2  MATERIALS PERFORMANCE  August 2010 NACE International, Vol. 49, No. 8

Accelerated Weathering

of Pipeline Coatings to Determine Dielectric Strength

BuddhadeB duari, Rustech Products Pte., Ltd., Kolkatta, India

Laboratory tests were made to assess the rate of

degradation of the dielectric strength of different

pipeline coating materials including tapes, epoxies,

polyurethane, and polyethylene, after

accelerated weathering.

An accelerated weathering test to determine the dielectric strength of pipeline coatings is important since the corrosion control sys-

tems of most pipelines consist of a com-bination of coating and cathodic protec-tion (CP). It is desirable that the dielectric properties of the coating deteriorate at the minimum rate possible. The deterio-ration of a coating over time leads to the need for additional CP current, which could be a technical and economic bur-den on the pipeline operator.

Insulating materials are used to isolate components of an electrical system from each other and from the ground. It is generally desirable to have the insulation resistance as high as possible, consistent with acceptable mechanical, chemical, and heat-resistant properties. Coated pipelines are cathodically protected, and it is desirable to check the dielectric strength of the coating at regular intervals.

To have good dielectric properties, coatings must possess the following char-acteristics:

• Prevent electrolytic discharge of current from the steel surface of the pipe

• Maintain substantially constant electrical resistance with time. The effective electrical resistance of a coating per average square foot depends on the following:— Resistivity of the coating mate-

rial— Coating thickness— Resistance to moisture absorp-

tion— Resistance to water vapor per-

meability— Frequency and size of holidays— Resistivity of the electrolyte— Bond or adhesion of coating

• Resist holidays caused by stress and soil contaminants

This analysis was made to study the deterioration of the dielectric strength of

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NACE International, Vol. 49, No. 8  August 2010 MATERIALS PERFORMANCE  3

C O A T I N G S & L I N I N G S

different coating materials, viz. coal tar enamel, hot-applied coal tar tape, cold-applied polyolefin tape, high-build epoxy, coal tar epoxy, polyurethane (PUR), and three-layer polyethylene (3LPE) tape. Results were based on 500 h accelerated ultraviolet (UV) weathering using a UVB-313† lamp, maintaining a cycle of 4 h UV and 4 h condensation, and also after 500 h salt spray per ASTM B117.1

Sample PreparationTwo-component systems such as

coal tar epoxy, high-build epoxy, and PUR were prepared by mixing the two

†Trade name.

Dielectric strength machine.

UV-accelerated tester.

Conditions of samples after 500 h of accelerated UV weathering exposure.

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4  MATERIALS PERFORMANCE  August 2010 NACE International, Vol. 49, No. 8

C O A T I N G S & L I N I N G S Accelerated Weathering of Pipeline Coatings to Determine Dielectric Strength

TAbLE 1

Dielectric strength (in KV) of different coating materials before and after accelerated testing

Sl. No.Type of Coating

MaterialOriginal Sample

After 500 h Accelerated Weathering

in UVAfter 500 h Salt Spray

1 Coal tar enamel (thickness 2.15 mm)

10.0 8 9.0

2 Hot-applied coal tar tape (thickness 2.15 mm)

9.0 7 8.0

3 Cold-applied polyolefin tape (thickness

1.01 mm)

25.0 23 20.0

4 PUR coating (thickness 2.10 mm)

17.5 16 12.5

5 Coal tar epoxy (thickness 1.5 mm)

14.0 10 12.0

6 High-build epoxy (thickness 1.5 mm)

12.5 10 11.0

7 3LPE coating (thickness 4.50 mm)

25.0 23 24.0

Dielectric Strength in KV

components in the defined ratio. The mixed components were then drawn over silicon paper to the desired thick-ness. Coal tar enamel samples were prepared by pouring and drawing the hot enamel over a silicon paper. Coal tar tape, cold-applied PE tape, and 3LPE samples were taken as is for testing.

Testing ProcedureOne sample of each type of coating

was taken for dielectric strength (dielec-tric breakdown voltage) testing at com-mercial power frequencies per ASTM D149.2 The dielectric strength of most solid coating material is influenced by temperature and moisture content. Be-fore testing, materials were brought to equilibrium in an atmosphere with con-trolled temperature and relative humid-ity. During testing, the voltage was in-creased from zero or from a level well below the breakdown voltage, until dielectric failure of the test specimen occurred. The test voltage was applied using simple test electrodes on opposite faces of the specimens. The dielectric

strength of each sample was measured and recorded using the machine shown in Figure 1.

Ultraviolet TestingFour samples of each type of coating

were subjected to accelerated weathering for 500 h in an accelerated weathero-meter with a UVB-313† lamp (Figure 2). The samples were subjected to attack by degrading elements of the weather, par-ticularly UV light, oxygen, and water.

After 500 h, samples were taken out and analyzed for surface defects such as chalking, cracking, blistering, wrinkling, flaking, and any other defects (Figure 3). The dielectric strength (breakdown volt-age) of each coating material was mea-sured and recorded after 500 h of acceler-ated weathering.

Salt Spray TestingSimilarly, two samples of each coating

material were taken and subjected to salt spray testing (fog chamber) for 500 h to provide a controlled corrosive environ-ment per ASTM B117.

After 500 h of salt spray, samples were taken out and dielectric breakdown volt-ages were measured and recorded.

ResultsTable 1 presents the results of the di-

electric strength tests subsequent to the UV and salt spray tests. Figure 4 shows the same data graphically.

The dielectric strength of all the coat-ings decreased from the original value. No significant changes were noticed in the dielectric property of coal tar enamel, coal tar tape, coal tar epoxy, and 3LPE coating after salt spray testing. No ap-preciable changes were seen in the dielec-tric property of cold-applied polyolefin tape and PU coating after UV exposure.

ConclusionsThe tests concluded that coal tar

enamel, coal tar tape, coal tar epoxy, and 3LPE coatings maintain good dielectric strength underground. PU and polyolefin tapes show better UV resistance than the preceding materials.

Dielectric failure involves several fac-tors such as surface discharge, thermal and chemical deterioration, and elec-tronic breakdown. Resistance to elec-tronic breakdown is the intrinsic dielec-tric strength of the material. The other factors involve influences other than the electric field.

Further studies on the suitability of proper coating material with respect to CP are underway in the author’s laboratory.

References

1 ASTM B117, “Standard Practice for Operating Salt Spray (Fog) Apparatus” (West Conshohocken, PA: ASTM International, 2009).

2 ASTM D149, “Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies” (West Con-shohocken, PA: ASTM, 2009).

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BuddhAdeB duArI is the managing director of rustech Products Pte., Ltd., 162 B 337 Lake Gardens, Kolkatta, West Bengal 700045, India. he is a NACe-certified Corrosion Specialist and Protective Coating Specialist with more then 25 years of industrial experience. he has worked on coating projects in chemical and petrochemical facilities, refineries, power plants, etc. he is carrying out research work at the department of Metallurgical & Material engineering, Jadavpur university, Kolkata, India and assisting one M. Tech student in his dissertation work in the field of pipeline coatings.

Results of dielectric strength tests as reported in Table 1.

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