Saad Arfan, Great Man-Made River Authority Corrosion and Inspection Center Benghazi - Libya

Transcription

1 Paper No EVALUATION OF A COAL TAR EPOXY COATING PERFORMANCE FOR PRESTRESSED CONCRETE CYLINDER PIPE OF THE GREAT MAN-MADE RIVER PROJECT (GMRP) PIPE LINES IN LIBYA Saad Arfan, Great Man-Made River Authority Corrosion and Inspection Center Benghazi - Libya saad_qa@yahoo.com Salah Elkoum Great Man-Made River Authority Corrosion and Inspection Center Benghazi - Libya selkoum@yahoo.com ABSTRACT Coal-tar epoxy (CTE) coating was selected to protect the external surfaces of the pre-stressed concrete cylinder pipe (PCCP) in aggressive ground conditions of Libya. This coating was selected based on cost and application efficiency. CTE has good track record and has good dielectric properties. However, it ages fast, cracks easily and is brittle. This paper compares two CTE coatings, from different suppliers. It also presents and discusses the results of the testing programs carried out to evaluate the performance of CTE after fifteen (15) years of service. The test results showed acceptable performance of the coal tar epoxy coating. Keywords: coating systems, coal tar epoxy, coating qualification, coating service life, weathered and non-weathered samples. INTRODUTION The Great man made River (GMRP) project in Libya is the world's largest water supply project. The water is conveyed from the aquifers located in Libyan south Sahara to reservoirs and treatment plant in the north coastal area, intended for municipal, industrial and agricultural use. Major equipment were designed to last for 50 years with minimum maintenance in the demanding environments in Libya. Figure 1 shows different phases of the project. Copyright 2009 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 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. 1

2 Pipelines constructed for this purpose are principally 4.0 meters internal diameter Pre-stressed Concrete Cylinder Pipe (PCCP) as shown in Fig. 2. Typical PCCP is generally 7.5 m in length and manufactured accordance to AWWA standard C301 as shown in Fig. 3. Necessary features in the construction of the pipes result in the pre-stressing wires covered with 19 mm thick cement mortar layer and with no corrosion allowance. The pipes in the coastal route are exposed to highly corrosive salt marches and crossed with and gas pipelines, water distribution and waste disposal lines. In addition, the high voltage overhead power lines cross GMRA pipes in many locations. Eastern line Spread 6 FIGURE 1 - GMRP Phases 2

3 FIGURE 2- Pipe construction FIGURE 3- AWWA C301 Embedded Cylinder pipe and joint details The pre-stressed wires of the pipeline were protected by cement mortar coating with a minimum thickness of 19mm, to provide mechanical strength and to protect it from corrosion. In aggressive ground, this was considered sufficient for the 50-year design life. However, additional corrosion protection was required to both prevent degradation of the cement mortar by sulphates, and to eliminate chloride attack on the pre-stressing wires (1). 3

4 A logic diagram developed from geotechnical surveys of the pipeline route was used to determine if additional corrosion protection, i.e., application of coatings, was required. Geotechnical surveys included stray current interaction, groundwater, soil natural moisture content, soil resistivity, sulphate and chloride concentrations, and soil ph. CHARACTERISTICS OF BARRIER COATING The barrier coatings: Can be applied on both green and dry mortar in a single coating operation, Can withstand steam-curing operations Possess adequate elongation properties to be capable of bridging mortar crack widths up to 0.5 mm occurring after applications (2). Possess adhesive properties capable of withstanding maximum predicted tensile strain by the mortar under the most severe design conditions, without sustaining rupture or loss of adhesives to the mortar. Possess mechanical properties capable of withstanding handling, transportation, laying and backfilling operations during the construction of the pipeline. Possess very low permissibility to water, moisture and oxygen (2) as well as to chloride and sulphate ions. Be inert to acid soil attack. Possess high dielectric strength. provide protection to PCC pipes buried in aggressive ground in Libya, with a design life of 50 years (2). BASIS OF COAL TAR EPOXY COATING SELECTION In order to select the appropriate coating system for PCCP two coatings evaluated were: coal tar epoxy and polyurethane / polyurea protective coatings. The trial program covered site application, testing include PCCP transportation and installation and laboratory testing. The coating selected for further evaluation and application was coal tar epoxy. This coating was selected based on cost and application efficiency. CTE has good track record and has good dielectric properties. QUALIFICATION OF COAL TAR EPOXY COATING SYSTEMS Qualification tests were carried out on two coal tar epoxy coating systems (two different suppliers), designated O and X. The coal tar epoxy coating was applied onto the external mortar layer on prestressed concrete cylinder pipes. The following qualification tests were carried out: Dry film thickness Moisture vapor permeability* (according to EN ISO ). Oxygen diffusivity* (according to ASTM 6539 (Darcy method)). Chloride ion diffusivity* (according to ASTM C1202). Chloride ion permeability*. Electrical continuity (according to EN 1081, Procedure C). Hardness* (according to DIN 53505; Shore A+D). Adhesion to mortar substrate* (according to EN 1542) Impact resistance* (according to EN 6272) Alkali resistance, storage in 10% sodium hydroxide (according to EN 2812 ((modified). Acid attack resistance (acetic) (according to EN 2812 (modified)). *Carried out on samples weathered at 0, 500, and 2000 hours 4

5 Test Tensile EN ISO 526 elongation at break TABLE 1 PROPERTIES OF COAL TAR EPOXY COATINGS ( O & X )" Coating System O Coating System X Modules of elasticity, N/mm Hardness Shore A Shore D Remarks 6.82% 0.82% The two coating systems can be categorized as inflexible plastics system Mandrel flexibility, mm Electrical continuity Not electrically conductive 75 Not electrically conductive Diffusion resistance water vapor 10,969 8,349 Resistance to aggressive media ( Water, Sulphate,Chloride & Caustic soda solution ) Have Resistant to these media Have Resistant to these media Bond strength, N/mm Shear strength, N/mm Abrasion resistance Taber Abrasion, mg per 100 revolutions System X is more brittle compared to system O. A crack is expected to occur in the mortar substrate, and with the lack of flexibility of the base material, and with low bridging characteristics of the coating system the crack will continuously open. Materials with a Shore A hardness > 80 are judged hard. This applies to both of the coating systems examined. Deep cracks formed with a bending radius of 32 or 25 mm. A flexible system must exceed a minimum bending radius of 15 mm without damage. The electrical continuity was determined in accordance with EN 1081, Procedure C, using three point electrodes. Both systems are judged not electrically conductive. System X has higher diffusivity with water vapor than system O. With the exception of acetic acid, both materials are resistant to these media. Usually bond strength of 1.0 N/mm2 is required for coating systems that are to be applied to mortar or concrete. For the system "O", the fractures occurred in both the mortar substrate and the coating - mortar bond. For the system "X", The fractures occurred 100 % in the coating - mortar bond. The two systems exhibit good resistance to mechanical abrasion. 5

6 Exposure of the coating to UV light for 500 and 2000 hours did not change the resistance of the coating to the diffusion of oxygen, water, and other chemicals. The exposure had no effect on the adhesive strength but reduced the elasticity. PERFORMANCE OF COAL TAR EPOXY COATING AFTER FIFTEEN YEARS OF SERVICE After 15 years of service samples were cut out from GMRP pipelines and the following tests were conducted: Coating thickness, Moisture vapor transmission, Oxygen diffusion coefficient, Sulphate ion diffusion coefficient, Sulphate ion permeability, Chloride ion diffusion coefficient, and Chloride ion permeability coefficient. The test results were compared with those obtained 15 years ago. TABLE 2 "COATING THICNESS OF COAL TAR COATS AFTER 15 YEARS OF FIELD EXPOSURE Measurements Coating Thickness (microns) Mean 971 6

7 TABLE 3 "MOISTURE VAPOR TRANSMISSION OF COAT TAR COATINGS AFTER 15 YEARS OF FIELD EXPOSURE" Properties Sample 1 Sample 2 Sample 3 Flux (g/m 2 24hr) DH 2 O composite (cm 2 s -1 ) 3.08 x x x 10-4 DH 2 O coating (cm 2 s -1 ) 2.39 x x x 10-5 Composite thickness (mm) Composite thickness (µm) TABLE 4 "OXYGEN DIFFICIENT COEFFICIENT OF COAT TAR COATINGS AFTER 15 YEARS OF FIELD EXPOSURE" " Oxygen diffusion coefficient Values, cm 2 /s Composite Coal tar mortar coat 8.19 x x x 10-6 TABLE 5 "SULPHATE ION DIFFUSION OF COAT TAR COATINGS AFTER 15 YEARS OF FIELD EXPOSURE (SAMPLE 1)" Duration (days) Depth Increment (mm) Mass Used in Analysis (g) Sulphate By Dry Mass of sample (%)

8 TABLE 6 "SULPHATE ION DIFFUSION OF COAT TAR COATINGS AFTER 15 YEARS OF FIELD EXPOSURE (SAMPLE 2)" Duration (days) Depth Increment (mm) Mass Used in Analysis (g) Sulphate By Dry Mass of sample (%) Chloride Ion Diffusion Coefficient No steady state chloride transmission rate was achieved within the 28 days test period. Therefore, no diffusion coefficient was calculated. Chloride Ion Permeability Coefficient No steady state chloride transmission rate was achieved within the 28 days test period. Therefore, no permeability coefficient was calculated. DISCUSSIONS Film thickness Thickness of the coatings after 15 years of field exposure were less than minimum required 900 micron in some cases (Table 2). Moisture Vapor transmission The moisture vapor transmission increased from 1.21/1.31 x 10-5 cm 2 /s (1994 test samples) to 3.3 x 10-5 cm 2 /s (2006 test samples (Table 3)) indicating decrease in coating resistance, but still the values meet the BS EN :2004 requirements. Oxygen diffusion The oxygen diffusion increased from 8.5/8.7 x 10-7 cm 2 /s (1994 test samples} to 44.1 x 10-7 (2006 test samples (Table 4)) indicating that coatings had good resistance to oxygen diffusion after 15 years of field exposure. Sulphate ion diffusion The percentage of acid soluble sulphate (SO 4 ) content of 2006 test samples were slightly higher (Table 5& 6) than 1.49% recorded 15 years ago. Prolonged exposure had not changed the resistance of the coatings to sulphate diffusion. Chloride ion diffusion and permeability No evidence for the presence of chloride in the tested samples. 8

9 Conclusions System "O is slightly more flexible in comparison to system X. The abrasion resistance of system X is higher when compared to system O. Other properties of these coatings were comparable including resistance to permeation of water, sulphate, chloride, and caustic. Both coatings were electrically non-conductive and were sensitive to impact Weathering up to 2000 hours did not affect the properties of the coatings. Even after being in service for fifteen years these coatings: o These coatings had good resistance to moisture vapour transmission and oxygen diffusion. o The percentage of acid soluble sulphate content remained acceptable and o There was no evidence of the presence of chloride in the coatings. References 1 - PCCP corrosion protection study report, GMRP, Specification for corrosion control, Exhibit B-34, Corrosion prevention by protective coating. BY Charles G. Munger,