Thermal Spray Coatings (Metallizing)

Thermal spray — also called metallizing or metal spray coating — protects steel by melting a corrosion-resistant metal and spraying it onto the surface. For steel the metals are zinc, aluminum and zinc-aluminum alloys. It is an established method for long-term protection of structures in aggressive environments, and a standard part of duplex (metal plus paint) systems.

Source for process and performance statements on this page: the Steel Construction Institute's Guidance Note 8.04, Thermally sprayed metal coatings (UK bridge-steelwork guidance), plus the standards named below.

How thermal spraying works

The metal, in wire or powder form, is passed through a heat source that melts it, and the hot molten particles are projected by compressed air onto the surface. They hit, flatten and solidify as overlapping platelets. The equipment is a hand-held gun that uses either a gas flame or an electric arc for heat. The sprayed metal solidifies immediately, so there is no drying time, and it does not sag or run.

Compared with hot-dip galvanizing, the guidance note lists these advantages: the steel is not heated significantly, which matters for parts that could distort; the work is not limited by part size; it can be done in the shop or on site; and even geometrically complex components can be coated with the right spray set-up. For how galvanizing itself works, see our hot-dip galvanizing guide.

Flame spray vs electric arc

• Gas flame spraying is long-established and uses powder or wire. It is less tolerant of inadequate surface preparation than arc, particularly on areas locally hardened by flame cutting.
• Electric arc spraying melts twin wire electrodes in an arc. It offers high application speed and improved adhesion to steel, particularly with sprayed aluminum. It is generally not as flexible as flame spraying for intricate articles or where equipment access is constrained, and it tends to be rough around web/flange intersections and stiffener corners — normal, and not detrimental to the coating.

Other thermal-spray processes exist for wear-resistant coatings; this page covers the zinc and aluminum corrosion-protection processes above.

Zinc, aluminum or Zn/Al — the metals compared

The total thickness of a system is usually set by the required life to first maintenance; a typical UK specification for sprayed aluminum or zinc in a multi-coat system is a minimum of 100 µm. Thickness is project-specific — take it from the specification, not from a general figure.

Surface preparation and sealing

Preparation decides the result. The surface must be cleaned of rust and mill scale and given an angular blast profile for mechanical key. The guidance note says sprayed metal needs an angular profile with a greater amplitude than other coatings, and it asks for a clean surface ideally to Sa 3, with Sa 2½ satisfactory for zinc. The grades and what they mean are in our steel surface preparation standards guide.

Sealing. After spraying, a thin sealer coat is usual: it impregnates the natural pores so moisture and oxygen cannot reach the steel. Sealers are low-viscosity materials — vinyl, phenolic and polyurethane types are common — and over-thick application is to be avoided — over-applying an epoxy sealer can leave a smooth, glass-like surface that hurts adhesion of the first paint coat. Seal as soon as practical: if the sprayed surface is left in damp conditions, dark staining can appear, which indicates corrosion of the steel underneath. Faying and abutting surfaces of slip-resistant bolted joints are left unsealed apart from a small margin at the edges, because sealer reduces slip resistance.

Repair and touch-up

Damaged areas can generally be repaired by local blast cleaning followed by more thermal spray, or by a zinc-rich epoxy paint over zinc or aluminum coatings; the same applies at site-welded joints. Where the structure will see high service temperatures, an inorganic zinc silicate repair coating is preferred. Repair detail always follows the project specification.

Standards

• ISO 2063-1:2019 — design considerations and quality requirements for corrosion protection systems of zinc, aluminum and their alloys applied by thermal spraying, covering service from −50°C to +200°C taking sealers into account. Its scope excludes hot-dip galvanizing, sherardizing, electroplating, organic coatings and paints, and heat-resistant aluminum coatings (covered by ISO 17834). ISO 2063-2 covers execution of the systems.
• NACE No. 12 / AWS C2.23M / SSPC-CS 23.00 — the joint standard for applying thermal spray coatings of aluminum, zinc and their alloys to protect steel. It covers surface preparation, application, repair of defects, thickness measurement, adhesion testing, and sealers and topcoats. Check which edition the project specification calls for.
• ISO 14918 — qualification testing of thermal sprayers. The guidance note recommends qualified applicators; in the UK it is mandatory for railway bridges.

The ISO and AMPP/AWS standards are paywalled; the scope descriptions here come from their catalog listings (ISO 2063-1 on Genorma, NACE No. 12 on AMPP), not the full text. Always work from the current edition named in your specification.

Quality control and inspection

Two points matter on any job. First, coating thickness can only be assessed, not directly measured: the rough sprayed surface on a grit-blasted substrate makes digital gauge readings vary, so ISO 2063-2 sets out an averaging method, and recording the blast profile before spraying gives a more realistic figure. Second, adhesion is checked by pull-off testing on representative test panels; ASTM D4541 is one of the methods used in practice (the UK highway specification uses Type III), and some parts use the grid test in ISO 2063-2. Inspection is done by a trained thermal metal spraying inspector.

Where it fits — and where it doesn't

Thermal spray is used on steel bridges and structures — in the UK guidance note: railway-bridge components, bridge decks, bearings, CCTV masts and lighting columns — usually as part of a duplex system, with paint over the sealed metal for added durability and a choice of colour. It is best suited to shop application, because the equipment is bulky; on site it is used for touch-up on new construction and for large areas on maintenance contracts.

The same guidance note lists the limits to plan around:

• Recesses and deep holes are hard to coat, because the spray must be directed near perpendicular to the surface. Pipes and hollow sections can be coated with special equipment, but only before fabrication, and unlike hot-dip galvanizing the inside of a fabricated hollow section cannot be coated.
• It needs a high standard of blasting and an angular profile, and takes more time and skill to apply; in typical multi-coat systems it is described as relatively expensive.
• Spraying zinc produces fumes that require personal protective equipment.

Related guides and services

• Dipped zinc instead of sprayed — galvanizing and the hot-dip galvanizing guide
• Blast grades and profile — steel surface preparation standards
• Whole-structure coating choices — structural steel coating and protective coating
• Not sure which route fits — industrial coating contractors

Specifying thermal spray zinc or aluminum for a structure? Use the form below with the asset, service environment and metro, and we'll match the request with a vetted applicator.