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:
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.
Coal Tar Enamel (CTE)
- 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)
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).