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Wednesday, 15 July 2020
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Raw materials & technologies, Technologies, Powder coatings

Preparing HDG steels for powder coating is now standardized

Monday, 28 January 2013

Bridges are one example of construction projects that are often built from hot-dip galvanized steel. How to coat this steel without adhesion failure of the powder system is explained in the new ASTM standard.

New standard set for preparing HDG steel for powder coatings

Source: Fotolia/Mitar Holod

New standard set for preparing HDG steel for powder coatings

Source: Fotolia/Mitar Holod

The new standard, ASTM D7803, "Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Powder Coating" 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.

"ASTM D7803 explains the steps to achieve adherence of powder coating over galvanized steel so there is no adhesion failure of the powder system,” says Thomas Langill, technical director, American Galvanizers Association, and an ASTM member. "There has been increased interest in powder coating since it has no volatile organic compounds over corrosion protection, such as galvanized coating for color and appearance.”
Langill notes that D01.46 is currently seeking powder coaters and inspectors to contribute to ongoing standards activities.

The new standard was developed by Subcommittee D01.46 on Industrial Protective Coatings, part of ASTM International Committee D01 on Paint and Related Coatings, Materials and Applications.

Zinc coating is very resistant to atmospheric corrosion

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.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.
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.

When patina appears the adhesion gets lost

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.

Pinholes are the basis for corrosion

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.

Different zinc surfaces lead to different adhesion of powder coatings

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.

Powder coatings adhere due to electrostatics

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.

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