Steel Coating Systems
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Transcript of Steel Coating Systems
Steel Coating Systems
By: Ivan Silbernagel, P.E.Structure Coatings EngineerODOT Construction Section
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)
1. Hot-Dipped Galvanizing Valmont Coatings Pacific States Galvanizing
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
Sulfuric Acid Pickling
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)
Galvanizing (Hot-Dipped)44' x 5'3" x 8'9" deep
Material Handling (racks or hooks)
Vent and Drainage
Oversize dipping
Material thickness
Different thickness– Reach bath temperature at different times– Cools at different rate– Maintaining original shape
Estimated Service Life of HDG
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
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
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)
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
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?
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
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
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)
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
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
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
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
Blasted sign structure
Phosphate treatment
Powder Coating Application
Preheat surface Apply primer (within 4 hrs of cleaning)Heat to partially cure primerApply topcoatHeat to finish cure
Application of Zinc Rich Primer
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
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
Duplex System vs HDG
AdvantagesAdvantagesService lifeService lifeColor choiceColor choiceSmooth finishSmooth finish
DisadvantagesDisadvantagesHigher initial costHigher initial costQuality control?Quality control?
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
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.
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.