AMPP Pipeline Coatings cheat sheet

A corrosion cell requires four elements: an anode, a cathode, an electrolyte, and an electron-conducting metallic path joining anode and cathode; removing any one element stops the electrochemical reaction.

Anode (oxidation): Fe → Fe²⁺ + 2e⁻. Cathode in neutral or alkaline water: O₂ + 2H₂O + 4e⁻ → 4OH⁻; in acidic water: 2H⁺ + 2e⁻ → H₂.

A pipeline coating protects by three mechanisms: a barrier to water, oxygen, and ions; adhesion that resists disbondment; and high electrical resistance that limits galvanic current. Inhibitors are a fourth, less common mechanism for line pipe.

Cathodic protection polarizes the steel to a more negative potential (commonly −850 mV vs CSE, IR-free), and the coating's high resistivity reduces the CP current required; coating and CP are complementary, not alternatives.

Cathodic disbondment: OH⁻ generated at the cathode raises pH and attacks the bond at a coating holiday; resistance depends on coating chemistry, adhesion, and temperature, and is evaluated by ASTM G8/G95 or CSA Z245.20.

Coating permeation is governed by water vapor transmission rate and oxygen transmission rate; lower permeation slows underfilm corrosion but does not compensate for poor adhesion.

Soil resistivity is a key corrosivity indicator; low-resistivity (for example below 2000 Ω·cm), high-chloride, or sulfate-reducing-bacteria-active soils are aggressive and demand a robust coating plus CP.

Faraday's law: metal loss (g) = (I × t × M)/(n × F), where I is current, t is time, M is atomic mass, n is electrons transferred, and F is 96,485 C/mol; it links CP current demand to predicted metal loss.

Fusion-bonded epoxy (FBE): single layer, typically 250–500 µm (10–20 mil), applied to preheated steel; excellent adhesion and high-temperature capability; the main risk is cathodic disbondment at damage.

Three-layer polyethylene (3LPE): FBE primer plus copolymer adhesive plus extruded PE topcoat; widely used for buried onshore lines; upper service temperature is typically about 60–85 °C depending on grade.

Three-layer polypropylene (3LPP): the same architecture with a PP topcoat; higher service temperature (often up to about 110–140 °C) and better mechanical and abrasion resistance than PE.

Dual-layer FBE (DLFBE): an FBE base plus a tough FBE or elastomeric overlay; used where abrasion or impact is expected; an all-epoxy system with no thermoplastic layer.

Coal tar enamel: a legacy hot-applied coating with good water resistance; it contains polycyclic aromatic hydrocarbons and is largely phased out for health, safety, and environmental reasons.

Tape coatings: cold- or hot-applied polyethylene or polypropylene tapes with butyl or mastic adhesive; sensitive to application tension and overlap, so quality depends heavily on field workmanship.

Concrete weight coating: applied for negative buoyancy and mechanical protection on subsea and water-crossing pipelines; it is not the primary corrosion barrier, so the corrosion coating beneath remains essential.

Liquid-applied coatings (epoxy, polyurethane, polysiloxane, acrylic) are used for above-ground piping, stations, and field joints; selection depends on UV exposure, temperature, and chemical resistance.

Selection drivers include operating temperature, design life, soil and water corrosivity, CP compatibility, mechanical loads during installation, field-joint compatibility, and total installed cost.

NACE No. 1/SSPC-SP 5 is white metal blast; No. 2/SSPC-SP 10 is near-white; No. 3/SSPC-SP 6 is commercial; No. 4/SSPC-SP 7 is brush-off. ISO equivalents are Sa 3, Sa 2½, Sa 2, and Sa 1.

Typical anchor profile for FBE is 40–100 µm (1.5–4 mil); 50–75 µm is common. Profile is measured with ISO 8503 comparators or replica tape per ASTM D4417.

Abrasive must be clean, dry, and free of oil and salts; angular steel grit cuts a sharper profile than round shot; ISO 11124/11125 specify metallic and non-metallic abrasive requirements.

Salt contamination is measured by the Bresle method (ISO 8502-6 sampling, ISO 8502-9 conductivity); chloride limits are often 20–50 mg/m² or lower depending on the coating specification.

Blasting should not proceed when the steel surface temperature is less than 3 °C (5 °F) above the dew point, and relative humidity must stay within the product specification, commonly ≤ 85%.

Mill scale, rust, and old coating must be removed to the specified grade; residual mill scale is cathodic to steel and promotes localized corrosion beneath a coating.

Weld spatter, slivers, scabs, and laminations must be ground or repaired before blasting; sharp weld beads need feathering because coatings are thinner over edges.

The hold period between blasting and coating application must be controlled; visible rust bloom or recontamination means the surface must be re-blasted before coating.

FBE is applied by electrostatic spray or fluidized bed to steel preheated, often to 200–260 °C depending on the product, so the powder melts, flows, and cures; gel time and cure are temperature-controlled.

3LPE/3LPP application: FBE primer is sprayed onto heated pipe, then adhesive and topcoat are extruded through a crosshead die or by side extrusion, followed by water quench.

Wet film thickness = dry film thickness ÷ volume solids fraction; for example, 400 µm DFT at 60% solids requires about 667 µm wet film.

Plural-component spray equipment must maintain the specified mix ratio; verify ratio, pot life, and induction time before production, and flush before mixed material gels.

Field-joint coatings must be compatible with the line-pipe coating; typical choices are FBE, heat-shrink sleeves, tape, or liquid epoxy/polyurethane, and joint surface preparation is critical.

Heat-shrink sleeve installation requires preheat, correct overlap onto the mainline coating, and controlled shrinkage with a soft flame or induction; the adhesive must melt and flow.

Coating defects arise from wrong preheat, contaminated abrasive, moisture on the substrate, or incorrect powder; runs, sags, pinholes, and thin edges are common application defects.

Amine blush on epoxy or polyurethane forms in cool, humid conditions; it must be removed before recoating because it blocks intercoat adhesion.

Cure schedules must be followed; forced cure or premature water quench can cause incomplete crosslinking and later disbondment.

Dry film thickness is measured with magnetic pull-off or eddy-current gauges per SSPC-PA 2; Type 1 is magnetic and Type 2 is electronic; readings are taken on a grid and compared with specification limits.

Holiday detection: low-voltage wet-sponge testing for thin films, high-voltage spark testing for thicker coatings; relevant standards include NACE SP0188, NACE SP0490, and ASTM G62.

Adhesion tests: pull-off (ASTM D4541) gives quantitative bond strength; cross-cut (ASTM D3359) and knife tests are qualitative; record the failure mode as adhesive or cohesive.

Cathodic disbondment testing per ASTM G8/G95 and CSA Z245.20 measures disbondment radius at a holiday under impressed CP current at elevated temperature; FBE is often tested 28 days at 65 °C and 1.5 V.

Mechanical tests: impact (ASTM G14), flexibility (ASTM D522), abrasion (ASTM D4060 Taber), and bend tests (CSA Z245.20) assess resistance to handling and installation damage.

Weathering and immersion tests such as salt spray (ASTM B117), QUV, and immersion screening are comparative; they do not directly predict field service life.

Visual inspection looks for runs, sags, pinholes, orange peel, contamination, and incomplete coverage, especially at field joints and weld seams.

Records should capture preheat, blast, and application parameters, batch and lot numbers, environmental conditions, and test results for traceability; nonconforming coating is quarantined.

Key coating standards: ISO 21809-1 (3LPE), -2 (FBE), -3 (3LPP); CSA Z245.20 (plant-applied external coatings); CSA Z245.30 (field-applied); NACE SP0109 (tape); NACE SP0188/SP0490 (holiday detection).

Surface preparation standards: SSPC-SP 5/10/6/7 and NACE No. 1–4; ISO 8501-1 (rust and preparation grades); ISO 8502 (contaminants); ISO 8503 (profile); ISO 11124/11125/11126 (abrasives).

The cathodic protection standard for buried piping is NACE/AMPP SP0169; coating and CP design should be evaluated together rather than in isolation.

Blasting safety: crystalline silica dust causes silicosis; controls include NIOSH-approved respirators, ventilation, and air monitoring; noise and abrasive injury are also hazards.

Coating safety: solvent-based products present VOC and flammability hazards; check the SDS, flash point, and ventilation; FBE powder can form explosive dust clouds.

Environmental controls: spent abrasive and removed coating may be hazardous waste containing lead, chromium, or PAHs; contain blast debris and dispose of it per local regulations.

High-voltage holiday testers are electrical hazards; only trained operators should use them, with equipment isolated and no contact with wet ground or other workers.

Confined space entry into coating tanks or pipes requires atmospheric testing, permits, and rescue planning; H2S may be present in live lines.

Specification limits for temperature and humidity exist for technical reasons; deviations should be documented and approved rather than worked around.