What Is Joint Coating? Field Application

Field joint coating (also called joint coating or girth weld coating) is the corrosion protection applied to the uncoated area at each pipe joint after field welding. Plant-applied coatings such as FBE and 3LPE leave a cutback (150-300 mm of bare steel at each pipe end) to allow welding during pipeline construction. After the girth weld is completed and inspected, the exposed steel must be coated in the field to restore the continuous corrosion barrier.

When Joint Coating Is Used

Every welded joint on a coated pipeline requires field joint coating. This applies to:

• Onshore buried pipelines (thousands of joints per project)
• Offshore pipelines (applied on the lay vessel during pipelay)
• Tie-in joints and road/river crossings
• Repair joints where damaged coating has been removed

The field joint is considered the weakest point in the pipeline coating system because it is applied under field conditions (weather, dust, humidity) rather than the controlled environment of a coating plant. Proper application and inspection are critical.

Joint Coating Method	Compatible With	Thickness	Max Temp	Application Time
Heat-shrink sleeve (HSS)	3LPE, 3LPP	2-4 mm	80-110°C	10-20 min
Liquid epoxy (FBE-compatible)	FBE	0.5-1.0 mm	80-110°C	5-15 min (heat cure)
Injection-molded polypropylene (IMPP)	3LPP	3-6 mm	140°C	3-8 min
Flame-spray FBE	FBE	0.3-0.6 mm	85-110°C	5-10 min
PE/PP tape wrap	3LPE, tape systems	1-2 mm	50-80°C	10-15 min
Petrolatum tape	Any (low-temp)	1.5-3 mm	50°C	5-10 min

Heat-Shrink Sleeve (HSS) Application

The heat-shrink sleeve is the most widely used field joint coating for 3LPE pipelines. The process:

1. Weld inspection: Girth weld NDT completed and accepted
2. Surface preparation: Power tool clean or abrasive blast the weld area and coating overlap zone
3. Preheat: Heat the pipe surface to 60-80°C using propane torch or induction coil
4. Sleeve positioning: Wrap the closure sheet (HSS) around the joint, centering on the weld
5. Shrinking: Apply heat uniformly with a propane torch; the sleeve shrinks radially, and the adhesive melts and flows to bond with the steel and coating overlap
6. Inspection: Visual check for wrinkles, channels, and lifting; peel test on witness strip; holiday detection

The sleeve must overlap the factory-applied 3LPE coating by at least 50-75 mm on each side to ensure a continuous seal.

Offshore Joint Coating

On offshore pipelay vessels, field joint coating must keep pace with the welding cycle (typically one joint every 3-8 minutes for S-lay). This time constraint drives the selection of fast-curing systems:

• Injection-molded polypropylene (IMPP): A mold is clamped around the joint, and molten PP is injected, filling the cutback area. Cures in minutes. Used for 3LPP-coated deepwater pipelines.
• Flame-spray FBE: Portable FBE spray equipment applies powder to the preheated joint, curing on contact. Fast but requires skilled operators.

Pro Tip

The most common joint coating failure is adhesion loss at the overlap between the field joint coating and the plant-applied coating. This overlap zone is where moisture ingress starts if the bond is poor. Always verify that the chamfered edge of the plant coating is clean, dry, and free of wax or release agents before applying the field joint system. Conduct adhesion testing (peel test per ISO 21809-3) on every 20th joint or per the project specification to catch problems early.

Field joint coatings must match the performance of the parent pipeline coating system in terms of adhesion, cathodic disbondment resistance, and temperature capability.

Read the full guide to pipe coatings