TWI Knowledge Summary

Flame spraying

Flame spraying is part of a wider group of coating processes known as thermal spraying. In these processes, a consumable (usually a powder or a wire) is heated and propelled onto a substrate to form a coating. Flame spraying is the oldest of the thermal spraying processes. A wide variety of materials can be deposited as coatings using this process and the vast majority of components are sprayed manually. 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. Table 1 highlights the typical performance of flame spraying compared with the other thermal spraying processes.

Table 1. Comparison of thermal spraying process and coating characteristics

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. The consumable types give rise to the two process variants:

• Powder flame spraying
• Wire flame spraying

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). Alternatively, in some basic systems, powder is drawn into the flame using a venturi effect, which is sustained by the fuel gas flow. 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.

	Schematic of the powder flame spray process
	Schematic of the wire flame spray process

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.

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, offshore platforms, LPG bottles) with aluminium or zinc coatings. Aluminium is more expensive, but has resistance to acidic gaseous atmospheres (such as those associated with the products of fossil fuel combustion), as well as neutral solutions, such as salt water. 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.

Developments

Recent developments have looked at increasing the particle velocities. Such processes are known as high velocity wire or flame spraying. These processes use internal combustion of the fuel and oxygen gases to create higher gas velocities and give much higher particle velocities. Improvements in adhesion and oxide content are claimed, exceeding those obtained by conventional flame spraying and arc spraying.

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 why flame spraying may be selected over other surface engineering techniques and they are likely to include a combination of the following:

• The part geometry or the working environment requires manual spraying - flame spraying offers the easiest and most tolerable working conditions
• There are cost concerns and the area is large and complex
• The required coating performance is reached with flame spraying
• Dust and fume levels associated with arc spraying are too high

Risks

As with all the thermal spraying processes there are particular health and safety issues that should be addressed before spraying -

Extraction of dust and fume
All thermal spraying processes produce dust and fume. Therefore, adequate extraction or ventilation to remove this dust and fume from the working environment should be provided. In addition, if the operator is exposed to this environment then the provision of suitable breathing equipment should be considered.

Fire or explosion of dust and fume
The build up of some metals in the form of dust can lead to fire and explosions. This especially is of concern for powders of aluminium and zinc. To avoid this, adequate provision for extraction and filtration of the spray dust, along with regular cleaning of any ducting, should be provided. The dust should not be allowed to get damp as this can lead to the evolution of hydrogen gas. Additionally, sources of ignition should be avoided such as static charges from rotating equipment, for example fans in the ducting. Depending on the circumstances, it may be necessary to carry out a 'Hazardous Area Classification'.

Further information

FAQ: What types of coating can I apply with flame spraying?
FAQ: What are the advantages of flame spraying?
FAQ: What are the disadvantages of flame spraying?
FAQ: What are the most common applications of flame spraying?
FAQ: What types of flame spraying equipment are available?

A comprehensive Best Practice Guide on Thermal Spraying and Best Practice Report on Thermal Spraying of Aluminium, Zinc and Their Alloys is available to TWI Industrial Members.

You can use the Weldasearch literature database to supplement what you find in JoinIT.

For specific enquiries please e-mail Surfacing: surfacing@twi.co.uk