Active Standard ASTM D7803 | Developed by Subcommittee: D01.46
Book of Standards Volume: 06.02
Significance and Use
4.1 This practice describes the methods of preparation of hot-dip galvanized surfaces prior to the application of powder coating. The key to achieving proper adhesion between powder coatings and galvanized steel is surface preparation. The surface must be entirely free from metal oxides prior to powder coating. Any metal oxides that remain on the surface of the galvanized steel can potentially retain air or moisture. Upon heating during the curing stages of the powder application, the oxides may release water vapor or air, which can expand and penetrate the powder coating, causing blisters or voids.
4.2 The zinc coating is constantly in a state of change. From the time the steel part is removed from the galvanizing kettle, the exposed zinc coating interacts with the environment to form, first zinc oxides and zinc hydroxides, and then zinc carbonates.4 The process of complete conversion of the outer layer of zinc carbonates can take up to two years of exposure to the environment, depending on the local weather and moisture conditions.
4.3 The zinc surface after full weathering is very resistant to atmospheric corrosion because the tight patina that is formed (zinc oxide, zinc hydroxide and zinc carbonate) is dense and tenacious. However, during the formative stages of patina development, the oxide/hydroxide layer is poorly adhered and must be removed in order for the powder coating to adhere properly to the galvanized coating. The second is pinholing/blistering of the coating which can severely limit its potential performance, especially in aggressive chloride environments. Entrapped gasses developed during the galvanizing process escape the surface through the coating as it cures at high temperatures. If these volatile materials are not removed through an outgassing process prior to the baking of the powder, then pinholing or blistering can occur. The presence of pinholes gives chlorides and other corrosive agents access to the zinc substrate consequently producing zinc corrosion products which may leach out through the coatings. While the presence of these corrosion products may not result in associated delamination of the coating, unsightly white staining of the coating can occur. Blisters are defects that are not adhered to the surface and may easily be broken into or off during handling, which creates performance and aesthetic issues. The proper preparation of the galvanized coating surface can increase the adhesion and coverage necessary to overcome these problems and results in a satisfactory service life of the powder coating and the galvanized coating together.
4.4 Variations in surface preparation produce end conditions that differ as far as surface roughness and zinc composition, hence they do not necessarily yield identical results when powder coatings are subsequently applied. The age of the zinc corrosion products on the galvanized coating will dictate the type of surface preparation to be selected.
1.1.1 Powder coating is a dry finishing process which uses finely ground particles of pigment and resin, electrostatically charged, and sprayed onto a part to be coated. The parts are electrically grounded so that the charged particles projected at them adhere to the surface and are held there until melted and fused into a smooth coating in the curing oven.
1.1.2 Hot-dip galvanized iron or steel is produced by the immersion of fabricated or un-fabricated products in a bath of molten zinc, as specified in Specification A123/A123M or A153/A153M. This practice covers surface preparation and thermal pretreatment of iron and steel products and hardware which have not been painted or powder coated previously (Practice D6386). Galvanized surfaces may have been treated with protective coatings to prevent the occurrence of wet storage stain. This practice neither applies to sheet galvanized steel products nor to the coil coating or continuous roller coating processes.
ICS Number Code 25.220.40 (Metallic coatings)
UNSPSC Code 11101713(Iron); 11101704(Steel)