Powder coating on steel, including duplex systems over galvanizing

Process

Powder is a dry mix of resin, curing agent and pigment. It is sprayed from an electrostatic gun onto earthed steel, where the charge holds it in place until curing. Overspray can be recovered and reused.

In the oven the powder melts, flows and cross-links into a hard, continuous film. Common chemistries are epoxy (good chemical resistance, poor UV resistance, so mainly indoor or primer use), polyester and TGIC or TGIC-free polyester (good exterior durability), and epoxy-polyester hybrids for interiors.

Film thickness and pre-treatment

Single-coat films are typically 50–125 µm. Thin films risk voids that expose bare metal; very thick films can lose impact resistance and adhesion. The powder manufacturer's data sheet sets the thickness range and cure schedule.

Performance depends mainly on pre-treatment. Steel is degreased and then either abrasive blast cleaned or given a conversion coating such as iron or zinc phosphate. Blast cleaning gives the best adhesion for exterior and heavy-duty work.

Powder coating is a barrier coating. If it is broken, bare steel underneath starts to rust and corrosion can creep under the film. For exterior steel, it is often applied over a zinc-rich primer or over galvanizing.

Duplex systems: galvanized plus powder

A duplex system combines hot-dip galvanizing with a powder or paint topcoat. The topcoat protects the zinc from the environment, and the zinc stops corrosion at any damage in the topcoat. The American Galvanizers Association states the combined life can be 1.5 to 2.3 times the sum of the two individual lives.

Galvanized steel needs specific preparation before powder coating. The surface is lightly sweep blasted or chemically treated, then preheated so air and moisture in the zinc are driven out before the powder is applied. Without this, gas escaping during cure causes pinholes and blisters (outgassing).

ASTM D7803 sets out surface preparation, including the thermal pretreatment, for powder coating galvanized steel. In Europe, EN 15773 sets the agreements and quality requirements between client, galvanizer and powder coater, and EN 13438 specifies the powder coatings for galvanized or sherardized steel products used in construction.

Standards and quality labels: scope matters

Several schemes often quoted for powder coating apply only to particular substrates or uses.

• ISO 12944 covers protective paint systems for steel structures and is widely used to state corrosivity categories and durability. Powder coatings are outside its scope, although some suppliers test powder systems to its test durations.
• Qualicoat is a quality label for coatings on aluminium for architecture. AAMA 2603, 2604 and 2605 (now under FGIA) cover organic coatings on aluminium extrusions and panels. Neither applies to steel.
• Qualisteelcoat is the corresponding quality label for coating steel, including hot-dip and continuously galvanized steel, with systems rated by corrosivity category.
• IS 13871 is the Indian specification for powder coating materials.

When to choose powder coating

Choose powder coating when colour, appearance and a smooth, uniform finish are needed together with good resistance to chipping, for example on fencing, palisade pales, brackets, enclosures and street furniture.

For exterior steel in corrosive atmospheres, specify a duplex system rather than powder alone. Powder coating is less suited to threads and close-fitting parts, because the film is thick, and to parts that will be cut or drilled after coating.