Many parts are exposed to moisture, salt or chemical attack in industry. As a result they can corrode. Thermal spraying provides one of the better protection methods available. By understanding how a thermal sprayed coating functions, it is possible to choose the most appropriate specification.
How the Layer Seals the Substrate
A thermally sprayed coating forms a physical barrier between the substrate surface and the environment surrounding the part. Moisture and oxygen are unable to penetrate through a properly applied coating to react with the substrate surface thereby preventing rust and corrosion from forming.
Sacrificial Protection When the Coating Is Damaged
Zinc and aluminium primers also offer Sacrificial Protection. As both of these metals are anodic to steel, they will corrode before the steel in the event of damage to the surface of the coating. This means that even if the coating is scratched or chipped, the steel edges of the damaged area will still be protected by the sacrificial metal from the coating. This is especially important in harsh environments where the cost of repair could be severe. A simple scratch or chip could have serious consequences if the coating is not designed to protect the steel in this way.
Managing Porosity to Prevent Corrosion Pathways
The coating applied by thermal spraying can have pores in it. Such pores can be a start for corrosion in very aggressive environments. Sealing of such coatings by a layer of sealer is very effective for preventing such penetration of corrosion to the substrate in environments like chemical plants and offshore.
There is more on Thermal Sprays at www.poeton.co.uk/surface-treatments/thermal-metal-sprays.
Which Environments Benefit Most
Applications where corrosion of structures in industrial environments is well known to cause failure, and where the potential for damage is high, are where the greatest benefit will be realised by thermal spray corrosion protection. Large steel structures, pipework, and fabrications are typical applications that can benefit from these coatings.
Choosing Between Zinc, Aluminium, and Zinc-Aluminium Alloy
The material of choice for the coating is critical. Pure zinc is often used for corrosion protection in moderate environments. It has excellent sacrificial corrosion protection of steel. Aluminium can be used at higher temperatures and in environments that contain acids. A Zinc-Aluminium alloy, typically of 85/15 composition, offers a compromise between the two and is particularly effective in salt contaminated atmospheres and in marine environments. It provides a longer service life than either pure Zinc or Aluminium in such environments and is a good all round material for offshore structural work.
Matching the coating material to the actual service environment is the most important feature of any corrosion protection specification.
