Surface Preparation for Pipeline Field Joints and Corroded Welds

Why Field Joints Are Particularly Difficult

A pipeline field joint presents several surface preparation challenges that do not exist at a fabrication facility:

The heat-affected zone. Welding creates a heat-affected zone (HAZ) on either side of the weld bead where the thermal cycle changes the metallurgical structure of the steel and produces iron oxide scale at the surface. This HAZ scale is chemically and mechanically different from mill scale on parent metal — it is more tightly adherent and requires more aggressive mechanical action to remove. Any surface preparation method that leaves HAZ oxide in place risks coating adhesion failure at the weld — precisely the highest-stress location on the joint.

Weld spatter. Manual metal arc (MMA) and flux-cored arc welding produce weld spatter — small solidified metal droplets that adhere to the pipe surface adjacent to the weld bead. Spatter must be removed before coating because it creates point contacts under the coating film that act as stress concentrators and early corrosion sites. Spatter is harder than the surrounding parent metal and resists most surface preparation tools unless approached at the correct geometry.

Differential corrosion at the joint. On an in-service pipeline undergoing rehabilitation, the field joint area typically shows a different corrosion grade than the adjacent coated pipe. The coating cutback zone and bare weld area may have been exposed for years — often without cathodic protection coverage — resulting in ISO 8501-1 Grade C or Grade D corrosion at the joint while the coated parent pipe remains at Grade A or B beneath its coating. The preparation method must address the worst-case surface condition at the joint.

Restricted access and confined geometry. Field joints are prepared in the trench during construction, or at elevated or restricted locations during above-ground pipeline maintenance. Blast equipment — compressors, blast pots, containment shrouds — cannot always be deployed in these geometries. In ATEX Zone 1 classified areas, abrasive blasting is categorically prohibited.

Applicable Preparation Standards

The surface preparation standard for pipeline field joints depends on the field joint coating system and the pipeline owner’s specification:

Pipeline owner specifications such as NACE SP0169 (external corrosion control of underground metallic piping), DNV RP-F102 and company-specific coating specifications govern the requirements. Always confirm against the project specification — do not assume a universal standard applies across pipeline operators or coating systems.

Why Abrasive Blasting Is Often Unavailable at Field Joints

In-trench construction. During new pipeline construction, field joints are prepared in the trench immediately before coating. Deploying a blast pot and compressor in a 1.5–2 m trench, with grit containment and recovery, is operationally impractical on a moving pipeline construction spread where coating must keep pace with welding. Mechanical tools that produce no grit and require minimal setup are the standard solution.

ATEX Zone 1 environments. Gas pipeline compressor stations, processing terminals, offshore platforms and petrochemical facilities are classified ATEX Zone 1 or Zone 2. Abrasive blasting — which generates friction and impact sparks, and disperses grit that can become a fuel-air-solid ignition risk — is prohibited in these environments. The pneumatic Bristle Blaster® (certified Ex II 2G c IIA T4 X) is the confirmed ATEX Zone 1 solution for SP10 preparation. For Tercoo® ATEX Zone 1 certification status, confirm directly with MontiPower.

In-service rehabilitation. Rehabilitating the external coating of an in-service gas or oil pipeline above ground requires working around the operating pipe. Grit blasting on a live operating pipeline presents an additional operational risk from grit ingestion into instrumentation, valve bodies and flanges adjacent to the work area. Grit-free mechanical preparation eliminates this risk.

The Two-Step Procedure for Pipeline Field Joints

Step 1: Surface assessment and classification

Before beginning preparation, assess each field joint visually and record:

• ISO 8501-1 rust grade of the bare metal area
• Presence of weld spatter, HAZ scale and condition of weld cap
• Any existing coating at the cutback zone — condition and adhesion
• Evidence of mechanical damage, gouging or previous repair

Photograph each joint before preparation. This documentation is typically required by the pipeline owner’s inspection protocol and provides baseline data for the quality record.

Step 2: Tercoo® disc pass — corrosion and contamination removal

For field joints with Grade C/D corrosion, weld spatter, HAZ scale or deteriorated coating remnants, fit the Tercoo® disc to the Bristle Blaster® drive unit and apply in systematic passes over the entire joint preparation zone. The Tercoo® disc pass removes:

• Heavy rust and laminar corrosion at the bare metal zone
• Weld spatter from the parent pipe surface adjacent to the weld bead
• HAZ oxide scale in the heat-affected zone on both sides of the weld
• Loosely adherent coating fragments at the cutback edge

Work methodically around the pipe circumference. On small-diameter pipe, complete the lower quadrant before repositioning. Swap to the Bristle Blaster® belt on the same drive unit — approximately 30 seconds — and follow immediately in the same working area.

Step 3: Bristle Blaster® belt pass — SP10 finish and anchor profile

After the Tercoo® disc pass has cleared the bulk corrosion and contamination, fit the Bristle Blaster® belt to the same drive unit and apply to the same area. Work in overlapping passes around the circumference, maintaining consistent contact and pressure. The wire tips impact the cleared steel to create the SP10 cleanliness level and the angular anchor profile in the 65–85 µm Rz range.

Pay particular attention to the weld bead itself and the immediate HAZ zone — the irregular topography at the weld cap requires multiple passes and careful tool positioning to ensure full contact with the surface. The Bristle Blaster® tip geometry allows effective preparation of the weld bead profile that flat tools cannot reach consistently.

Step 4: Visual and profile verification

After the Bristle Blaster® pass, verify the prepared surface against the joint coating specification:

• Visual cleanliness: compare against SSPC-VIS 1 for SP10 — minimum 95% of surface free of all visible residues
• Anchor profile: apply Testex Press-O-Film® X-Coarse replica tape (range 40–115 µm) per ASTM D4417 Method C at three to five locations around the joint circumference; subtract 50 µm tape substrate; target 65–85 µm Rz (or the range specified by the field joint coating manufacturer)
• Soluble salt: measure per ISO 8502-6 / ISO 8502-9 at the joint and parent pipe areas; confirm against the project specification maximum (typically 20 mg/m² chloride equivalent for buried pipeline service)

Record all readings on the joint quality record. Do not proceed to coating application until all parameters are within specification.

Step 5: Apply field joint coating promptly

Flash rusting on mechanically prepared steel begins within 30–60 minutes in humid or marine atmospheric conditions. Apply the joint primer or base coat within the minimum window specified by the coating manufacturer — often 2–4 hours in standard conditions, less in aggressive environments. If the joint cannot be coated within the specified window, the surface must be re-prepared.

Documented Field Applications

Frequently Asked Questions

Can the Two-Step Method meet the preparation requirements of a pipeline owner’s specification that references NACE SP0169 or a company standard?

NACE SP0169 references the surface preparation standard as specified by the coating manufacturer or the pipeline owner’s coating specification — it does not prescribe a specific method of achieving that standard. Where the coating manufacturer’s data sheet permits mechanical preparation to SSPC-SP10, the Two-Step Method is a compliant preparation method. The documentation requirements — visual comparison records, Testex tape readings, soluble salt measurements — are the same regardless of whether SP10 was achieved by blasting or by the Two-Step mechanical method. Confirm with the pipeline owner’s inspection engineer before proceeding if any doubt exists.

How is the Bristle Blaster® used on the curved surface of a pipe?

The Bristle Blaster® can be applied to pipe surfaces of any diameter. On large-diameter pipe (above 400 mm / 16 inch), the flat wheel face contacts the curved surface adequately for full preparation coverage. On small-diameter pipe (below approximately 150 mm / 6 inch), the tool operator maintains contact by following the curvature — the wheel edge contacts the pipe rather than the full face, requiring more overlapping passes to ensure complete coverage. The Bristle Blaster® has been used in documented pipeline applications from 6-inch transmission pipe to large-diameter offshore risers.

What is the correct soluble salt limit for a buried pipeline field joint?

The soluble salt limit for buried pipeline service is typically set by the pipeline owner specification or the field joint coating manufacturer. A common specification reference is 20 mg/m² chloride equivalent (measured by Bresle patch / ISO 8502-6 and ISO 8502-9). Some specifications also set limits for sulphate contamination. Where the project specification does not define a specific limit, the coating manufacturer’s data sheet should be consulted — this is the binding technical reference for coating system performance and warranty.

Does the Two-Step Method work on pipe in the ditch during new construction?

Yes — the Two-Step Method was developed in part to address the field joint preparation challenge in construction trenches where blast equipment cannot be deployed. The Tercoo® disc and Bristle Blaster® belt run on the same drive unit — a single hand-held machine that can be operated in a trench at grade, overhead on elevated pipe, or in other restricted geometries that are impractical for blast equipment. The pneumatic drive unit requires only a compressed air supply — typically already present on the coating spread for other tools.