How the process works
Wire or powder is fed into a spray gun. In arc spray, two wires carrying an electric current meet and melt in an arc; in flame spray, a fuel gas flame melts the material. Compressed air atomizes the molten metal and propels it to the surface. Each droplet flattens and solidifies on impact, and successive passes build a lamellar coating. Because the steel itself stays relatively cool, there is no heat distortion of the part and no size limit imposed by a bath, which is an advantage over hot-dip galvanizing for large or field-installed structures.
Metals used
| Material | Main protective behaviour | Typical settings |
|---|---|---|
| Zinc | Strongly sacrificial to steel | Atmospheric exposure, bridges, structural steel |
| Aluminum (TSA) | Barrier with some sacrificial action | Marine, immersion, high temperature, under insulation |
| Zinc-aluminum alloys | Combination of both | Atmospheric and marine exposures |
The choice of metal and thickness comes from the exposure, the expected service life and the project specification.
Surface preparation
Thermal spray bonds mechanically, so preparation is critical. The joint standard SSPC-CS 23.00/AWS C2.23M/NACE No. 12 calls for an abrasive blast to white metal or near-white metal, depending on the service, with an angular profile produced by angular abrasive. The profile is checked per ASTM D4417 against the specification, and spraying begins before the steel can rerust or become contaminated.
Sealing and topcoating
The sprayed coating has some porosity. A thin, penetrating sealer is normally applied to fill the pores and slow the ingress of moisture. Where colour, additional protection or chemical resistance is needed, a compatible paint system is applied over the sealer, forming a duplex system.
Quality control
Typical checks, as set out in SSPC-CS 23.00 and ISO 2063, include:
- cleanliness and profile before spraying;
- coating thickness measurements;
- adhesion testing, for example per ASTM D4541;
- a bend test on coupons to confirm application quality;
- visual inspection for blisters, loose particles or cracks.
Thickness targets and acceptance criteria come from the project specification, which normally refers to the joint standard for minimum values by service.
See our thermal spray metallizing service.