Nitrogen ‘shroud’ improves thermal spraying process

A new development in thermal spraying will see cost reductions, a widening of its potential applications and, crucially, improved corrosion protection in comparison with existing processes.

The process involves melting metal feedstock and firing it from nozzles at a velocity of 1,100ms at the target material. On impact the particles spread and freeze, adhering to the surface.

The properties of the particles determine the quality of the final coating. Coatings are especially important for boilers, coal burning equipment and sea valves or chemical plant equipment that are susceptible to corrosion.

The thermal spraying process was invented in 1912 and has been in limited use since. But three years ago an international consortium of seven companies, including UK firm Metallisation, forged ahead with research to improve the process.

There is an optimum temperature for the sprayed particles of coating material. If the correct temperature is not achieved cold particles will not deform properly on impact, causing porosity around them. If particles are too hot they will oxidise in flight, causing oxide inclusion and therefore weakening the corrosion resistance.

The research, to resolve hot particle oxidisation, led to a new process known as ‘functional shroud arc spraying’. The ‘shroud’ is an inert gas that envelopes the coated material and spray stream.

A Cray supercomputer was used to model the shroud that would be required to minimise spray oxidation. Nitrogen proved to be the best gas for this.

Testing has shown that the new process gives much reduced degradation and oxidation for the coated part ensuring a potentially longer performance life. As well as corrosion resistance, the electrical conductivity and mechanical properties of the coated material is also improved.

Importantly, there is a cost saving of 10% compared with more conventional arc spray technologies. The new process achieved protection performance that was better than other thermal coating techniques that would cost five times as much, according to Metallisation.www.metallisation.com