EXTERNAL PIPE
COATINGS
EXTERNAL PIPE COATINGS
To protect the buried line pipes from external corrosion, the line pipes are coated. A good quality pipeline coating shall have following characteristics:
Adherence to pipe Non-porous
Free of defects, pores, flaws, holidays Should act as an insulating barrier Should be inert to environment
Coefficient of thermal expansion of the coating film > that of pipe metal High resistance to abrasion
Resistance to UV degradation
Following types of coatings are mainly used for coating of line pipes: Coal Tar Enamel Coating
3 Layer Polyethylene Coating Fusion Bonded Epoxy Coating
COAL TAR ENAMEL COATING
Reference Codes
BS 4164 : Specification for coal-tar based hot applied coating materials for protecting from iron and steel, including suitable primer.
AWWA C-203-86 : AWWA standard for Coal Tar Protective coatings & linings for steel water pipelines, enamels and tape-hot applied.
Materials
Primer : Fast drying synthetic primer Type B for cold application.
Coal Tar : The coal tar enamel shall consist of a uniform mixture of a specially modified coal tar and inert non-fibrous filler.
The primer & coal tar enamel shall conform to BS 4164.
Wrapper
Inner Wraps : The inner wrap glass fibre reinforcement material shall conform to AWWA C-203. The glass fibre reinforcement material is reinforced in the longitudinal direction.
Outer Wraps : The outer wrap material shall conform to AWWA C-203. The outer wrap material is coal tar impregnated asbestos felt or glass fibre felt.
Coal Tar Enamel Coating Process
1) Surface Preparation
Oil, grease or other contaminants are removed by solvent cleaning prior to blasting. Exterior surfaces of the pipes to be coated are blast cleaned by sand blasting to near white metal finish to Grades 2 1/2, SIS 0550900. Prior to primer application, the pipe is cleaned to remove any wetness or other contamination.
2) Priming
Primer material compatible with the enamel coating is used. Primer is applied on sand blasted pipe in an uniformly thick film to completely cover the circumference of the pipe and all surfaces which are to be coated. The freshly primed pipes are dried on racks until they are no longer wet, sticky or tacky.
3) Heating of Coal Tar Enamel
The chopped Coal Tar Enamel is heated in kettles to maintain a continuous supply of hot enamel. When the enamel is in a molten condition in the kettle, it is continuously agitated.
4) Coating of Pipes
The coating system consists of applying over the primed pipe, a coat of coal tar enamel having a minimum thickness of 2.4 mm and one wrap of glass-fibre mat, followed by a second coat of coal-tar enamel having minimum thickness of 2.4 mm and wrap of glass fibre mat followed immediately by an outer wrap of coat tar impregnated asbestos felt/glass fibre felt.
Enamel is applied by pouring on pipes and spreading to the specified thickness. Enamel is applied so that each spiral resulting from the spreading operations overlaps the preceding spiral producing a continuous unbroken film.
Glass fibre mat is applied simultaneously with the first coat of coal tar enamel. The roll of glass fibre mat is maintained under sufficient tension to embed the mat in the enamel before the latter sets or cools. The second coat of hot coal tar enamel is then applied simultaneously with the second layer of glass fibre mat. The impregnated outer wrap is then applied immediately behind the glass fibre reinforcement in a tight uniform spiral.
5) Inspection & Testing
Holiday Detection: Coated and wrapped pipes which have been
supported on racks or skids are subjected to a test with an electric holiday detector to detect all coke particles and other inclusions such as pinholes, voids, holidays, air bubbles, cracks and other breaks.
Measuring coating thickness: The thickness of the coating is measured
by Elcometer.
Testing the adhesion of the coating: The adhesion of the coating is
determined at three points of the coated surface, one in the centre and two at about 500 mm from the bevelled ends. After the coating has cooled sufficiently two parallel incisions, at least 25 mm apart, are made in the coating right down to the metal and two more incisions at right angles to and intersecting the first two incisions. An attempt shall then be made with the aid of a stopping knife or similar tool to peel the coating off the pipe by applying a uniform force on the coating directed away from the pipe. The adhesion is unsatisfactory if the layers of coating part from the primer coat or if the primer coat parts from the steel.
3 LAYERS POLYETHYLENE COATING
Reference Codes
i. DIN 30670 Polyethylene-Coatings For Steel Pipes And Fittings
Requirements And Testing
iv. ASTM D - 543 : Standard Method of test for Resistance of Plastics to
Chemical Reagents.
v. ASTM D - 570 : Standard Method of Test for Water Absorption of
Plastics.
vi. ASTM D - 638 : Standard Method of test for Tensile Properties of
Plastics.
vii. ASTM D - 792 : Standard Test Method for Specific Gravity and
Density of Plastics by Displacement.
viii. ASTM G - 42 : Tentative Methods for Cathodic disbonding of
Pipeline coatings subjected to Elevated or Cyclic temperatures.
x. DIN 53735 Testing of Plastics: determination of Melt Index of
Thermoplastics.
Materials
Polyethylene compound Epoxy powder
Adhesive
Properties of Polyethylene Compound
S. No Properties Unit Requirement Test Method
1. Tensile Strength at +25°C Psi 2900 min ASTM D 638
2.
Melt index. Test conducted at
a temperature of +190°C
under a total load (including piston weight of 2.16 Kg)
g/10 min. 0.25 min. ASTM D 1238
3. Specific Gravity at +25°C - 0.945 min. ASTM D 792
4. Hardness at +25°C Shore D 60 min. ASTM D 2240
5. Water Absorption, 24 hours,
+25°C % 0.01 max. ASTM D 570
S. No Properties Unit Requirement Test Method
7. Dielectric withstand, 100
Volt/sec rise at +25°C Volts 30,000 min. ASTM D 149
Properties of Epoxy Powder
Epoxy powder shall be chosen such that it complies with the functional requirements and properties of coating system.
Properties of Adhesive
Adhesive shall be chosen such that it complies with the functional requirements and properties of coating system.
Properties of Coating system
S. No Properties Unit Requirement Test Method
1. Resistance to indentation at
70 ± 2°C mm 0.3 max. DIN 30670
2. Resistance to Impact (Min.
of 30 impacts at 70°C. No
holiday allowed when
tested at 25 Kv)
NM per mm of coating thickness
5 min. (for dia <200 mm, 6 min
(for dia ≥200 mm)
DIN 30670
3. Resistance to Peeling. Test
Method at 50°C Kg/cm 7.0 min. DIN 30670
4. Elongation due to tearing % 500 DIN 30670
5. Water Soak Test. Sample
immersed 100%, at 80°C for 7 days - No de-lamination, No loss of peel strength tested at 50°C Modified British Gas BGS PS/CW6, Part 1, App. E
6. Resistance to thermal aging - Retain 70% of Melt
index DIN 30670
7. Resistance to Exposure to
Light - Retain 70% of Melt Index DIN 30670
8. Cathodic Disbondment
after 28 days, at +70°C radius of mm
disbond
3 Layer Polyethylene Coating process
1) Surface preparation
Solvent cleaning is done in accordance with SSPC - SP 1. Prior to abrasive blast cleaning, the pipes are preheated to 65°C to 85°C for removal of any moisture, raising of silvers, laminations etc. The external surface of the pipe is cleaned by abrasive blast cleaning machine for white metal finish to SA 2 ½ of Swedish Standard SIS 055900.
2) Coating application
The external surface of the pipe shall is heated to about 190°C followed by application of following three layers:
I. Electrostatic application of epoxy powder (100 - 175 micro meter thick or as recommended by the manufacturer)
II. Crystalline Co-polymer Adhesive applied by extrusion (min. 0.20 mm thick)
III. Polyethylene coating by extrusion.
The coated pipe is subsequently quenched and cooled in water for a period, which shall sufficiently lower the temperature of pipe coating to permit handling and inspection.
The copolymer adhesive is applied by extrusion within 15 seconds of the fusion bonded epoxy (FBE) powder application and before the solidification of the FBE so that the two materials will interact by chemically bonding.
3) Inspection and Testing
Visual inspection: of finished coating for colour, blisters, sags, porosity,
burns and handling damage during coating, stacking and loading.
Thickness: The coating thickness is measured by Elecometer.
Holiday Detection: Each coated pipe is checked over 100% of coated
surface by means of a “holiday detector” for pinholes, voids, holidays, air bubbles, cracks and other breaks.
Peel test: Peel test is carried out as specified in DIN 30670 for testing
adhesion. The system shall fail only in the adhesive layer. Failure of either adhesive to steel or adhesive to backing is not acceptable.
Indentation Resistance: Indentation resistance test is carried out as
specified in DIN 30670.
Impact Resistance: Impact resistance test is carried out as specified in
DIN 30670. Impacts, located equidistant along the length of coated pipe
are performed. Immediately after testing, the test area is subjected to holiday detection. The pipe is rejected if any holiday is noted in the test area.
COMPARISON OF COATINGS
Description Coal Tar Enamel coating 3 Layer PE coating
Service Conditions
Temperature Suitable up to 45°C Suitable up to 60°C
Anti-corrosion Properties
Resistance to Moisture Good Very good
Cathodic Disbondment Fair Good
Volume Resistivity Fair Good
Electrical Properties
Coating Resistance 1200 Ohms-m² 12000 Ohms-m²
Mechanical Properties
Impact Resistance Fair Excellent
Elongation Fair Excellent
Indentation Resistance Poor Very good
Penetration Resistance Poor Good
Abrasion Resistance Fair Good
Resistance to Peel Fair Excellent
Resistance to Soil Stresses Poor Excellent
Long Term ageing
Heat and Light ageing Fair Very Good
Application and Field Handling
Application Easy to apply at yard Easy to apply at yard Damages during handling
and transportation High Low
Ease of repair Easily repairable Easily repairable
Cathodic Protection Design
Current Density 500 micro A/m² 50 micro A/m²
CP Stations At every 15-20 km At every 30-40 km
Cost
Coating Relatively Low Relatively High
Cost of CP Stations Approx. 3 times 3LPE Normal Cost of Transportation/
EXTERNAL FBE (FUSION BONDED EPOXY) COATING
The FBE is an electrostatically changed powder, which is pneumatically sprayed onto the hot pipe, and contains excellent passive corrosion protection properties.
Single Layer Coating
The system was developed for the protection of pipelines operating at elevated temperatures where coating adhesion and resistance to soil stresses and cathodic disbondment are required. It consists of a single layer of cured thermosetting epoxy resin powder as the corrosion barrier (no other mechanical protection) applied by electrostatic charge to a preheated pipe. As per CSA Z245-20, the thickness requirement for Single
Layer Coating shall be 350 - 500 micron.
Advantages
Service temperature up to 90°C maximum (110°C intermittent in dry soil conditions)
Excellent corrosion resistance Excellent flexibility
Excellent adhesion to steel and extremely resistant to soil stresses Compatible with most known field joint protection systems Does not shield cathodic protection
Disadvantages
Susceptible to damage during handling, transportation & pipeline construction Limited impact resistance
Hydroscopic material leading to increased cathodic protection demand as pipeline ages
Dual Layer Coating
Dual layer fusion bonded epoxy is a 2-component powder coating designed to provide long term corrosion resistance and mechanical protection to steel pipe. The system was principally developed for road/river crossing and HDD (Horizontal Directional Drilling) applications where resistance to abrasion and gouging are considered important. It consists of:
i) A primary layer of thermosetting epoxy resin powder as the corrosion barrier applied by electrostatic charge to a preheated pipe and
As per CSA Z245-20, the thickness requirement for Dual Layer Coating shall be 750 -
800 micron.
Advantages
Service temperature up to 90°C maximum (110°C intermittent in dry soil conditions)
Excellent corrosion resistance
Provides excellent abrasion resistance
Excellent adhesion to steel and extremely resistant to soil stresses Compatible with most known field joint protection systems Does not shield cathodic protection
Disadvantages
Poor flexibility at higher coating thicknesses More expensive than most coating systems Limited impact resistance
Hydroscopic material leading to increased cathodic protection demand as pipeline ages
FIELD JOINT COATING
Field joint coatings are carried out using Heat Shrinkable Sleeves.
Reference Code
DIN-30672-1991 : Coating of Corrosion Protection Tapes & Heat Shrinking Products for Pipelines - Operational Temperatures upto 50º C.
Materials
Heat shrinkable wraparound sleeve consists of radiation cross-linked, thermally stabilised, ultraviolet resistant semi-rigid polyolefin backing with a uniform thickness of high shear strength thermoplastic/copolymer hot melt adhesive. The joint coating system may consist of a solvent free epoxy primer applied to the pipe surface prior to sleeve application. The backing is coated with thermochrome paint which will change colour when the desired heat during shrinking is attained. The wraparound sleeve is supplied in pre-cut sizes to suit the pipe diameter and the requirements of overlap.
The thickness of field joint coating material is such that an average thickness of 2.0 mm is guaranteed on the complete finished field joint coating.
Properties of Joint Coating Material
Sr.
No. PROPERTY UNIT REQUIREMENT METHOD TEST
1. Tensile Strength @ 25º C. Psi 2200 ASTM D-638
2. Ultimate Elongation % @ 25º C. % 400 min DIN 30672
3. Dielectric withstand with 1000
Volt/Sec KV > 30 ASTM D-1249 OR
DIN 30672
4. Water absorption 24 hours % < 0.05 ASTM D-570
5. Volume Resistivity @ 25ºC. Ohm-cm > 1x10/15 ASTM D-257
6. Resistance to Thermal Aging @
100º C. % Elongation, Change in
<25%
DIN 30672
Properties of Joint Coating System (As applied)
Sr.
No. PROPERTY UNIT REQUIREMENT METHOD TEST
1. Resistance to impact @ 23ºC Class
C, 30 Blows - No Holidays DIN 30672
2. Resistance to peel Class C
@ 23 ºC.
With Metal Surface With Mill Coating At Layers Overlap @ 50ºC.
With Metal Surface With Mill Coating At Layers Overlap N/cm Width >19 >10 >19 > 4 > 2.5 > 4 DIN 30672
3. Cathodic Disbondment (after 30
days) Test Method A, @ 60ºC. mm <30 ASTM G-42
4. Resistance to Indentation, @ 50ºC. mm 0.3 max. DIN 30672
5. Lap Shear Strenth @ 50ºC. Between
Wrapping & Metal Surface and Mill Coating.
N/ cm2 > 6 DIN 30672
Field Joint Coating procedure
1) Surface Preparation
Oil, grease, salts and other contamination are removed from steel surface by wiping with rags soaked with suitable solvents such as naphtha or benzene. Solvent cleaning procedure is in accordance with SSPC-SP 1. Surface is then blast cleaned to a finish equivalent to SA 2½ of Swedish Standard SIS-055900. The ends of existing pipe protective coating are inspected and chamfered prior to application and cleaned as necessary. Joint coating shall be applied immediately after completion of surface preparation.
2) Application of Joint Coating
The wrap around sleeve of size such that a minimum overlap of 100 mm is ensured (after shrinking) on both sides of the yard applied corrosion coating of pipe is selected. Before centering the wraparound sleeve, the bare steel surface shall be preheated over the surface to remove the moisture. The minimum preheat temperature as recommended by the manufacturer and shall be checked by means of pyrometer or temperature crayons. The solvent free epoxy primer (if applicable) is applied prior to sleeve application.
The wraparound sleeve is then entirely wrapped around the pipe positioning the closure patch off to one side of the pipe in 10 o’clock or 2 o’clock position, and the edge of the undergoing layer facing upward. A heat shrinking procedure is applied to shrink the sleeve in a manner such that all entrapped air is removed using gloved hands and hand rollers. Heating is carried out until the thermochrome paint has completely changed its colour. Resulting coating must be free of wrinkles, cold spots and weld profile visible on the sleeve.
3) Inspection
A visual inspection is carried out for the following:
Mastic extrusion on either ends of the sleeve.
Punctures or pinholes or bend failure. The external appearance of the sleeve shall be smooth, free of dimples, air entrapment or void
Weld bead profile shall be visible through the sleeve.
The entire closure patch shall have changed colour uniformly.
Holiday Inspection: The entire surface of the joint section is inspected by
a full circle holiday detector after the joint has cooled below 50ºC.
4) Testing
From each test sleeve, one or more strips of size 25 mm x 200 mm is cut perpendicular to the pipe axis and slowly peeled off. The required peel strength shall meet the requirements of DIN 30672 as applicable for 230 C or 500 C. For qualification, peel test must be carried at 230 C. and 500 C. After removal of strip the bulk of adhesive shall remain adhered to the pipe showing no bare metal, otherwise, test is considered to have failed. The adhesive layer that remains on the pipe surface shall be free of voids resulting from air or gas inclusion.