Zinc Rich Primers for Corrosion Protection in Marine Environments J. Peter Ault, P.E.Elzly Technology CorporationZinc-Rich Coating Types Type 1: Inorganic Zinc Coatings 1-A: Inorganic post-cure (water-soluble) 1-B: Inorganic self-cure (water-reducible) 1-C: Inorganic self-cure (solvent-reducible) Type 2: Organic Zinc-Rich Coatings Zinc Rich Epoxy Zinc Rich Moisture Cure Urethane Other, less common Organic Zinc-richZinc-Rich Coatings Coatings with greater than 65% zinc by weight in the dried film are considered Zinc-Rich Zinc content is important, SSPC Paint 20 defines 3 levels of zinc in the dried filmLevel 1 equal to or greater than 85%Level 2 equal to 77% to 85%Level 3 equal to 65% to 77% Zinc particle size and purity are also issues which may impact performanceInorganic Zinc Powdered metallic zinc Mixed with a reactive silicate solution Ethyl Silicate Potassium Silicate Other Alkalai Silicates Zinc becomes part of the film Not merely encapsulated in a binder Resists higher temperature 750 F/ 400 CInorganic ZincCrosssection of IOZ after applicationSandblasted surfaceCourtesy of Corrosion Prevention of Protective Coatings, C.G. Munger, 1984, NACE InternationalInorganic Zinc: Type 1-A Two component material zinc powder and waterborne silicate Post Cure Cures by subsequent application of a curing compound Acidic postcuring solution is common Limited use todayInorganic Zinc: Type 1-B Two component material zinc powder and waterborne alkali silicate After water evaporates from coating, carbon dioxide from the air reacts with the silicate to cure coating Surface alkalinity created during cure may interfere with topcoat adhesionInorganic Zinc: Type 1-C Two component material zinc powder and solventborne ethyl silicate Depends on moisture in atmosphere to complete hydrolysis Do not cure well at low temperatures or low relative humidities Cure may be accelerated by light misting with waterOrganic Zinc-Rich Available as one, two- or three-package products Two component epoxy with zinc dust Moisture cure urethane with zinc dust Performance related to zinc content in cured dry film Must be low enough for binder to create a uniform film Greater than 85%/90% recommended Performance also related to the binder material Zinc content in dry film is not the same as zinc content in wet productZinc Rich CoatingsCourtesy of Corrosion Prevention of Protective Coatings, C.G. Munger, 1984, NACE InternationalNOTE CRITICAL PARTICLE-TO-PARTICLE CONTACTApplication Considerations Zinc Dispersion in Film Proper mixing during application Weathering of Zinc Primer prior to topcoating Secondary surface preparation Degree of Cure Especially for IOZ silicate Resists chemicals, to a degree (zinc is amphoteric reacts with acids and bases)DoD Specifications for Zinc Rich PrimersA-A-59745, Commercial Item Description, Zinc-rich Coatings Pertinent Characteristics 90% or greater zinc content Capable of being applied by spraying or brushing Compatible with CARC primers Performance Requirements Accelerated corrosion (cyclic) Adhesion pull-off Adhesion cross-cut Mandrel bend Typically Moisture Cure Urethane BinderZinc Rich Coatings for Navy Ship Use DOD-P-23236, Type 3 (circa 1982) Inorganic Zinc Ch 631 (1982) specified for use in chain lockers, non-floodable voids and floodable but normally empty voids CCIMS lists tanks with zinc rich coatings MIL-DTL-24441, formula 159 Epoxy zinc rich Approximately 91% zinc in dry film by weight Pre-construction primer Thin film build, lower zinc loadsPerformance Of Zinc Rich Coatings in a Marine Environment5-Year Marine ExposureW/O Zinc Primer W/ Zinc Primer19-Months Simulated Catapult TroughZinc Containing PrimerWithout Zinc Containing Primer18Zinc Rich Primers for VehiclesZinc-RichEpoxy/CARCZinc-RichCARCEpoxy/CARCControlZinc-RichOnly Two organic zinc rich coatings (MCU) under conventional CARC coating system GM 9540P testing of panels with coating damage 9-year exposure of 5-ton truck doors at Cape CanaveralZinc Rich Primers for VehiclesEpoxy Primer OnlyZinc Rich AZinc Rich B Galvanized SheetZinc Rich Primers for VehiclesZinc-RichEpoxy/CARCZinc-RichCARCEpoxy/CARCControlZinc-RichOnly1 year9 yearsZinc Rich Primers for VehiclesZinc-RichEpoxy/CARCZinc-RichCARCEpoxy/CARCControlZinc-RichOnly1 year9 yearsHow Do Zinc Rich Coatings Work?22BarrierSacrificial???? Perform a variety of experiments Polarization resistance Open Circuit Corrosion Potential Electrochemical Impedance Spectroscopy Galvanic Current (Interaction with steel)Electrochemical Data23Effect of Environment00.10.20.30.40.50.60.70.80.90 5 10 15 20 25Current -mAElapsedTime - DaysGalvanic Current FlowSeawaterTap Water24Effect of Environment250.30.40.50.60.70.80.910 5 10 15 20 25Volts vs SCEElapsedTime - DaysTap Water CellE(off), zincE(off), steelCouple Potential 0.30.40.50.60.70.80.910 5 10 15 20 25Volts vs SCEElapsedTime - DaysSeawater CellE(off), zincE(off), steelCouple Potential Polarization Behavior(9 days in seawater)26-1.200-1.000-0.800-0.600-0.400-0.2000.0001.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00Potential vs SCE -VoltsCurrent - AmpsGalvanizedZinc AZinc BZinc CZinc DEvans Diagram of Steel-Zinc Couple27Based on data in "Atlas of Polarization Diagrams for Naval Materials in Seawater," Harvey P. Hack, April 1995-1200-1000-800-600-400-20001.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03Potential, mV vs Ag/AgClCurrent - mAzincsteel1/100 zincEffect of Metallic Surface Area Ratio28-0.100.10.20.30.40.50.60 5 10 15 20 25 30 35Steel Polarization -VTime - DaysEffect of Area Ratio(Zinc Surface as percent of Steel Surface)0.17%0.44%0.44%0.7%1.8%2.7%7.1%7.1%11%28%44.4%100%How Much Zinc is Effectively Protecting the Steel? Many Variables Binder type Packing factor Zinc content Defect geometry Zinc coating thickness Electrolyte conductivity Zinc particle purity Zinc particle size Surface chemistry29Electrochemical Observations Lower resistivity electrolyte Better sacrificial protection, shorter life Over time, steel polarization decreases Indicative of zinc consumption Zinc coatings behave differently Need to determine sources of differences Is specifying zinc content sufficient?30Summary/Conclusions Zinc rich coatings can help increase coating service life in marine atmosphere exposure Zinc rich coatings have unique issues which must be considered Todays coating community is more sophisticated than 30 years ago