What is metal spraying?
Metal spraying, also called thermal spraying or metallising (Dutch: schooperen, after its inventor), is the application of a metal coating such as zinc, aluminium or a zinc-aluminium alloy to a surface that has first been blast-cleaned to bare metal. The metal is melted in a spray gun and propelled by compressed air in fine droplets onto the workpiece (the substrate), where it instantly solidifies into a well-bonded, tough layer. The process was developed in the early 20th century by the Swiss engineer Max Ulrich Schoop, which is why it is called “schooperen” in Dutch.
Techniques
- Flame spraying with wire or powder: the metal is melted in a fuel gas-oxygen flame.
- Arc spraying: an electric arc between two metal wires melts the wire tips.
- Plasma spraying and high-velocity oxy-fuel (HVOF): for special, very dense or ceramic coatings.
Application
Metal spraying is widely used to protect steel against corrosion, for example on bridges, locks, steel facades, railings and components too large for a galvanising bath. Requirements for thermally sprayed zinc and aluminium coatings are set out in ISO 2063.
Benefits and points to watch:
- No size limit: unlike hot-dip galvanising in a bath, any size can be treated, including on site.
- No distortion: the workpiece hardly heats up, so thin sections do not warp.
- Preparation is everything: the surface must be clean and blast-cleaned to a rough profile. Poor blasting is the main cause of coating failure.
- Seal it: the sprayed layer is porous. A sealer is usually applied straight away, often followed by a paint or powder coating system (duplex system).
- Cathodic protection: zinc (and to a lesser degree aluminium) protects steel sacrificially, so small areas of damage do not rust straight away.
- Specialist work: the process requires special equipment, extraction and protective gear. It is not a DIY job.
Related Terms
- Galvanising
- Corrosion
- Grit blasting
- Airless spraying
- Cathodic protection
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