In the past, protecting steel from corrosion typically involved either the use of hot-dip galvanizing or powder coating alone. However, with greater emphasis on long-term performance of both methods, many industries started using a combination of both methods in what is commonly referred to as a "Duplex coating".
This powder coating on hot-dip galvanizing process provides exceptional flexibility to architects, designers, and builders through a wide colour palette and long part life.
What is Hot-Dip Galvanizing?
Galvanizing is a four-step process that requires exceptional attention to details and procedures to ensure quality in the finished part. The fundamental steps in the galvanizing process are:
Cleaning. The first step involves dipping the steel into a hot alkaline solution to remove dirt, oil, and grease. However, this solution is unable to remove some surface residues such as welding slag, water-insoluble paints, adhesives used in the making of castings.
Galvanizing. The part is immersed in a bath of molten zinc. During galvanizing, the zinc bonds to the steel, creating a series of highly abrasion-resistant zinc-iron.
Finishing. After the steel is withdrawn from galvanizing bath, excess zinc is removed by draining and then galvanized part is air-cooled. Quenching is omitted for Duplex coating.
Inspection. Surface condition and coating thickness inspections complete the process. Because of the immediate solidification of the zinc upon contact with the air, a galvanized coating may have runs or drips or other imperfections. These imperfections don‘t affect corrosion prevention properties and can be smoothed out before powder coating.
In the photo: Hot-Dip Galvanized Steel Poles.
What is Powder Coating on Hot-Dip Galvanized Steel?
Powder coating over hot-dip galvanizing is a two step coating process. The fundamental steps in the powder coating on hot-dip galvanized steel process are:
Light Blasting. In the first step, the steel part is prepared by light blasting to smooth out imperfections before powder coating.
In the photo: Steel Poles after light abrasion.
Powder Coating. The second step is the application of a powder coating to the galvanized part. The powder coating is applied to the part and then heat cured, which bonds the powder to the galvanized part.
In the photo: Completed part.
The finished coating protects both the steel part and the galvanized layer, and provides an attractive exterior finish to the part.
What is the Downside to Powder Coated Hot-Dip Galvanized Steel?
The powder coating process can be difficult to perform properly if 48 hours have passed since the galvanization of the part or it is exposed to weather and environs. Before the powder coating process all parts must be free of dust or grease, moisture, oxides and other contaminants as this can lead to poor coating and tiny air bubble spots. These problems can be avoided if galvanizer simply uses consistent cleaning procedures, control methods and storage.
Bubble spots
Tiny air bubbles can form under the powder coating during the stoving/curing process in the curing cycle. The outgassing that causes these tiny bubbles can be generated by contaminants that are not removed from the steel before galvanising or before the powder coating is applied.
In the photo: Tiny bubble spots.
Special Effect Powder Coatings
As said hot-dip galvanizing can cause issues with outgassing and can create tiny air bubble spots or uneven coating.
In the photo: Substrate defects.
Special effect powder coatings can be applied on dip-galvanized steel to cover up surface imperfections.These special effect powder coatings are usually driven by architects and designers aiming for particular looks. Rusty looking finish, or a rough texture, makes the part stand out from the crowd. It offers a really unique look and it‘s also a great method for hiding substrate defects such as welding clean up, minor scratches, lumpy zinc etc.
The Main Benefits of The Combination of The Two Surface Finishes
The combination of the two surface finishes provides a much longer life of the part than using either of the processes alone. While galvanizing prevents the steel from rusting, the topcoat powder, will act as a barrier providing additional protection from the elements and create an attractive exterior finish to the part. This results in low maintenance structures, architectural pieces, and fabrications for many years in most locations.