TWI Frequently asked questions
The main situations are summarised below:
Operating temperature - if it is above the working range of the paint. Some paints have very high temperature ranges, e.g. 200 to 300°C. However, you have to consider the environment. The temperature limit ofpaint systems in sea water can be around 60 to 70°C, after which they lose their corrosion protection properties. For higher temperatures you should use a metallic coating, e.g. for sea water you could use thermally sprayedaluminium.
Wear - in abrasive wear situations, a paint system would offer little or no protection compared with an appropriate metallic coating. For sliding contact, surface treatments and metallic coatings give excellentperformance. Some parts can benefit from a solid lubricant such as PTFE, and this may be added to metallic coatings e.g. electroless nickel. Only in very low load situations are polymers used, but rarely paints.
Corrosion - paints can offer excellent corrosion protection. However, when the conditions become more aggressive, metallic coatings are used. Metallic coatings used for corrosion resistance fall into two categories:low temperature aqueous and high temperature. Aqueous corrosion-resistant coatings form a barrier to corrosion; they include cladding and weld overlay, and self sealing coatings like aluminium. For high temperature exposure, thecoatings need to provide a diffusion barrier against active species such as oxygen. Thermally sprayed coatings are prominent choices for this type of application e.g. gas turbine blades.
Specialist applications - metallic coatings and ceramic coatings offer a different set of physical properties compared to polymers. For this reason they are used in applications such as medical implants where they formbioactive surfaces.
Further information can be found in many of the recent handbooks and introductions to surface engineering e.g. 'Engineering coatings. Design and application' second edition. Stan Grainger and Jane Blunt. ISBN 1 85573 369 2. Contact Woodhead Publishing Ltd or TWI.
Copyright by TWI, 1999
