Background
Fusion Bond Epoxy (FBE) has a long history as the anticorrosion coating of choice for pipe lines in North America and other parts of the world. The developments in the technology have focused for a long time in improving the basic technology by making it more reliable. These improvements have involved not just material changes but also improvements in the processing with an increased awareness of the needs within the application process to ensure the long term adhesion performance of the coating system. Specifications have become more demanding of the Fusion Bonded Powder and the applicators ability to apply them1.
Physical damage has been regarded over the years as a weakness in fusion bond epoxy anti corrosion systems. The development of mechanical protection systems has been a method to reduce this weakness and provides an economic solution. The approach takes full advantage of the FBE anti corrosion properties at the same time as eliminating the concerns for shielding raised by the use of other polyolefin based coating systems.
These dual layer systems, often called "ARO", were developed primarily as a defense against 3 layer polyethylene coatings, particularly in North America.
The improved performance of these systems has been described and qualified in many papers2. These papers and other sources have demonstrated, through evaluations using methods such as stone drop testing and gouge testing, performance better than standard epoxy.
The recent CSA Z245.20.06 specification is the 1st national standard to try and create specifications for such coating systems.
When the performance of these systems is compared to standard FBE and to 3 - layer PE coatings
3 it can be seen that although they give clear benefits over standard FBE they do not fully achieve the performance of 3 layer PE.
|
|
FBE
|
Dual Layer
|
3 – Layer PE
|
|
Gouge Resistance
|
50 kg continuous
fail holidays
|
50 kg no holidays
|
50 kg holiday
|
|
Impact resistance 40mm backfill
|
3360 holidays
|
2 holidays
|
0 holidays
|
Additionally when various dual layer FBE systems arc compared we can clearly see a tradeoff between mechanical resistance and flexibility.
|
Property
|
Dual layer FBE system
A
|
Dual layer
FBE system
B
|
Dual layer
FBE system
C
|
Dual layer
FBE system
D
|
|
Flexibility – 30 C 3.2%
|
Pass
|
Fail
|
Fail
|
Fail
|
|
Flexibility 0 C 3.2%
|
Pass
|
Pass
|
Fail
|
Fail
|
|
Flexibility 0 C 2.2%
|
Pass
|
Pass
|
Pass
|
Fail
|
|
Gouge resistance-load to failure at
600 micron system
|
45 Kg
|
50 Kg
|
55 Kg
|
60 Kg
|
| |
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A well applied 3 layer coating is still an excellent choice for corrosion protection of pipelines. However, the advantages of a 3 layer system with respect to impact damage have been tainted over the last few years when 3- layer coatings have come under greater scrutiny and discussion regarding adhesion failures. 4 5
Fig 1. Pipe end Disbondment
Failures have been reported from a wide range of applicators on a wide range of 3 layer PE systems
Various theories have been ex pounded on the ca use of disbondment including -
1. Thickness of FBE, originally in 3 layer this was 60 microns but du e to failures this has been increased and now thickness is more typically 150 microns. Several studies have demonstrated that a minimum of 150 microns of FBE is required for optimum performance of 3 layer pipe coatings.
2. Application temperature, lower application temperatures are commonly used when FBE is applied in a three layer coating system primarily to aid in intercoat adhesion between the adhesive and FBE. This has led to speculation that the lower application temperature results is poorer wetting of the substrate and compromises the adhesion of the FBE.
3. Cyclic stress due to the differential contraction between steel, FBE and PE.
A residual strain in the region of 2% is developed when an MDPE coating system is at 20°C.
This relates, in the case of 3mm of the PE to a sheer force in the region of 140-110 Kg/cm2 from the stress required to elongate the PE by 2%. A a measure of comparison the lap shear strength requirement for FBE in AWWA in the region of 2 10 Kg/cm2 psi.
Adhesion values are also known to fall in moist conditions. Even dolly pull adhesion strengths can be below 210 Kg/cm2 after water immersion. 6
Anecdotal support for this residual stress being one of the key factors is the apparent increase in reported failures as the industry has moved towards HDPE from LDPE.
4. The water permeability of PE. The rate of water permeability through PE has come under discussion as a contributing factor in the reduced adhesion levels.
With all these theories it is clearly apparent that the cause of the failures are not simple and not due to a single cause but most probably a combination of factors which the quality assurance regime used for 3 layer polyethylene coating doe s not currently evaluate. Thus making it difficult to quality assures the coating system from any pipe coating plant with short term testing methods.
FBE as a standalone coating does not appear to suffer this type of disbandment failure and the quality assurance regimes appears to give a good level of quality assurance.
If only the mechanical handling was better!
A new system for a multilayer FBE based coating has been developed as a solution to this using a different concept resulting in a tough resilient anti-corrosion coating (TRAC).
The concept can be visualized with the following model.
A system consisting of three layers each with a different characteristic which work together to give a high level of performance.
The outer layer is a tough layer which resists penetration.
The middle layer is a resilient layer which absorbs shocks.
The inner layer is an FBE optimized for adhesion and anti corrosion properties designed to work with the protection of the outer layers.
The three layers are applied in a continuous coaling operation similar to that currently employed for dual layer epoxies. This results in the three layers being fused together.
How does the system work to deliver performance
Impact resistance
The outer rough layer stops penetration into the film.
The middle resilient layer spreads the impact over a wider area and absorbs the energy.
The inner layer is therefore protected from damage.
The value of an energy absorbing process can be clearly seen in the comparison of two well know impact t test procedures. A standard FBE panel tested for impact, for example in accordance with the AWWA C2l3 method is required to meet a requirement of 11. J but the same coating tested using the CSA 2245.20.06 method will only yield 2-3 J. Why the difference? It is because the energy is absorbed differently. in AWWA method allow s the energy to be absorbed by deforming the metal panel in CSA it is absorbed only by the FBE. With the new system it is absorbed by the resilient layer which also spreads the load which does get through to the anticorrosion layer.
Gouge Resistance
In this case the tough outer layer resists penetration and the inner resilient layer absorbs and spreads the load.
If the load is sufficient to penetrate the outer layer then the middle layer acts as a lubricant to smooth the gouge and protect the anticorrosion inner layer.
Flexibility
The middle layer absorbs the bending force which result s in a more even distribution of the bending force with less chances of reducing stress raisers which ca n break the outer layer.
Comparison of panels on key tests gives the following results:
|
|
Method
|
|
FBE
|
Dual FBE
|
3LPE
|
TRAC
|
|
Thickness
|
|
(micron)
|
450
|
700
|
3000
|
800
|
|
Gouge
|
|
(Kg load to penetration)
|
50
|
70
|
55
|
100
|
|
CD
|
CSA Z245.20
|
(mm 28 Days 65 C)
|
8
|
6
|
5
|
4
|
|
Impact
|
CSA Z245.20
|
(Joules)
|
3
|
8
|
10
|
10
|
|
Flexibility
|
CSA Z245.20
|
3% PPD
|
-30oC
|
0oC
|
-30oC
|
-30oC
|
A full performance evaluation of TRAC against CSA Z245.20-06 gives the following performance:
|
|
Results
|
Specification
|
Comments
|
|
24hr CDT 65C
|
4.0mm
|
6.5mm
|
System 2 test
|
|
28day CDT 65C
|
4.0mm
|
20mm
|
System 2 test
|
|
28day CDT amb
|
2.0mm
|
8.5mm
|
System 1A test
|
|
X-section porosity
|
1
|
1-4
|
System 2 test
|
|
Interface porosity
|
1
|
1-4
|
System 2 test
|
|
2 deg flexibility at -30C
|
pass
|
no cracking
|
System 2 test
|
|
3 deg flexibility at -30C
|
pass
|
no cracking
|
System 1A test
|
|
3.0J Impact resistance
|
pass
|
no holidays
|
System 2 test
|
|
24hr 75C adhesion
|
1
|
1-3
|
System 2 test
|
|
28 day 75C adhesion
|
1
|
1-3
|
System 2 test
|
|
1.5 deg strained CDT
|
pass
|
no cracking
|
System 2 test
|
|
Cure conversion
|
>95%
|
>95%
|
System 2 test
|
Conclusion
The TRAC system features a combination of fusion bond powders providing all the benefits and none of the weaknesses of stand-alone FUE and multi layer extruded coatings.
In addition to long term anti-corrosion protection the TRAC system provides:
• Mechanical protection to match or exceed other multilayer system
• Advanced abrasion and impact performance
• Improved flexibility at low temperatures
• Easy application with minimal plant changes
• Excellent raised weld coverage
• Coating system for small and large diameter pipes
• Compatible girth weld coating solutions
• Negligible damage on concrete weight coating application
References
1. A review of developments in fusion bonded epoxy 10 meet new challenges M. Wilmott, D. Grimshaw, Jotun Powder Coatings. BHR group 16th International Conference on Pipeline Protection 2005
2. Dual-layer fusion bonded epoxy (FBE) coatings protect pipelines - A Kehr, M Dabiri, R Hislop - 15th International Pipeline Conference Oct 2003
3. Use of multilayer coatings to minimize coating damage during pipeline construction - S Edmunson, R Espiner, I Thompson, 15th International Pipeline Conference Oct 2003
4. Are we protecting our assets D Nonnan BHR Group 15th International Conference on Pipeline Protection 2003
5. Recent experience with pipeline coating failures M Roche, D Melot, G Paugam, Total E&P, 16th International Conference on Pipeline Protection 2003
6. Testing of external Pipeline Coatings for High Temperature Service, Howard R. Mitschc and Paul R. Nichols, Shell Global Solutions. NACE 2006 paper 06040
