Why the Pipe and the Coating Are Specified Separately
The steel pipe and the anti-corrosion coating are two separate purchases. API 5L defines the requirements for the line pipe itself: grade, chemistry, mechanical properties, dimensions and testing. The external coating is specified separately against a coating standard or a project specification, and a complete order therefore states the pipe standard and grade, the coating standard, the coating class, the required thickness and the inspection requirements.
This distinction explains why a line item described only as an anti-corrosion pipe is not sufficient for manufacture. Coating performance depends on the substrate preparation, the pipe surface temperature during application and the coating materials, all of which need to be fixed before production.
Structure of a Three-Layer Polyethylene Coating
Layer 1, epoxy primer: fusion-bonded epoxy powder applied to the blast-cleaned, heated pipe surface. It provides the primary corrosion barrier and bonds chemically to the steel.
Layer 2, adhesive: a copolymer adhesive that bonds the epoxy primer to the outer polyethylene layer and absorbs shear between the two materials.
Layer 3, polyethylene topcoat: an extruded polyethylene layer that provides mechanical protection against impact and abrasion during handling and installation.
The three layers work as a system. The epoxy controls corrosion, the adhesive controls interlayer bonding, and the polyethylene carries mechanical loads. Failure of any one layer, or of the bond between them, reduces the service life of the coating.
Standards and Coating Classes
| Standard | Scope | Typical use |
|---|---|---|
| ISO 21809-1:2018 | Plant-applied three-layer polyethylene and polypropylene external coatings for welded and seamless steel pipe used in buried or submerged pipeline transportation systems | International pipeline projects |
| DIN 30670 | Polyethylene coatings on steel pipes and fittings, requirements and testing | European and project specifications referring to the DIN system |
| DIN 30678 | Polypropylene coatings on steel pipes and fittings | Higher operating temperature applications |
ISO 21809-1:2018 classifies coatings according to design temperature and testing requirements, and it specifies the minimum total coating thickness as a function of the pipe outside diameter. The purchaser states the required class and thickness in the project specification, and the manufacturer demonstrates compliance by measurement and test.
Pipe Dimensions and Coating Thickness Control
Coated line pipe is supplied in the sizes produced by the mill. A typical capability range for 3LPE coated pipe is an outside diameter from 159 mm to 3,200 mm with wall thickness from 5 mm to 30 mm, depending on the manufacturing route chosen. These are supplier capability ranges rather than requirements of API 5L or ISO 21809-1, so the final dimensions are always confirmed against the project specification.
Coating thickness is measured after application to confirm that the specified minimum has been achieved and that the maximum has not been exceeded. The required thickness depends on the coating class, the pipe diameter, the design temperature, the handling conditions during installation and the project specification: there is no single thickness that applies to every size. Measurement is normally carried out on the finished pipe at locations defined by the project, because the thickness of the epoxy primer, the adhesive and the polyethylene topcoat are verified as separate layers as well as in total.
Application Process
Surface preparation: the pipe surface is cleaned and abrasive blast cleaned to remove mill scale, rust and contaminants and to achieve the specified surface profile and cleanliness grade.
Heating: the pipe is induction heated to the temperature range specified for the coating materials.
FBE application: fusion-bonded epoxy powder is applied and melts into a continuous film on the hot surface.
Adhesive and polyethylene application: the copolymer adhesive and then the polyethylene are applied by extrusion so that the layers bond while the substrate is still at the specified temperature.
Cooling and end treatment: the coating is cooled under controlled conditions, and the pipe ends are left uncoated to a defined cut-back length so that field welds can be made.
Inspection and Testing of a Plant-Applied Coating
Testing verifies both the coating and the bond to the steel. Depending on the standard and the project specification, the programme includes coating thickness measurement, holiday detection, impact resistance, indentation or penetration testing, peel strength at ambient and elevated temperature, cathodic disbondment testing, flexibility testing and, where relevant, hot water immersion. Peel strength is particularly important, because a pipeline protected by cathodic protection relies on a coating that resists disbondment at coating defects, so the standard requires the coated pipe to be assessed under conditions that represent this service.
Applications
3LPE coated pipe is used for buried and submerged pipelines carrying oil, gas, water and slurries, for onshore gathering and transmission lines, for offshore pipelines and risers, and for municipal and industrial water transmission mains. It is normally combined with cathodic protection, which protects bare steel at coating defects and at field joint locations. Internal corrosion is a separate matter and is addressed by material selection, internal lining or chemical treatment.
FAQ
Q: Does API 5L cover the 3LPE coating?
No. API 5L covers the steel line pipe. The coating is specified separately, most often against ISO 21809-1, against DIN 30670 or against a project-specific coating specification.
Q: Is there one standard coating thickness for all sizes?
No. The minimum total thickness depends on the coating class, the pipe outside diameter, the design temperature and the handling conditions, and it is stated in the project specification.
Q: What does the three-layer system actually consist of?
A fusion-bonded epoxy primer, a copolymer adhesive layer and an extruded polyethylene outer layer, each with a defined function in the corrosion protection and mechanical protection of the pipe.
Q: How is the quality of the coating verified?
By a combination of thickness measurement, holiday detection, impact and indentation tests, peel strength testing, cathodic disbondment testing and visual inspection of the finished pipe.
Q: Can 3LPE be used at high operating temperatures?
Polyethylene coatings have a temperature limit that depends on the grade of the coating material, and polypropylene systems are used where higher continuous operating temperatures are required.
Q: What is the cut-back at the pipe ends for?
The coating is stopped short of the pipe end so that the field weld can be made on bare steel. The exposed area is then coated with a compatible field joint system after welding.
