Coatings and Wrapping for Corrosion

Coatings and wrapping systems protect buried and submerged infrastructure from corrosion by creating physical barriers between metal surfaces and corrosive environments. These systems work through multiple mechanisms: polymer coatings block moisture and oxygen penetration, cathodic protection wrapping provides sacrificial anodism, and fusion-bonded epoxy creates chemically resistant barriers. Properly applied coating systems can extend pipeline service life from 20-30 years to 75-100 years in municipal applications. However, coating effectiveness depends entirely on application quality and substrate preparation - even minor holidays or disbondment can accelerate localized corrosion beneath the protective layer.

• Raw Water Intake Structures: Steel intake pipes and screens (24"-72" diameter) receive fusion-bonded epoxy (FBE) or coal tar enamel coatings to withstand aggressive raw water conditions. Selected for 30-50 year design life in buried or submerged environments where replacement costs are prohibitive

• Clarifier Mechanisms: Steel center columns, rake arms, and drive assemblies use polyurethane or vinyl ester coatings in primary and secondary clarifiers. Critical because these components operate continuously in corrosive mixed liquor (pH 6.5-8.5) where downtime costs $10,000-50,000 per day

• Digester Structures: Anaerobic digester steel components receive specialized H2S-resistant coatings like glass flake epoxy or polyurea. Essential due to hydrogen sulfide concentrations of 1,000-3,000 ppm that rapidly corrode unprotected steel

• Force Main Piping: Ductile iron and steel force mains (6"-36" diameter) use polyethylene wrapping or liquid epoxy coatings for soil-side protection, with cement mortar linings for internal protection against sewage at velocities of 2-8 fps

Daily Operations: Visual inspection of coated surfaces during routine rounds, checking for chalking, cracking, or disbondment. Monitor cathodic protection readings on wrapped piping systems. Document coating condition in maintenance management systems. No active process control required, but early detection prevents costly repairs.

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Maintenance: Annual detailed inspections using NACE coating condition assessment protocols. Touch-up repairs require surface preparation, primer, and topcoat matching original system. Full recoating intervals: 15-25 years for immersion service, 10-15 years atmospheric. Requires confined space entry permits, respiratory protection (half-face minimum), and certified coating applicators for warranty compliance.

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Troubleshooting: Coating failures manifest as rust bleeding, blistering, or delamination. Premature failure (under 10 years) indicates inadequate surface prep, contamination, or wrong coating selection. Holiday detection reveals pinholes.

• Surface Preparation Equipment: Blast cleaning systems (SSPC-SP10 near-white metal) using steel grit or garnet media. Municipal projects typically require 2-4 mil profile depth. Selection based on coating manufacturer specifications and substrate condition

• Primer Systems: Zinc-rich primers (75-95% zinc content) for galvanic protection, or epoxy primers for barrier protection. Typical dry film thickness 2-5 mils. Zinc primers selected for immersion service; epoxy for atmospheric exposure

• Topcoat Materials: Polyurethane, epoxy, or polyurea systems with 5-20 mil thickness. Polyurethane for UV resistance, epoxy for chemical resistance, polyurea for rapid cure (30-60 seconds). Selection based on service environment and application window

• Wrapping Systems: Polyethylene tape (20-40 mils) with butyl adhesive for buried piping, or shrink sleeves for irregular shapes. Include cathodic protection integration points and holiday detection compatibility

• Quality Control Tools: Holiday detectors (wet sponge or spark testing), dry film thickness gauges, and adhesion test equipment for ASTM D4541 pull-off testing to verify 350+ psi bond strength

• Pipeline Parameters: Pipe diameter: 4-72 inches (typical municipal distribution/transmission), Operating pressure: 50-200 psi (distribution), 200-400 psi (transmission), Service temperature: 32-140°F (ambient to heated service), Soil resistivity: 500-50,000 ohm-cm (affects coating selection)

• Coating Thickness Requirements: Fusion-bonded epoxy (FBE): 12-16 mils (AWWA C213), Polyethylene tape: 20-30 mils minimum, Coal tar enamel: 1/8 inch minimum, Polyurethane: 40-60 mils for severe environments

• Performance Criteria: Holiday detection voltage: 67.5V per mil thickness, Adhesion strength: minimum 400 psi (ASTM D4541), Cathodic disbondment: <6mm radius after 30 days, Impact resistance: 2-4 J minimum (ASTM G14), Service life expectancy: 50+ years for transmission, 30+ years distribution

• Environmental Factors: Soil pH range: 4.5-10.0, Groundwater chloride content: <500 ppm preferred, Burial depth: 3-15 feet typical

• What coating system matches the corrosion environment severity? Soil resistivity <2,000 ohm-cm or pH <5.5 requires premium systems like 3-layer polyethylene or polyurethane. Standard FBE adequate for resistivity >5,000 ohm-cm with neutral pH. Wrong choice leads to premature failure and costly repairs within 10-15 years versus expected 50-year life

• Does the installation method allow proper coating application? Directional drilling requires flexible coatings (polyethylene tape, polyurethane). Open-cut allows any system. FBE cannot handle tight radius pulls <100 pipe diameters. Improper matching causes coating damage during installation, requiring expensive re-coating or replacement

• What holiday detection and repair procedures are specified? Detection voltage must match coating thickness (67.5V/mil). Repair materials must be compatible with base coating system. Inadequate QC allows pinholes that cause localized corrosion and potential pipe failure

• Are cathodic protection requirements integrated with coating selection? High-resistivity coatings reduce CP current requirements from 1-5 mA/ft² to 0.1-0.5 mA/ft². Disbondment-prone coatings interfere with CP effectiveness. Poor coordination leads to inadequate protection or coating damage from excessive current

• Primary: 33 05 23 - Trenchless Utility Installation

• Secondary: 33 11 00 - Water Utility Distribution Piping (for pipe coating specifications)

• Related: 33 71 02 - Cathodic Protection (for coordinated corrosion control systems)

• Material/Equipment Verification: Surface preparation standards (SSPC-SP6 minimum for steel), Environmental conditions during application (temperature, humidity limits), Coating thickness verification with wet film gauges

• Installation Requirements: Certified applicator requirements per manufacturer specifications, Surface preparation equipment (sandblasting, power tools), Environmental controls (tenting for weather protection)

• Field Challenges: Weather delays extending project schedules 2-4 weeks, Achieving proper surface preparation on existing structures, Coating compatibility with existing systems

• Coordination Issues: Scheduling around plant operations for in-service repairs, Lead times: 2-3 weeks for standard coatings, 6-8 weeks for custom colors

• 3M - Scotchkote 206N liquid epoxy for steel pipes and structures, widely specified in municipal applications

• Denso North America - Petrolatum tape systems like Densyl for buried pipelines

• Carboline - Carboguard 890 series for concrete structures and Carbothane urethane topcoats

• Sherwin-Williams Protective & Marine - Macropoxy 646 Fast Cure for municipal tank interiors and Acrolon 218HS for exteriors

• Cathodic Protection Systems - Preferred for buried pipelines over 1,000 LF; higher upfront cost but 30+ year life versus 15-20 years for coatings

• Stainless Steel Cladding - Cost-effective for high-wear areas like valve vaults; 3-4x coating cost but eliminates maintenance

• Fiberglass Reinforced Plastic (FRP) - Excellent for chemical exposure; 2x epoxy cost but superior chemical resistance for chlorine contact structures

Establish relationships with certified applicators early - qualified contractors are limited in most markets. Request sample panels for large projects to verify color match and performance. Consider bulk purchasing for multi-year programs to reduce costs 15-20%. Schedule coating work during optimal weather windows (spring/fall) to avoid temperature and humidity delays that can double project duration.