Flame Spraying

Flame spraying uses the heat from the combustion of a fuel gas (usually acetylene or propane) with oxygen to melt the coating material, which can be fed into the spraying gun as a powder, wire or rod. Flame spraying is the oldest of the thermal spraying processes. Flame spraying has distinct advantages, including ease of application and low cost, compared with the other spraying processes. These benefits make it a widely used process.

How does Flame Spraying work?

For the powder flame spraying process, powder is fed directly into the flame by a stream of compressed air or inert gas (argon or nitrogen). It is important that the powder is heated sufficiently as it passes through the flame. The carrier gas feeds powder into the centre of an annular combustion flame where it is heated. A second outer annular gas nozzle feeds a stream of compressed air around the combustion flame, which accelerates the spray particles towards the substrate and focuses the flame.

In the wire flame spraying process, the wire feed rate and flame settings must be balanced to produce continuous melting of the wire to give a fine particulate spray. The annular compressed air flow atomises and accelerates the particles towards the substrate.

Coating Quality

Two key areas that affect coating quality are surface preparation and spraying parameters. Surface preparation is important for coating adhesion and can affect the corrosion performance of the coating. The main factors are grit blast profile and surface contamination. Spraying parameters are more likely to affect the coating microstructure and will also influence coating performance. Important parameters include gun to substrate orientation and distance, gas flow rates and powder feed rates.

The bond of a thermally sprayed coating is mainly mechanical. However, this does not allow the bond strength to remain independent of the substrate material. There are significant differences between spraying onto carbon steel and stainless steel, for example. This may be due to surface oxide or thermal expansion, and such factors should be considered before applying the coating.

All thermal spray coatings contain a degree of internal stress. This stress gets larger as the coating gets thicker. Therefore, there is a limit to how thick a coating can be applied. In some cases a thinner coating will have a higher bond strength.

Benefits

There are particular reasons (advantages) why flame spraying may be selected:

The part geometry or the working environment requires manual spraying – flame spraying offers the easiest and most tolerable working conditions, all purpose spraying
There are cost concerns and the area is large and complex
Coating can be applied rapidly
The required coating performance is reached with flame spraying
Sprayed metal is usually harder than the same metal in wrought form and has higher lubrication holding properties.
Dust and fume levels associated with arc spraying are too high
Powder: Wide range of coated materials.
Well suited for applications requiring protection against heavy wear, corrosion, heat, oxidation, and electrical conductivity.

Applications

Flame spraying is widely used were lower coating costs are desired and a lower coating quality can be tolerated. Some typical applications include:

Corrosion protection of structures and components (e.g. bridges) with aluminium or zinc coatings. Zinc has resistance to alkaline corrosion. Flame spraying is also used to spray corrosion resistant thermoplastic polymer coatings.
Reclamation of worn shafts, particularly of bearing areas with materials such as stainless steel or bronze alloys. The coatings produced are quite porous and lubricants can be absorbed into the coating, enhancing the performance of the bearing.

The wire materials utilized are Lead, Tin, Cadmium, Nickel, Babbitt, Zinc, Brass and Bronze.
The powder materials utilized are Aluminum Oxide, Nickel Graphite, Plastic Base Powders and Babbitt.