How it works
TSA is applied by metallizing. In arc spraying, two aluminum wires meet in an electric arc and compressed air atomizes the molten metal; in flame spraying, wire or powder is melted in a fuel-gas flame. The droplets strike the steel, flatten into splats and freeze, building the film pass by pass. Adhesion is essentially mechanical, which makes surface preparation decisive: a high blast-cleanliness grade and an angular surface profile, both defined in the governing specification, with spraying done before the steel can flash rust.
Aluminum is anodic to carbon steel in most natural electrolytes. Where the film is porous or locally damaged, the aluminum corrodes in preference to the steel, and its corrosion products tend to fill the pores. That combination of barrier and galvanic action is what distinguishes TSA from organic coatings, which protect only as long as the film is intact.
Why it is sealed
As-sprayed TSA contains interconnected porosity. A sealer — a thin, low-viscosity coating that penetrates the pores rather than building a film on top — is usually applied to slow the ingress of moisture and contaminants. The sealer has to be compatible with the service temperature; formulations for hot service differ from those for ambient exposure. A topcoat may follow for colour coding or specific chemical exposure, as the specification requires.
Where it is used against CUI
Insulated carbon steel piping and vessels that operate in, or cycle through, the temperature range where water stays liquid on the metal are prime candidates. TSA tolerates higher temperatures and thermal cycling than many organic systems and keeps protecting at small damage, which matters when the surface cannot be inspected without stripping insulation. AMPP SP0198 lists it among the coating options for corrosion under insulation. Offshore structures and splash zones are other common uses.
How it is specified
AWS C2.23M/NACE No. 12/SSPC-CS 23.00 is the joint specification for the application of thermal spray (metallized) coatings of aluminum, zinc and their alloys for the corrosion protection of steel. It covers surface preparation, application, thickness measurement, adhesion (tensile bond) and bend testing, sealing and quality control records. Thickness, sealer and acceptance values come from that document and the project specification.