steel coating systems
TRANSCRIPT
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Steel Coating Systems
By: Ivan Silbernagel, P.E.Structure Coatings EngineerODOT Construction Section
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Coating Options
1. Hot-Dip Galvanizing (spec 2530)2. Liquid Coatings (spec 594)3. Powder Coatings (spec 593)4. Duplex Systems (spec 593 & 594)5. Thermal Spray Coatings (AWS)
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1. Hot-Dipped Galvanizing Valmont Coatings Pacific States Galvanizing
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HDG Surface Prep Steps
1. Degreasing - Removes dirt, oils, organic residue
2. Sulfuric Acid Pickling - Removes mill scale and oxides
3. Fluxing - Mild cleaning, provides protective layer
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Sulfuric Acid Pickling
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Galvanizing Process
Immersed in a bath of molten zinc (≈ 830 0F)> 98% pure zinc, minor elements added for coating properties (Al, Bi, Ni)Removed after reaching bath temperature (approx 5 to 7 minutes) Zinc reacts with iron in the steel to form galvanized coatingCreates 4 layers (3 zinc alloys w/ zinc topcoat)
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Galvanizing (Hot-Dipped)44' x 5'3" x 8'9" deep
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Material Handling (racks or hooks)
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Vent and Drainage
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Oversize dipping
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Material thickness
Different thickness– Reach bath temperature at different times– Cools at different rate– Maintaining original shape
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Estimated Service Life of HDG
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ASTM Standards for Galvanizing
Galvanized ProductsASTM A 123 – general ASTM A 153 – small parts (fasteners)ASTM A 767 – rebar
Supporting SpecificationsASTM A 143 – embrittlementASTM A 384 – distortionASTM A 385 – high-quality coatingsASTM A 780 – repairASTM D 6386 – surface prep for painting over
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Hot-Dip Galvanizing
AdvantagesAdvantagesBest protectionBest protectionMetallurgical bondMetallurgical bondQuality controlQuality controlDurableDurableInterior (pipe/tubes)Interior (pipe/tubes)
DisadvantagesDisadvantagesVenting/drainageVenting/drainageSize limitsSize limitsCrevicesCrevicesDeformationDeformationReduced slip coef.Reduced slip coef.Field weldsField welds
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2. Liquid CoatingsStandard Specification 594 covers
– shop and field applications1. "new steel"2. "existing steel" (maintenance)3. "rehabilitating coated steel"4. "non-steel metallic" (galvanized, aluminum)
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New SteelSpec 594– Near white blast– Shop application– 3 coat system from QPL
"Shop Coating" - primer is inorganic or "Shop & Maintenance Coating" - organic primer
– No separate measurement or pay
Designer must specify what to paint– General note and/or modify 594.00 Scope
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Existing SteelSpec 594– Near white blast– Mostly field applications– 3 coat system from QPL
"Shop & Maintenance Coating"
Designer must address– What to and not to paint (inaccessible areas?)– Do you want a warranty?
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Rehabilitating Steel (Removal or installation of components)
Spec 594– Commercial Grade Power Tool Cleaning– Field applications– 3 coat system from QPL
"Rehab Coating" - surface tolerant zinc primer
Designer must specify– What to and not to paint
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Non-Steel Metallic
1. Hot-dipped galvanizing– surface prep per ASTM D6386
2. Aluminum– light brush blast or hand sand
Shop or field application2 coat system from QPL
"Non-Ferrous Coating" - no zinc primer
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Liquid Coatings
Specification 594 does not cover: – 1 and 2 coat systems– overcoat systems– special uses (i.e. submerged, cables, anti-skid,
etc.)– special colors (Federal Color Standard 595B)– non-visible contaminates (salts)
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Liquid Coatings
AdvantagesAdvantagesAny size/shapeAny size/shapeShop or fieldShop or fieldRepairs matchRepairs matchLower initial costLower initial cost
DisadvantagesDisadvantagesVOC'sVOC'sQuality controlQuality control– contaminationcontamination– film thicknessfilm thickness– curecure
Shorter lifeShorter life
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3. Powder Coatings3. Powder CoatingsSpecification 593 covers applications on new and existing structures
used for small to medium sized parts (railing, sign structures, light poles, etc.)
PTI size limited to 52' x 14' x 12' high
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Powder CoatingsPowder Coatings
spec 593 requires a 2 coat system– Primer
zinc rich epoxy primer for steelepoxy primer for non-steel metallic
– Topcoatpolyester meeting AAMA Spec 2605
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Surface Prep Steps
Steel surfaces– "Near White" abrasive blast– Phosphate treat immediately and dry with heat
Galvanized surfaces– ASTM D6386
Other non-steel metallic surfaces– Light abrasive blast or hand sand
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Blasted sign structure
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Phosphate treatment
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Powder Coating Application
Preheat surface Apply primer (within 4 hrs of cleaning)Heat to partially cure primerApply topcoatHeat to finish cure
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Application of Zinc Rich Primer
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Powder Coatings
AdvantagesAdvantagesNo VOC'sNo VOC'sQuality ControlQuality ControlAdhesionAdhesionNo dry sprayNo dry sprayTransport Transport DamageDamage
DisadvantagesDisadvantagesSize limitsSize limitsDoesn't fill crevicesDoesn't fill crevicesCastingsCastingsField repairsField repairs
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4. Duplex SystemsAdditional Coating over HDGAddressed in spec 593 and 594
1. ASTM D 6386 - Standard Practice for Preparation of Zinc (Hot-Dip) Galvanized Coated Iron & Steel Product & Hardware Surfaces for Painting
2. Galvanizer & Painter must communicate
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Duplex System vs HDG
AdvantagesAdvantagesService lifeService lifeColor choiceColor choiceSmooth finishSmooth finish
DisadvantagesDisadvantagesHigher initial costHigher initial costQuality control?Quality control?
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5. Thermal Spray CoatingNo ODOT Standard SpecCan be done in shop or fieldSurface cleanliness criticalSeal with a topcoat for better performance AWS C2.18 - Protecting Steel with Thermal Sprayed CoatingsAWS C2.25 - Thermal Spray Feedstock
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System Costs ($/SF)*System Costs ($/SF)*
HDGHDG LiquidLiquid(new)(new)
LiquidLiquid(maint.)(maint.)
PowderPowder DuplexDuplex
7-15**7-15** 5-105-10 20-35***20-35*** 5-105-10 +3-5+3-5
** ApproximateApproximate cost, includes prep and application. cost, includes prep and application. Size, shape, and quantity affect costs.Size, shape, and quantity affect costs.
**** May require more SF than other systems. Use May require more SF than other systems. Use 0.45 to 0.60 $/lb. for estimating cost0.45 to 0.60 $/lb. for estimating cost
***Includes full removal and recoating. 60 to 80% of ***Includes full removal and recoating. 60 to 80% of cost is for containment and surface prep. cost is for containment and surface prep.
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System Service Life (yrs)*System Service Life (yrs)*
CoatingCoatingEnvironEnviron
HDGHDG LiquidLiquid(new)(new)
LiquidLiquid(maint)(maint)
PowderPowder DuplexDuplex
MarineMarine 25-4025-40 15-2515-25 10-2010-20 15-2515-25 40-6040-60W. Ore.W. Ore.(wet)(wet)
30-5030-50 25-3525-35 20-3020-30 25-3525-35 50-7050-70
E. Ore.E. Ore.(dry)(dry)
75+75+ 35+35+ 30+30+ 35+35+ 50-10050-100
**ApproximateApproximate time until corrosion at defects affect time until corrosion at defects affect structural integrity. Aesthetics may reduce life.structural integrity. Aesthetics may reduce life.