Abstract

GORLACH, I.A.. Evaluation of the HVAF Thermal Sprayed Coatings. R&D j. (Matieland, Online) [online]. 2008, vol.24, pp.4-8. ISSN 2309-8988.

Anticorrosion protection forms a large sector of the South African industry because of the scale of the mining industry, general maintenance of infrastructure and due to extra demand for anticorrosion protection in the coastal regions. In order to prolong service life and reduce maintenance costs, essentially all steel structures, bridges, wind power plants, electricity pylons, gas cylinders, air conditioners and heat exchangers need to be coated for protection against corrosion of various kinds. Due to size limitations, galvanizing is often not feasible, therefore large structures are usually painted or thermal sprayed with zinc or aluminium depending on the requirements. Thermal sprayed metal coatings protect the base material longer than paints, and they also can withstand highly corrosive environments. The quality of metal sprayed coatings depends mainly on the thermal and kinetic energy of the spray particles. High particle velocities result in dense coatings with higher bonding strength due to improved metallurgical bond between the substrate and the sprayed material. Traditional metal spraying techniques, which have been used in industry for decades, such as wire flame and twin-wire arc are classified as low velocity processes because the sprayed material is conveyed by compressed air having subsonic velocity. Recently, a new method, namely the high velocity air flame (HVAF) process, was introduced for thermal spraying for anticorrosion protection. In this paper, zinc-aluminium (Zn-Al) coatings thermal sprayed using the HVAF method are analysed. The thermal sprayed coatings were characterized by the standard techniques, such as light microscopy, scanning electron microscopy with energy-dispersive spectroscopy, X-ray diffraction, salt spray and bond strength tests. The results show that thermal sprayed coatings have a dense structure, a high bonding strength, low presence of oxides and high resistance to corrosion. This is attributed to highflow/particle velocities and relatively low combustion temperatures of HVAF in comparison with other thermal spraying technologies. High spray rate and good coating quality make the HVAF thermal spray method a viable alternative to the conventional wire flame and twin-wire arc methods.

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