High velocity thermal spray systems such as the High Velocity Oxy-Fuel (HVOF) and supersonic velocity continuous combustion (SVCC) systems have been successfully utilized for in-situ applications for the past decade. A supersonic velocity thermal spray process is one where the material is atomized and / or accelerated under supersonic gas velocities (greater than the speed of sound). For this reason, the energy of the gas stream is focused directly on the material to be sprayed, producing a coating that has low permeability, and has a homogeneous coating structure than a low velocity coating. Further the bonding strength is excellent.
Permeability of a correctly selected and applied coating, in a corrosion environment, is the primary characteristic that determines the coatings useful life. If the coating is permeated at a rapid rate by corrosive media, and if the media reacts with the substrate underneath the coating, the bond of the coating to the substrate will be degraded. This substrate degradation may result in the coating spalling from the surface. SVCC coatings have very low permeability due to lowest porosity and special sealing process utilized.
In corrosion applications, a finer more homogenous coating will be less likely to suffer metallurgical segregation effects. Segregation occurs to a larger extent in low velocity coatings where particles are larger, and low alloy, heavily oxidized layers form around each spray particle. Thus low velocity coatings will corrode internally at a much higher rate, leading to premature coating failure.