Pipelines Coating A comparative study

Various organic coatings are in use for pipeline passive external corrosion protection, which are supplemented with active corrosion protection i.e. cathodic protection. The choice and selection of particular coating system is dependent on the various considerations, such as external stresses the coating has to withstand, compatibility with cathodic protection and its current demand, soil characteristics, operating temperature etc.

Essential properties that a particular coating system should possess are as under:

  • Protective coating must remain defect free with in practical limitations over a period of time, specifically for areas where temperature fluctuations are wide.
  • Coating must be tolerant to handling during construction & field bending and in service deterioration.
  • Following are the essential properties of coating system:
  • – Impact Resistance
    – Penetration Resistance
    – Resistance to cathodic disbondment
    – Stability at elevated temperature
    – Resistance to soil stress
    – Resistance to water absorption
    – Chemical resistance (Acids & alkali)
    – Volume resistivity
    – Flexibility to bending
    – Hardness (abrasion resistance)
    – Maintenance and refurbishment frequency
    – HSE consideration
    – Resistance to damages during handling
(Some of the above properties are interdependent)


There are various types of coating system which are being applied on hydrocarbon pipelines. However, for the sake of this article, prediminanently there are following types of external anti-corrosion coatings:

  1. Fusion Bonded Epoxy (FBE) Coating
  2. Three Layer Poly Ethylene/ Propylene (3LPE/ 3LPP) Coating
  3. Coal Tar Enamel (CTE) Coating

Fusion-bonded epoxy powder coating
Fusion bonded epoxy coating, also known as fusion-bond epoxy powder coating and commonly referred to as FBE coating, is an epoxy-based powder coating that is widely used to protect steel pipe used in pipeline construction from corrosion. FBE coatings are thermoset polymer coatings. The name fusion-bond epoxy is due to resin cross-linking and the application method, which is different from a conventional paint. The resin and hardener components in the dry powder FBE stock remain unreacted at normal storage conditions. At typical coating application temperatures, usually in the range of 180 to 250 °C (356 to 482 °F), the contents of the powder melt and transform to a liquid form. The liquid FBE film wets and flows onto the steel surface on which it is applied, and soon becomes a solid coating by chemical cross-linking, assisted by heat. This process is known as “fusion bonding”. The chemical cross-linking reaction taking place in this case is irreversible. Once the curing takes place, the coating cannot be returned to its original form by any means. Application of further heating will not “melt” the coating and thus it is known as a “thermoset” coating.

Three Layer Poly Ethylene/ Propylene (3LPE/ 3LPP) coating
3LPE/ 3LPP coating system is a multilayer coating composed of three functional components: a high performance fusion bonded epoxy (FBE), followed by a copolymer adhesive and an outer layer of Medium Density Polyethylene (MDPE) or High Density Polyethylene (HDPE), which provides protection against external corrosion.

  • The codes and standards for 3LPE are ISO 21809-1, DIN 30670 or CAN/CSA Z245.21.
  • Fusion Bonded Epoxy (FBE): First layer of FBE shall be 150 microns thick.
  • Adhesive: Second layer of extruded co-polymer adhesive, 200 microns thick.
  • Polyethylene (MDPE/HDPE): Third layer of extruded polyethylene (MDPE/HDPE) or polypropylene (MDPP/HDPP), 1.6 – 2.95 mm thick.
  • Application of Coating: Surface preparation of pipe by blast cleaning, subsequent to pipe heating fusion epoxy powder shall be applied and after that extruded adhesive shall be applied over the pipe. Finally extruded polyethylene (MDPE or HDPE) layer shall be applied over the adhesive layer.
  • Total coating thickness is generally < 3.0 mm for pipe dia < 24" and 3.3 mm for pipe dia 32” & above)

Coal Tar Enamel (CTE)
CTE coating is a thermoplastic polymeric coating. The CTE coating system is made up of four main components: primer, coal tar enamel, glass fibre inner-wrap and glass fibre outer-wrap.

  • The codes and standards for CTE are AWWA C203, BS 4164 or IS 10221.
  • Primer: The primer shall be quick drying synthetic primer for cold application.
  • Coat Tar Enamel: Coal tar enamel shall consist of uniform mixture of modified coal tar and inert non-fibrous filler.
  • Inner Wrap: The fibre glass inner wrap shall be thin and flexible, uniform mat of compressed glass fibres. The inner wrap shall be reinforced type.
  • Outer Wrap: The outer wrap material shall be a coal tar impregnated glass fibre felt.
  • Application of Coating: Surface preparation of pipe by blast cleaning, primer shall be applied to the pipe surface by air-less spraying equipment, coal tar enamel shall be mechanically applied over the primed pipes with one wrap of glass fiber mat followed by coal tar enamel and followed immediately by an outer wrap of coal-tar impregnated glass fiber felt.
  • Total coating thickness is 4.8 mm (for all pipe sizes).

Property CTE coating FBE Coating 3LPE Coating
Service temperature : 45 oC 110 oC 80 oC
Impact resistance : Poor High Excellent
Penetration resistance : Initially good but changes to poor with passage of time High Excellent (No changes observed during service life)
Damages during handling & transportation : High High Low
Cathodic disbondment : Good Very good Good
Resistance to soil stress : Poor Good Excellent
Resistance to water and oxygen permeation : Good (initially) Good Excellent
Chemical resistance : Good Very good Very good
Volume resistivity : Good Excellent Excellent
Bending flexibility : No flexibility 1.5 oppd length 2.0 oppd length
Hardness : Less Excellent Excellent
Requirement for maintenance and refurbishment frequency of coating/ CP system : High High Low
Impact on human health : Highly injurious Less injurious Less injurious
Environmental impact : Highly polluting Less polluting Less polluting
Cathodic protection current demand (design) : 0.3 – 1.0 mA/ m2 0.3 – 1.0 mA/ m2 0.05 – 0.1 mA/ m2
Resistance to peel : Fair -- Excellent
Distance between CP stations : 20 km - 25 km 20 km - 25 km 40 km - 45 km