation

Figure 5. Results of sea water immersion test

W

F

H

DCBA

W

F

H

DCBA

8

Crack Resistance Test by Thermal Cycling Figure 2(b) shows T-bar specimens used to test for crack resistance. The coatings were prepared on

panels cleaned to Sa 2. The DFT of T-bar specimens ranged from 1,200m to 1,500m. After full curing of test panels, crack resistance test (thermal cycles : 6hrs at -20oC, 6hrs up to 60 oC, 6hrs at 60oC and 6hrs down to -20 oC) were carried out in a humidity controlled chamber. As shown in Figure 6 and 7, the cracks were found at the welding joint line coating material D, B, C and A in order. Coating D was very susceptible to crack, whereas coating A showed a good performance.

The thermal cyclic test results were well matched with the previous coating failure in actual ships. For example, in actual ships, coating D experienced a serious coating failure at erection joint seams as shown in Figure 1 and coating B also revealed a partial coating failure, but coating A and C showed better performance so far. Consequently, this result can be used as a protocol to determine coatings crack resistance, where the minimum acceptance cycle to resist crack generation is 30 cycles at 1,500m and 40cycles at 1,200m.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 100

1500

1200

1500

1200

1500

1200

1500

1200

Thermal Cycle(6hrs at -20'C, 6hrs up to 60'C, 6hrs at 60'C and 6hrs down to -20'C : 1cycle/day)

D

A

B

D.F.T(um)

Coat.

C

24 27

24 27

24 56

68

24 56

35 56

5643

Partial Crack All Crack

24 56

83

Figure 6. Result of crack resistance test

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Figure 7. Crack generated specimen after 85 cycles (D.F.T 1200m)

Coating analysis : Elongation and Thermal Expansion As shown in results of CD test, water immersion test and cracking test, coating D is more prone to

crack and/or delamination than others and coating A has very good performance. In this study, in order to evaluate the effect of a coatings physical properties on the coating performance, the elongation and thermal expansion of coating themselves were measured.

- Elongation Coating tensile tester is used to assess the resistance of a dry film of paint or related product to

cracking and/or detachment from a flexible substrate. The ability of a coating material to resist breaking under tensile stress is one of the most important and widely measured properties of materials used in structural applications such as ship building.

In this study, the elongation of samples was measured by using a testing machine (H25KS, HOUNSFIELD) in accordance with ASTM D882 as shown Figure 8(a). The initial distance of jaws was 25mm and the testing speed was 0.2mm/min, temperature was 25oC, and R.H. 50%. The average value of three readings for each test is shown in Table 4. From this result, each elongation of coating materials was 0.77%, 0.41%, 0.33% and 0.27% for A, C, B and D, respectively. Therefore, coating A was more flexible than other coating materials and had the more resistance to cracking and/or detachment, but coating D was more susceptible to crack and/or detachment than others.

- Thermal expansion coefficient In order to assess the effect of the coating material on coating performance such as CD resistance,

rust creepage and crack resistance, thermal expansion of each coating was measured by increasing the temperature at 10oC/min from 0oC to 60oC using Thermo-Mechanical Analyzer (TMA) as shown in Figure 8(b). It can be seen from Table 4 and Figure 9 that coating materials have different curves in TMA plot. In general, for the cargo oil tankers, the temperature of coating on the ballast tanks adjacent

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to the cargo oil tanks can reach up to 60oC and drop to 0oC in the cold sea. In this study, therefore, thermal expansion was measured from 0oC to 60oC. From Figure 9 thermal expansion at 60oC were 37.5, 32.0, 20.4 and 19.8 for A, C, B and D, respectively. Coating material A showed the higher thermal expansion. On the contrary, Coating D showed the lowest thermal expansion than others. In comparison with previous test result, it is found that an increase in elongation and thermal expansion improves the crack resistance and other performances of the coating.

Results in this study suggest that more elongation and thermal expansion have an effect on the coating performance. In other words, coatings that have more elongation and thermal expansion are more flexibility, furthermore have better coating performances. Therefore, coatings flexibility and thermal expansion are a good indicator of a coatings ability to withstand the disbonding, cracking, or other mechanical damage. In general, however, very higher thermal expansion may lead to higher residual stress and cause more cracking for coating of the same fracture strain. Therefore, the additional investigation is necessary to find out the real relation between coatings thermal expansion and coatings elongation & crack failure.

(a) Elongation tester (b) Thermo-Mechanical Analyzer (H25KS, HOUNSFIELD) (TMA, TA Instruments, U.S.A)

Figure 8. Elongation tester and Thermo-Mechanical Analyzer

Table 4. Results of elongation and thermal expansion of coating film

Coating A B C D Elongation (%) 0.77 0.33 0.41 0.27

Thermal Expansion* (m) 37.5 20.4 32.0 19.8 * Temperature varied from 0oC to 60oC

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Thermal Expansion

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60

Temperature('C)

Dim

ensi

on C

hang

e(um

)D

Steel

B

C

A 60'CSteel : 2.7 A : 37.5 B : 20.4 C : 32.0 D : 19.8

Figure 9. Thermal expansion of coating film

The Relationship between Surface Preparation and Coating Performance The surface profiles of specimens prepared by power tooling types H, F and W were measured by

surface roughness tester (Figure 10 and 11). As shown in Figure 4(c) and 5(c), the coating performance of the surface prepared by power tool

was better in the order of H, F and W and the differences were attributed to the resultant surface profile. However, as the previous results of CD test and sea water immersion test, the performance of coating materials is a great difference in comparison with those of the surface preparation methods. It was also found that quality of coating material itself is more critical to the long term performance of the coating film than types of surface preparation method.

Figure 10. Measurements of surface roughness (DIAVITE DH - 5)

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50m50m50m20m

(a) H(avg. 25.6m) (b) F(avg. 17.1 m) (c) W(avg. 13.6 m)

Figure 11. Surface profile after each surface preparation

SUMMARY OF TEST RESULTS

The performances of the coatings applied on the surface prepared by 3 types of power tooling

surface treatment methods were evaluated in terms of CD test, sea water immersion, etc. The test results are summarized in Table 5 and Table 6.

Table 5. Summary of test results - Surface profile & Profile

Power tool methods H F W Remarks Surface profile(m) 25.6 17.1 13.6

C.D.T 19.7 24.5 27.2 Creepage (mm) Sea Water Immersion 10.5 11.6 13.0

Table 6. Summary of test results - Coating materials

Coating types A B C D Creepage by C.D.T(mm) 15 27 18 35

Creepage by Water Immersion(mm) 7.1 12.7 12.6 14.5 Crack Resistance*

Elongation (%) 0.77 0.33 0.41 0.27

Thermal Expansion** (m, at 60oC) 37.5 20.4 32.0 19.8 Final Evaluation*** Best Bad Better Very Bad

* Rank of resistance : > > > ** Temperature variation from 0 oC to 60oC

*** Rank of quality : Best > Better > Good > Bad > Very Bad

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CONCLUSIONS

Evaluations were carried out to study the effect of the current surface preparation methods and to select proper coating materials for the erection joint seams of WBT. Each coating material performed quite differently, whereas the difference of the surface preparation

methods revealed less effects on the coating performance. Therefore, it was found that coating material itself is more critical to the performance of the coating film than the types of power tooled surface preparation. It was found that evaluation test such as the CD test, sea water immersion test, thermal cyclic test,

thermal expansion and elongation test results are consistent with each other. It was shown that an increase in a coating elongation and thermal expansion properties improves its

crack resistance and other performances. Therefore, this result suggests that higher elongation and thermal expansion is more beneficent to the coating performance and can be used as a screening test for material selection and/or as a quality control test to check coating in the shipbuilding. The coatings thermal cyclic test results were well matched with the previous coating failure in actual

ships. Consequently, this result can be used as a protocol to determine coatings crack resistance, where the minimum acceptance cycle to resist crack generation is 30 cycles at 1,500m and 40 cycles at 1,200m.

REFEFERENCES 1. US Army, Painting: New Construction and Maintenance, US Army Corps of Engineers document

EM1110-2-3400, 1995. 2. C. S. Park, et al., Symposium Paper No.05017 presented at NACE CORROSION/05, Houston,

March, 2005. 3. International Maritime Organization, Maritime Safety Committee 81st session, Agenda item 7. 4. C. G. Munger, Corrosion Protection by Protective Coatings, NACE International, pp.199-211, 1999. 5. ibid, pp.325-345, 1999. 6. S. W. Guan, The selection, application and inspection of 100% solids polyurethane coatings for

corrosion protection, SSPC2000, Nov. 12-16, 2002. 7. C. K. Clear et al., Performance of Epoxy-Coated Reinforcing Steel in Highway Bridges, National

Research Council, Transportation Research Board, NCHRP Report 370, Washington, National Academy Press, 1995.

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Text1: logo: number: 07004paper: Paper No.copyright: 2007 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACE International, Copyright Division, 1440 South creek Drive, Houston, Texas 777084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.cp: Copyright