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.