Steel Coating Systems

By: Ivan Silbernagel, P.E.

Structure Coatings Engineer

ODOT 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 coating

Creates 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 – embrittlement

ASTM A 384 – distortion

ASTM A 385 – high-quality coatings

ASTM A 780 – repair

ASTM D 6386 – surface prep for painting over

Hot-Dip Galvanizing

AdvantagesAdvantages

Best protectionBest protection

Metallurgical bondMetallurgical bond

Quality controlQuality control

DurableDurable

Interior (pipe/tubes)Interior (pipe/tubes)

DisadvantagesDisadvantages

Venting/drainageVenting/drainage

Size limitsSize limits

CrevicesCrevices

DeformationDeformation

Reduced slip coef.Reduced slip coef.

Field weldsField welds

2. Liquid Coatings

Standard Specification 594 covers– shop and field applications

1. "new steel"

2. "existing steel" (maintenance)

3. "rehabilitating coated steel"

4. "non-steel metallic" (galvanized, aluminum)

New Steel

Spec 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 Steel

Spec 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 application

2 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

AdvantagesAdvantages

Any size/shapeAny size/shape

Shop or fieldShop or field

Repairs matchRepairs match

Lower initial costLower initial cost

DisadvantagesDisadvantages

VOC'sVOC's

Quality controlQuality control– contaminationcontamination– film thicknessfilm thickness– curecure

Shorter lifeShorter life

3. Powder Coatings3. Powder Coatings

Specification 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

– Primerzinc rich epoxy primer for steel

epoxy 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 primer

Apply topcoat

Heat to finish cure

Application of Zinc Rich Primer

Powder Coatings

AdvantagesAdvantages

No VOC'sNo VOC's

Quality ControlQuality Control

AdhesionAdhesion

No dry sprayNo dry spray

Transport Transport DamageDamage

DisadvantagesDisadvantages

Size limitsSize limits

Doesn't fill crevicesDoesn't fill crevices

CastingsCastings

Field repairsField repairs

4. Duplex Systems

Additional Coating over HDG

Addressed 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

AdvantagesAdvantages

Service lifeService life

Color choiceColor choice

Smooth finishSmooth finish

DisadvantagesDisadvantages

Higher initial costHigher initial cost

Quality control?Quality control?

5. Thermal Spray Coating

No ODOT Standard Spec

Can be done in shop or field

Surface cleanliness critical

Seal with a topcoat for better performance

AWS C2.18 - Protecting Steel with Thermal Sprayed Coatings

AWS 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% ***Includes full removal and recoating. 60 to 80% of cost is for containment and surface prep. of cost is for containment and surface prep.

System Service Life (yrs)*System Service Life (yrs)*

CoatingCoating

EnvironEnviron

HDGHDG LiquidLiquid

(new)(new)

LiquidLiquid

(maint)(maint)

PowderPowder DuplexDuplex

MarineMarine 25-4025-40 15-2515-25 10-2010-20 15-2515-25 40-6040-60

W. 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.