Condition Assessment and Environmental Ranking of Weathered T&D Structures and Equipment; Evaluation and Testing of Coatings
Condition Assessment and Environmental Ranking of Weathered T&D Structures andEquipment; Evaluation and Testing of Coatings for Electrical Transmission and Substation Structures and Equipment
Topics For Today• Overview and History of T&D Coatings.• Focus on: Above-grade lattice Towers, Transmission Poles, Substation Structures.• Aged Galvanized Steel, Overcoating Aged Coatings, Rusted Carbon Steel Structures.• Evaluate Substrates and Environment.• Evaluate Coatings Specified.
The Basic Idea• Create a working maintenance painting program for electric utilities to protect hundreds of thousands of T&D assets utilizing a full survey of the environmental corrosion potential, the substrate conditions, and the priorities of the owner; normally resulting in specifications for one- coat systems, or minimal sealing/priming, after minimal (SSPC-SP2) Surface Preparation (maximum SSPC-SP3)..above-grade.
Common Coating Systems• One-coat coating systems for weathered galvanized steel. (SSPS-SP2)• Spot prime with rust penetrating sealant and full coat of one-coat tower paint. (SP2) and/or spot SP3 cleaning.• Full prime or penetrating sealant and one full coat of tower paint. (SSPC-SP3 full or partial as required)
Towers by Install Date USA30000250002000015000100005000 0 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Cost Effective Approach• Also for Communication towers, Railroad and highway bridges, highway sign posts, guard rails, chain-link fencing, quonset huts, some metal roofs, pre-fab buildings, and distribution poles. The list is limitless; and massive cost savings to the owners and tax-payers, and much more sustainable than replacement with new galvanized or painted carbon steel.
Back to the Future• Oldest surviving basic technology to still be sole- sourced since 1955. Many high-performance coatings developments are tied to significant cleaning and surface preparation requirements, which sometimes trigger extreme controls on environmental costs (full-containment) and other expensive requirements. Lowest cost per square foot per year @ 20-30 years life.
Ancient History• Resin System=Drying Oils= Pressed flax-seed oil= Linseed Oil+ modification=Excellent wetter.• In use for hundreds of years; first used as varnish.. Absorbs oxygen from the air and cures outside/in, or top down. 30 days to through dry and 1 year to harden enough for accelerated testing.• Originally used with lead pigments as industrial coating + barrier pigments.
Basics of This Type of Approach• Minimize Cleaning and Surface Preparation Costs.• Minimize Removal of Sound Existing Coatings.• Minimize Generation of Hazardous Waste.• Minimize Costs associated with full containments, air-fed respirators, etc.• Maximize corrosion protection with minimal life- cycle costs= Functional protection vs. aesthetics.
T&D Maintenance Program• Assess Stage of Aging • Survey Structures Past (New, Partially “Optimal Window of Weathered, toast. Opportunity.”• Predict “Time to First • Digitize data base, Maintenance” of all pictures, and T&D structures. spreadsheets.• Create Pro-Active • Continuously Monitor Corrosion Protection your Program. Program.
Standards, Classifications, Rankings, Visual Guides vs. 1980s (none)• ISO 9223: This standard classifies the corrosivity of an atmosphere based on measurements of time of wetness, and pollution categories (sulfur dioxide, airborne chlorides), UV, et.al..• ISO 12944: Basic C-1 through C5i and C5m.• NACE: Standards, Surface Prep Standards
More Standards Organizations• NACE: Standards, Surface Prep Standards• ASTM: Test Procedures, Standards (D610 -08), adhesion D-3359, et. al..• IEEE: Joint Standards with NACE for T&D!• EPRI: Scientific Research & advanced equipment.• SSPC: QP Programs, Surface Preparation Standards (SSPS-VIS2 and SSPC-VIS3)
NACE/IEEE• STG 41 - Electric Utility Generation, Transmission, and Distribution• TG 395 - Atmospheric (Above-Grade) Corrosion Control of Transmission, Distribution, and Substation Structures by Coating Systems• TG 386 - Below-Grade Corrosion Control of Transmission, Distribution, and Substation Structures by Coating Systems
Life-Cycle of New Galvanized• Removed from molten zinc bath: Phase N1• Pure zinc layer reacts with oxygen(O2)> forms zinc oxide (OH)2: First 48 hours. Phase N2• Zinc oxide reacts with moisture, and forms zinc hydroxide: Zn (OH)2. 48 hours to 6 months. Phase N3
Life-Cycle of New Galvanized• Zinc hydroxide exposed to more O2 and carbon dioxide (CO)2, and forms Zinc Carbonate. 2ZnCO3> Zn(OH)2. Now has a stable oxide (protective oxide or patina layer for up to 2 years zinc surface is active). Phase N4• “Aged galvanized” > 2 years.
Categories of Aged Galvanized• Up to 2 years: New (Special methods to clean and prepare surface) = “New Galvanized”= Phase A1• From 2 years to first signs of corrosion: Most of original galvanized layer remains; minimal to no zinc-iron alloy staining or rust is evident. Surface may stay this way for over 20 years. = “Partially Aged Galvanized,” or Phase A2.
Categories of Aged Galvanized• Aged Galvanized (First signs of corrosion pattern; such as edges. Galvanized layer becomes thinner; up to 10% rusting.= “Aged Galvanized,” Phase A3.
Condition Assessment• 10-50% Rusting and less than 2 mils of zinc-iron alloy layer remaining. Lots of staining and abrasive rusting. = Phase A4• Greater than 50% rusted. Minimal to zero zinc-iron alloy layer remaining. Staining and rust cover > 90% of the structure. = Phase A5• Beyond Condition 5 the structure may need to be repaired or replaced. Heavy pitted rusting over the entire structure. Phase A6
C1 Rural areas, low pollution. HeatedC2 buildings/neutral atmosphere.C3 Urban and industrial atmospheres. Moderate sulphur dioxide levels. Production areas with high humidity.C4 Industrial and coastal. Chemical processing plants.C51 Industrial areas with high humidity and aggressive atmospheres.C5m Marine, offshore*, estuaries, coastal areas with high salinity.
TOW: Time of Wetness• TOW units are hours per year (hours/year) when relative humidity (RH) > 80% and the temperature > 0oC.• TOW <= 10 T1• 10 < TOW <= 250 T2• 250 < TOW <= 2,500 T3• 2,500 < TOW <= 5,500 T4• 5,500 < TOW T5
Sulphur Dioxide (SO2) Levels• SD <= 10 P0• 11 < SD <= 35 P1• 36 < SD <= 80 P2• 81 < SD <= 200 P3• The units used for the sulfur dioxide categories in the ISO 9223 are as sulfate deposition (SD) rate in mg m-2 day-1.
Airborne Chlorides• The units used for the chloride categories (airborne salinity) in the ISO 9223 are as chloride deposition (CD) rate in mg m-2 day-1:• CD <= 3 S0• 4 < CD <= 60 S1• 61 < CD <= 300 S2• 301 < CD <= 1,500 S3
Atmospheric Contaminants• Atmospheric contaminants such as hydrogen sulfide, hydrogen chloride and chlorine present in the atmosphere can intensify atmospheric corrosion damage, but they represent special cases of atmospheric corrosion, normally related to industrial emissions in specific micro-climates.
Combine the Rankings• For example; if you have a lattice-tower in a C3 environment, and it is a Condition Ranking of A3, a Specification for surface preparation and coating would be generated by that combination:• SSPC-SP2 Hand Tool Cleaning• Primer/Finish: One coat of modified linseed oil MIO/Zinc dust tower paint applied at 8-10 mils= 20-25 years of corrosion protection.
UV Precipitation Acid Rain Airborne Air Chlorides INDEX TOW INDEX Corrosives Pollution Specific Index SourceCoastal Low Medium-High Medium P&P Mill Low- HighMaine MediumVegas Very Very Low Very Low HighLosAngelesNorth VeryPole LowChinaCaribbeanTexas High Low Low High High High
More Severe Combinations• S3 Ranking for Airborne Chlorides= Severe.• P3 Ranking for So2=Severe.• T5 Ranking for Time of Wetness= Severe.• UV5 Ranking For Ultra Violet Rays= Severe.• X5 Ranking for Other Atmospheric Contaminants= Severe.• Severe rankings in all categories are rare.
Micro-environments= Isolated Environmental Contaminants• Specific to small numbers of towers on a long line.• Chemical Plants• Pulp & Paper Mills• High-Humidity Low Rain Areas• May require more chemical resistant coatings; minor % of the total.
Essential Properties of Coating• Maximum wetting and penetration into tight corroded spaces.• High-build; 9- 11 mils WFT in one coat= 8-10 mils DFT.• Easy to apply by mitt or pound brush.• Maximum barrier properties to resist moisture, airborne chlorides, chemicals, air pollutants, and ultra-violet light.
Benefits of Penetrating Sealers• Tie-down existing old coatings; wick under loose edges and into tight crevices.• Very low viscosity.• No curing stresses that pull off old coatings.• 100% SBV• Tougher and more moisture and chemical resistant than alkyd primers.
MIO as Barrier Pigment• Chemically Inert and very resistant to pollution.• Does not react with high or low pH. (acid and alkaline resistant)• Labyrinth Effect and Shielding from UV, pollution, and especially moisture.• Absorbs UV and warms the coating; hence drying.• Promotes adhesion to substrate and itself.
MIO: 100 Years of Success• Is Non-Toxic, Non-Oxidizing, Non-Corrosive & Non-Flammable. With such excellent chemical & environmental properties, it is not a surprise that it is the world’s most favored barrier pigment for over 100 years. Formulators consider MIO to be a key weapon in their anti-corrosive arsenal.
MIO as Barrier Pigment• Improved mechanical properties.• Less cracking• Tougher coating all around.
Properties of Ceramic Microspheres• Reduces Undercutting and improves resistance to cathodic disbonding• Enhances Film-Build and especially edge retention on towers.• Lowers Permeability of the paint film.
Contractor Qualifications•Adult CPR, AED, and Community First Aid•High Voltage Electrical Safety for Power Generation, Transmission, &Distribution (required OSHA rule 1910.269)•Fall Protection•Tower / Pole Rescue•OSHA 10-hour Safety Training•Lead Awareness and Hazard Communication•*OSHA 30-hour Supervisor Safety Training•Lead Removal and Safe Operating Procedures•Hazardous Waste Operations and Emergency Response•* Lift / Boom Operations and Safety•* CCS-lhf1 and Permit Required Confined Space certification•* SSPC C-3 Supervisor / Competent Person•* NACE Certified Coating Inspector
Tower Footings• 1930s and 1940s: Dipped in hot tar. No longer specified. Under-film corrosion. Coal-tar epoxy used in 1960s-1980s. Not often specified; also strong cohesion vs. sub-film corrosion. Results improved when 110 micron HDG was increased to 200 microns. More awareness of conductivity (soil resistivity) pH values, types of soils, et. al.. 0.5 mm zinc plates welded to tower legs on all sides.
Tower Footing History• Zinc plates welded plus hot tar dips.• Tapes used 1955-1980s: Freeze-thaw issues.• Copper earth mats: tower leg becomes the anode= 500 microns per year of corrosion.• Moisture-cured coal-tar urethanes frequently used today (limitations on surface preparation costs and time)