Responsibility• Surface preparation of galvanized steel for

over-coating is often overlooked in the project bidding process. In the absence of up-front discussions and agreements, hot-dip galvanizers will process steel to conform to ASTM A 123 or A 153, both of which allow incidental protrusions which do not interfere with intended form, fit or function of the part. These incidental protrusions quite often do not conform to ASTM D 6386 and must be removed prior to over-coating. The remediation of these surface imperfections typically becomes the responsibility of the over-coat applicator

Selecting Coatings • There are 3 steps to follow when

selecting coatings for use over galvanized steel– 1. Determine the service environment– 2. Determine the condition of the

galvanized steel– 3. Use the Table in SSPC Guide 19 to

select an appropriate coating or coating system

Step 1: Determining the Service Environment• Service environments for coatings over

galvanized steel may be generally classified as:– Chemical Atmospheric– Buried– Marine Atmospheric– Immersion in Fresh or Salt Water– Under Insulation– Aesthetic Concerns, Atmospheric

Exposure

Chemical Atmospheric• Coating selected

protects the galvanized steel from premature zinc depletion as a result of exposure to acidic or alkaline chemical fumes and condensation– pH under 5 or

above 10

pH

Buried• Coating selected

protects the galvanized surface from premature depletion of the zinc as a result of high levels of moisture, alkalinity, or acidity in the soil

Marine Atmospheric• Coating selected must protect the

galvanized surface from premature depletion of the zinc due to high levels of humidity and salt mist

• Coating should be resistant to UV light

Immersion in Fresh or Salt Water

• Coating selected will keep the zinc carbonate from sacrificing the galvanized surface to prevent premature depletion of the zinc due to the presence of constant moisture

Under Insulation• Coating selected

must seal the galvanized surface to provide protection from corrosive conditions resulting from condensation in insulation materials at anticipated service temperatures

Aesthetic Concerns, Atmospheric Exposure• When a different finish color, other

than matte gray, is required for: – Improved appearance– Safety designation– Service designation

Safety MarkingSafety MarkingSafety MarkingSafety Marking

Color Color CodingCodingColor Color CodingCoding

Step 2: Determine Condition of Galvanized Substrate• Determine if the galvanized substrate is:

– New– Partially Weathered– Fully Weathered– Deteriorated

Newly Galvanized Steel• Zinc coated steel that has no

surface treatment after galvanizing and has been galvanized within the previous 48 hours

Partially Weathered Galvanized Steel

• The galvanizing is older than 48 hours or has zinc oxide or zinc hydroxide on the surface

Fully Weathered Galvanized Steel• Galvanizing has developed a stable

zinc carbonate surface that is very suited for paint coating adhesion

Deteriorated Substrates• Reddish brown discoloration on galvanized

surfaces indicate:– The complete loss of galvanizing and rusting

of the underlying steel– The reddish brown layer of zinc iron alloy

which forms at the interface between the galvanizing and the steel substrate that has been exposed due to the loss of surface zinc

Deteriorated Substrates• A DFT gage can be used to check for

the presence of zinc in discolored areas

• Note: Don’t wait until all or most of the zinc is depleted, if you do they you will have to sand blast before you paint over the HDG surface

Synergistic EffectSynergistic EffectSynergistic EffectSynergistic Effect

Step 3: Selection of Coatings• The table in SSPC Guide 19 shows

the generic coating materials typically used in conjunction with hot dip galvanizing in various service environments

Step 3: Selection of CoatingsCoating N PW/W D Aesthetic Chemical Buried Marine Immersion Insulated

Acrylics Y Y N Y N N Y N N

Bituminous Y Y Y N Y Y N Y N

Chlorinated Rubber

Y N N Y Y N Y Y N

Coal Tar Epoxies

Y Y Y N Y Y Y Y Y

Epoxies Y Y Y Note 3 Y Y Y Y Y

Inorganic Zinc Rich

N N N N N N N N N

Latex Acrylics

Y Y N Y N N N N N

Organic Zinc Rich

N N Y Note 4 N N Y Y N

Micaceous Iron Oxide Reinforced Polyurethane

Y Y Y N N N Y Y N

Vinyls Y Y Y Y Y N Y Y N

N = NEW, PW= PARTIALLY WEATHERED, W= WEATHERED, D=DETERIORATEDNOTE 3: EPOXY COATINGS CAN BE TOPCOATED WITH SYSTEMS THAT INCLUDE: POLYURETHANE, POLYSILOXANE, POLYASPARTIC, OR ACRYLIC FINISH COATS FOR IMPROVED AESTHETICSNOTE 4: SURFACE PREPARATION IN ACCORDANCE WITH SSPC SP 16

Adhesion of Coatings to Galvanizing

Example• Scenario: You have a galvanized

structure that was erected 3 miles from the coastline. The specification states that the exterior it is to be coated. Determine the coating options

Example• Step 1: Determine the service environment

= Marine atmosphere• Step 2: Determine the condition of the

galvanizing= Fully weathered

• Step 3: Use the Table and determine the possible coatings= Acrylic, Coal Tar Epoxy, Epoxy, Micaceous Iron

Oxide Reinforced Polyurethane, Polyaspartic and Vinyl=Acrylic, Micaceous Iron Oxide Reinforced Polyurethane, Polyaspartic are best choices due to the need for UV resistance

Summary• By following the three steps

described in this webinar and referencing SSPC Guide 19, choosing the best coating system for the application over galvanizing is achieved, providing years of corrosion protection

Questions?