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 Marking

ColorCoding Color Coding

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 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 Coatings CoatingNPW/WDAestheticChemicalBuriedMarine Immersion Insulated AcrylicsYYNYNNYNN BituminousYYYNYYNYN Chlorinated Rubber YNNYYNYYN Coal Tar Epoxies YYYNYYYYY EpoxiesYYYNote 3YYYYY Inorganic Zinc Rich NNNNNNNNN Latex Acrylics YYNYNNNNN Organic Zinc Rich NNYNote 4NNYYN Micaceous Iron Oxide Reinforced Polyurethane YYYNNNYYN VinylsYYYYYNYYN N = NEW, PW= PARTIALLY WEATHERED, W= WEATHERED, D=DETERIORATED NOTE 3: EPOXY COATINGS CAN BE TOPCOATED WITH SYSTEMS THAT INCLUDE: POLYURETHANE, POLYSILOXANE, POLYASPARTIC, OR ACRYLIC FINISH COATS FOR IMPROVED AESTHETICS NOTE 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?