Steel Pipe Anti-Corrosion Construction: Systems, Application and Inspection

Why Anti-Corrosion Construction Is a Discipline of Its Own

A buried or immersed steel pipeline usually fails from the outside in long before it fails from internal pressure. Soil, ground water, stray current and the joint trench are all corrosion sites, and a coating that is thin at one spot or damaged during hoisting becomes the starting point for a through-wall pit. For that reason coating work on a project is treated as one controlled process covering the material, the surface preparation, the application sequence and the inspection records, rather than as a finishing trade carried out after the pipe has been laid.

On a typical water, slurry or low-pressure process line, every steel pipe and every steel accessory is coated with the same two families of material: a non-toxic epoxy zinc-rich system for the inside, and a coal tar epoxy system reinforced with glass cloth for the outside.

Internal and External Coating Systems

Position	Coating system	Build-up	Key points
Internal	Non-toxic epoxy zinc-rich paint	Two primer coats followed by two finish coats	Each coat is applied only after the previous one has dried; primer must be even and complete with no misses
External	Coal tar epoxy with glass cloth reinforcement, four coats and three cloth layers	Colourless primer, then build coats with three interleaved glass cloth layers and two topcoats	Total dry film thickness of the external system is not less than 200 µm, with every cloth layer fully saturated
Fittings and accessories	Same system as the pipe body	Same build-up and same materials	Repair coatings must match the original system, not merely match the colour

Using one system across pipe, fittings and repairs matters because a mismatch in film build or in binder chemistry creates a galvanic or permeability difference at exactly the point where the line is most likely to be disturbed during handling.

Surface Preparation Before Coating

Adhesion is decided before the first coat is opened. Surfaces are cleaned of oil, dust, welding slag, mill scale and loose rust, and the steel is left dry, free of water marks and with a metallic lustre. The specified cleanliness class is Sa 2.5 near-white blast cleaning assessed to ISO 8501-1; where blasting is impractical, hand and power tool cleaning is used for local repair areas and for touch-up, removing all loose material and leaving a sound, roughened surface for the primer.

Preparation is also a site discipline. Pipe joints coated in the trench must be free of soil and water traces before painting, and work is stopped during rain, snow or fog when the work is done in the open, because moisture on the substrate is the most common cause of premature coating breakdown on a buried line.

Application Sequence Step by Step

Internal coating - apply two primer coats, then two finish coats. Primer is applied evenly and without misses; a coat is only followed once the previous one is dry. Voids, hollows, wrinkles and scratches in the finished film are repaired before the pipe leaves the coating bay.

External coating - apply the colourless primer and allow it to dry, then build the reinforced system so that three glass cloth layers are wrapped and fully impregnated with coal tar epoxy, finishing with the surface coats. The cloth must be saturated so that no dry fabric is left inside the film.

Handling - the coating is protected during transport, lifting and stringing. Any damage found is recoated immediately; pipe with unrepaired coating damage is not installed.

Joint coating and repair - repairs use the same structure and materials as the pipe body and are carried out after butt welding, once the pipe surface has cooled below 50 ℃.

Backfilling - backfill is placed so that the external coating is not scratched or crushed by rock or by the machine bucket.

Confined areas such as valve chambers and sleeves need artificial ventilation during coating, both for solvent control and to keep the film drying properly.

Inspection and Acceptance Criteria

Appearance - coated pipes are inspected one by one. The surface must be smooth with a bright paint film, uniform in appearance, free from wrinkles, hollows and lumps; the glass cloth reinforcement must be completely saturated with paint over the whole face.

Thickness - one pipe in every twenty is sampled. Three sections are measured on each sampled pipe, at both ends and in the middle, with four readings per section taken at the top, bottom and both sides. An electronic dry film thickness gauge is used and the lowest reading must still satisfy the specified anti-corrosion grade.

Continuity - every pipe receives a DC holiday test with a spark detector. No spark-over is permitted, which is what proves that pits, pinholes and mechanical damage have been found. For the reinforced external system the test voltage is normally 2500 V to 2800 V, matching a dry film thickness of about 450 µm.

Thickness and continuity are recorded together because a thick film that sparks is still a reject, and a film that passes the spark test at a low voltage may simply be too thin to meet the design life.

FAQ

Q: What coating is used on the inside of the pipe?
A non-toxic epoxy zinc-rich paint applied as two primer coats followed by two finish coats, with each coat applied only after the previous one has dried and all misses repaired.

Q: What does four coats and three cloth layers mean?
It describes the external coal tar epoxy system in which three glass cloth layers are wrapped and fully impregnated between coating passes and finished with surface coats, giving a total dry film thickness of not less than 200 µm.

Q: What surface preparation is required?
The specification calls for Sa 2.5 near-white cleanliness assessed to ISO 8501-1, with oil, dust, welding slag, mill scale and loose rust removed and the steel dry and showing a metallic lustre. Hand and power tools are used for local repairs and touch-up.

Q: How is coating thickness verified?
One pipe in twenty is sampled, three sections per pipe and four readings per section, using an electronic dry film thickness gauge. The thinnest reading must still meet the anti-corrosion grade specified for the line.

Q: What holiday test voltage is used?
Each pipe is tested with a DC spark detector at 2500 V to 2800 V and no spark-over is allowed; that voltage range corresponds to the reinforced system at roughly 450 µm dry film thickness.

Q: When can a damaged coating be repaired?
After butt welding, once the pipe surface has cooled below 50 ℃, using the same coating structure and materials as the pipe body and the surrounding factory coating.