Coating Types and Tests

Paper No.

613

LABORATORY COATING EVALUATION AND ITS RELATIONSHIP WITH THE SELECTION TO PROTECT PIPELINE AGAINST EXTERNAL

CORROSION

Valmore Rodriguez, Elias Perozo, Lino Castafieda, Elena Alvarez Material Technology Department

Intevep, S.A. P.O.Box. 76343 Caracas 1070 A.

Venezuela

There are several pipeline external coatings classified into different types, among them are extruded and tape polyethylene, fusion bonded epoxy (FBE), coal tar enamel, coal tar epoxy, polyurethane, wax, cementitious epoxy and multilayer systems. These coatings are assumed to protect the pipeline for a certain period of time. In order to select the most appropriate coating, several accelerated laboratory tests need to be carried out to obtain their properties and correlate them with the performance in the field.

Cathodic disbonding tests at different temperatures and voltages, adhesion at different temperatures, impact resistance, loss of adhesion against time under water immersion conditions, differential scanning calorimetry analysis and water uptake are the most important tests to carry out in this study. The results indicate that coatings perform differently in each test. For this reason the selection must be done according to the soil characteristics, considering the results obtained in laboratory.

This paper will provide a guideline to select a coating for a specific environment and the results that should be expected in the field. For example, the multilayer system presented the best performance like low cathodic disbonding, excellent adhesion, high impact resistance, excellent behavior under hot water immersion and high temperature resistance. This system can be used in all types of soil with high life time expectancy in protecting the pipeline. On the other hand, the polyurethane coating presented high impact resistance but very low adhesion and high cathodic disbonding that limits its use in corrosive soil.

Keywords: Coatings, Pipeline, Evaluation,

Copyright 01998 by NACE International. Requests for permlssion to publish thismanuscript in any form, in part or in whole must be made in writmg to NACE International, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the wews 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.

INTRODUCTION

Many types of anti-corrosion coatings are available in the market to be used on pipelines. The coatings act as an isolating material between the corrosive environment and the pipeline. It is important to select the best coating system that will have the longest useful life for the application, taking into account the economical factor. There are many variables that need to be considered when a coating decision has to be made. This implies that the adhesion, cohesion, chemical stability, water absorption, mechanical properties, soil stress resistance, application, microbiological attack resistance and cathodic disbonding should be necessary to maintain the integrity of the pipe.

Some coatings lose their integrity and ultimately fail under certain conditions to provide the service for which they were intended. A failed coating may be more harmful and costly than bare pipeline. It can be argued that a bare pipeline is more expensive to maintain than coated pipeline because of the increase expenses for a larger CP system and larger current demand [l]. However, if a coating fails, the cost can be much higher.

Each coating has a set of properties that describes its behavior under certain conditions. These properties are usually part of the manufacturer material specifications or laboratory coating evaluation.

This paper intends to provide a laboratory evaluation of the most frequently used organic coatings applied on pipelines and how this data should be used to select the most appropriate coating for a specific environment.

COATING SYSTEMS

Fusion bonded epoxv (FBE): This coating has been commercially available since the early 1960s. Since that time improvements have been made. Several brands are available in the market for one layer of FBE coating. Lately, coatings with two layers of FBE have been introduced in the market. FBE is considered a thin hard and relatively brittle coating. Most companies that use FBE coatings have in-house specifications to control its applications [2].

The FBE samples were obtained either from the manufacturer or application plants.

Coal tar epoxv: Coal tar epoxy is a relatively hard coating that can be applied in one or two layers. This coating has been used extensively to recoat existing linepipe and to repair coatings. In order to obtain good performance of this coating the surface needs excellent surface preparation. Several products were evaluated and the samples were supplied by coal tar epoxy manufacturers.

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Cathodic disbonding test.

The cathodic disbonding test is one of the most important tests for coating evaluation and selection. For this test the ASTM G-8 and ASTM G-42 standards were used. Two different temperatures were used to evaluate the coatings (25 and 95C). All the tests were run at -1,5 volts (vs SCE), 3% NaCl aqueous solution for 30 days.

Peeling Resistance test

This test was done only for the polyolefin tape, extruded polyethylene, wax and multilayer using tape as external layer due to the intrinsic characteristics of the coatings. Effect of the temperature on the adhesion was investigated in this study. All tests were performed using the DIN 30 670 standard.

Coating adhesion

Adhesion is another important property of coating. The adhesion test was done according to ASTM D-3359, before and after hot water immersion. For one layer FBE coating, adhesion to steel substrate was investigated using pull-off strength (ASTM D-4541).

Impact Resistance

Mechanical damage is reported to be one of the major causes of coating breakdown and resistance to impact will therefore be an important coating parameter to be analyzed. The impact resistance was obtained according to ASTM G-14 standard.

Water absorption

The amount of water that a coating system can absorbe has a negative effect on the coating life time. A high water absorption will cause low performance. The test was done at 25,60 and 95C for 48 hours.

Hot water immersion

This test was done at 50 and 95C for 48 hours depending at the condition where the coating will be exposed. The samples were analyzed before and after the test in order to find blistering, color change, softening, hardness, etc.

The adhesion after hot water immersion (90C) was determined as a function of time. The adhesion test after the immersion period was carried out under ASTM D-4541 standard specification.

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Soil stress

The soil stress effect on tape coating was study in the field. For this test, coated pipe was exposed to high soil stress clay for 3 years. After this period, the coatings were visual inspected and the results were analyzed.

RESULTS AND DISCUSSION

FBE coatings

Table 1 shows the properties of FBE coatings evaluated. It can be observed in this table that the FBE coatings have excellent adhesion, relatively low impact resistance, moderate water absorption and low cathodic disbonding. For one layer FBE, the coatings are susceptible to loss of adhesion and the cathodic disbondment increases significantly when temperature increases. On the other hand, for two layers FBE coatings showed good performance in the high temperature test.

In order to investigate the adhesion under hot water immersion for one layer FBE coating, an experiment was carried at 90C and the adhesion was obtained at different times. Table 2 shows the adhesion values obtained for one of FBE coatings. It is observed in this Table there is a loss of adhesion with exposure time at high temperature (9OC) and this loss of adhesion appeared after 10 days of testing time. This indicates that most of the adhesion test time recommended in the literature for hot water immersion may not be appropriate to evaluate this property. It was shown in previous work that one layer FBE coating had low performance on a hot pipeline [3].

Water absorption is a very important parameter to study to assure the performance of the coating. Figure 1 shows the water absorption study of a two layered FBE coating at 25,60 and 95C as a function of time. As it is expected, the water absorption increases with temperature, nevertheless, the values obtained are considered in the acceptable range. It is important to mention that at a higher value of water absorption the performance of the FBE decreases. An intensive study of the performance of 2 FBE layers was published by Rodriguez et.al. [3,4]. In this study it was shown that the properties of the two layered FBE coating will perform well under a high temperature regime.

Tape coatings

The properties for tape coatings are shown in Table 3. This coating presents high impact resistance (the values shown in table 3 seem to be low but 2 inch diameter pipes were used), low water absorption, medium-high cathodic disbondment and a moderate value of adhesion. All tape products presented

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low properties at high temperature, the cathodic disbondment increases and the adhesion decreases significantly (see Figures 2). Results on cathodic disbonding test at 65 C in this study has shown that the disbonded diameter passes acceptance range (> 40 mm). From these results the tape coatings are not considered for high temperature (greater than 60C).

One of the properties that need to be investigated is the resistance to soil stress. In the laboratory, this test is not easy to do. For this reason, it was decided to carry out a test in the field under high soil stress clay. This was done to obtain the effect of the soil stress on the performance of the tape coating samples. In Figure 3 can be observed the low resistance of tape to soil stress, this behavior has been consistent to all type of tape coating that have been tested. From these results, the tape samples have been found to be susceptible to soil stress. Several authors have reported this phenomenon previously [5,6]. The selection of type of tape coating should be not used in those environments with soil stress.

Multilayer system

The multilayer coatings performed well under the laboratory test (see Table 4). The coatings have high adhesion to steel, high impact resistance, excellent performance under hot water immersion, low cathodic disbonding, low water absorption and kept the adhesion after being immersed in hot water. The cathodic disbondment values for high temperature test were in the acceptance range. This indicates that the coating will perform well in the field under high temperature operation.

Coal tar epoxy

The characteristic of the coal tar epoxy evaluated are the following (see Table 5): good adhesion, very low impact resistance, medium values of cathodic disbondment, low water absorption and fairly good behavior in hot water immersion. Due to the properties described above this coating is not recommended for new construction, it is recommended for repair or recoat existing pipe.

Urethane

The Urethane coating evaluated presented high impact resistance, low adhesion, bad performance under hot water immersion, high cathodic disbondment and low water absorption (see Table 5). The product evaluated does not satisfy or guarantee the protection of the pipeline against external corrosion.

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Extruded polyethylene

The extruded polyethylene coating evaluated has the characteristic of good adhesion, good impact resistance, low water absorption, low performance under hot water immersion, acceptable cathodic disbondment value (see table 5). This coating has performed quite well in the field for more than 20 years. Nevertheless, problems of microbiological attack have been found under the conditions of Maracaibo Lake [7].

Cementitious coating

This coating is a cement based with polymer additive. The properties of this coating is presented in Table 6. The most important aspects of this coating are: high impact resistance, good adhesion, moderate value of cathodic disbondment, loss of adhesion after hot water immersion and relatively high water absorption. It cracked during the cathodic disbondment test as shown in Figure 4. This coating needs some improvement to be acceptable as a reliable one to protect pipeline against external corrosion.

Concrete epoxy on FBE

The combination of concrete epoxy on a layer of FBE gives excellent results as is shown in Table 6. The most important properties are: high impact resistance, excellent adhesion, excellent behavior under hot water immersion, very low cathodic disbonding value and relatively high water absorption. Furthermore, this coating has the advantage of high abrasion resistance. This product was successfully applied on a directional drilling pipeline where a high resistance to abrasion was needed. The concrete epoxy was also evaluated as primary coating and results obtained were under specification. For this reason the concrete epoxy coating is not considered to be used alone due to low performance in the laboratory, but it is highly recommended on top of FBE in those projects that require high mechanical properties, especially abrasion resistance for the coating.

Wax

This is the most difficult coating to evaluate due to intrinsic properties of the wax. Nevertheless, effort was applied to characterize it since this coating is used to do maintenance and repair in some pipelines. This coating has very low adhesion to steel (almost none), very low water absorption, very low impact resistance and the cathodic disbonding value seemed to be acceptable (see Table 6). It is very important to mention that it is very difficult to determine the cathodic disbondment value since this coating does not have adhesion. This coating is used for maintenance and minor repair of pipeline.

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CONCLUSIONS

According to the results obtained in this study, the following conclusions are made:

Multilayer coatings have the best properties and it can be used in all environments, especially for those with high temperature. Nevertheless, its selection has to be made with economical studies, since it is considered a high initial cost coating.

The one layer FBE coating can be selected in new construction basically in all environments, except for those pipelines that transport high temperature. fluids (it should be selected below SOOC). Especial attention needs to be paid in those environments where the coating will be in continuous contact with water since an inadequate degree of curing will cause premature failure.

The two layers of FBE coatings have overall good properties and can be selected in the majority of the environments including high temperature pipelines.

Tape coatings can be used in those environments where soil stress is not present and the temperature of the pipeline has to be below 60-C. Special attention should de paid in those humid environments since this coating has shielding effect.

Extruded polyethylene coating can be selected in those new projects where the temperature is below 60C and there is low soil stress effect.

Coal tar epoxy coating can be used to repair or recoat existing pipelines, taking into consideration the surface preparation and the low impact resistance of this coating.

Urethane and cementitious coatings need more development to increase the performance to be considered as a reliable coating.

The use of concrete epoxy on FBE coating improves the performance of it. It is recommended for those projects where high impact resistance and high abrasion resistance are needed. There is still a limitation with the temperature. The selection should be limited to those projects under 70-C.

Wax coating should be used for maintenance and minor repair of pipelines.

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1. Werner D.P, Lukezich SJ., Hanckock J.R. and Yen B.C. Survey results on pipeline coatings selection and use, Material Performance, 11 (1992):~ 16.

2. Griffiths B. and Edwards H.S., The quality control of factory applied epoxy coating to linepipe Third International Conference on the internal and External ptotection of pipes, London-England, 1979.

3. Rodriguez V., Perozo E., Gil R., Paiva A., External coating evaluation for high temperature pipeline, NACE Corrosion95, Paper No. 370.

4. Rodriguez V., Perozo E., Alvarez E., Coating application and evaluation for heavy wall thickness, temperature and pressure pipeline, NACE Corrosion97, Paper No.577.

5. Rodriguez V., Paiva A., Castaneda L., External coating failure in Venezuelan pipeline, NACE Corrosion94, Paper No. 571.

6. Coulson K.E.W., Nunez S., Choate L.C., Banach J.L., A review of gas industry pipeline coating practice NACE Corrosion90, Paper No. 242.

7. Rodriguez V., Paiva A., Astudillo M., New approach to control microbiological attack to high density polyethylene external coating, NACE Corrosion 94, Paper no. 572.

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Table 1. Properties of the FBE coatings evaluated

A: One layer FBE coating company A AT: One layers FBE coating company A for high temperature AZL: Two layers FBE coating company A B: One layer FBE company B BZL: Two layers FBE company B

Table 2. Adhesion for one layer FBE coating at 90C at different exposure time.

Time (days) 0 1 3 7 10 21 28 Adhesion > 1000 > 1000 > 1000 > 1000 > 1000 720 432 fvsij

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Table 3. Properties for Polyolefin tape coatings evaluated

Coating 1 TA-1 1 TA-2 1 TA-3 [ TA-4 1 TB-1 ( TB-2 1 TC-1

immersion

TA-1,2: Tape coating from company A, normal temperature coating TA-3: Tape coating company A, high temperature coating. TB-1,2: Tape coating company B, normal temperature coating. TC-1: Tape coating company C, normal temperature coating

Table 4. Properties of the multilayer coatings evaluated

Coating MA

Thickness (mild 1 49.1

Peeling resistance 62.4 (N/cm) @ 25C

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Table 5. Properties of coal tar epoxy, urethane and extruded polyethylene.

Coating

Thickness (mils) Adhesion (rating: Impact resist. (lbf.inch) Hot water immersion Adhesion after hot water Cathodic disbonding (mm), diameter Water absorption (%) Peeling resistance

/ (Nkm)

CTA

22.3 good

15

Visual Changes

good

20

1.54

61 B and C. CTA, 8, C: coal tar epoxy company 1

UA: Urethane coating company D EPE: Extruded polyethylene company

CTB CTC UD EPE

19 16 37.1 41 good good 4 good

26 18 56 36

Visual Visual No visual No visual Changes Changes changes changes

good good bad fair

33 14 > 47 22

1.1 1.1 1,61 very low

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Table 6. Properties of cementitious, FBE-concrete epoxy and wax.

(Ldf.inch) Hot water No visual No visual immersion changes changes

Adhesion after 2,3* 1 hot water (N/mm2) Cathodic 22 6 disbonding (mm), diameter Water 2,75 2.3 63 absorption (%) 70C

Peeling resistance (N/cm) CA: Cementititous coating company A CEA: Concrete epoxy coating company A WA: Wax coating

I WA 90 ---

17

23

0.23

0.8

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Figure 1. Water absorption for two FBE layers coating at 25,60 and 90C.

5.0

4.0

3.0

2.0

1.0

0.0 ,...m...n...,... 0 20 40 60 80 100 120 1

Time (hours)

14

12

10

8

6

4

2

0

-t- 25C

--m- 94C

0 5 10 15

Time (days)

Figure 2. Peeling resistance of a tape coating at 25 and 94C for different exposure time.

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Figure 3. Effect of the soil stress on the performance of tape coating.

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Figure 4. Crack found in the cathodic disbonded test on Cementitious coating.

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