Protection against corrosion : The benefits of sprayed metallic layers

  1. There is almost no limitation on the size of component or structure which can be treated.
  2. The materials are simple to apply using Metal Coat combustion flame or electric arc spraying equipment. Operators can be trained in a relatively short time and with a small amount of practice are capable of producing consistently sound and even coatings on properly grit blasted surfaces.
  3. Where large areas or large numbers of components are to be sprayed, the wire fed spraying equipment is easily mechanised or fully automated. Both the combustion gas and electric arc spraying systems have efficient stop/start devices for production economy.
  4. The process itself is simple involving only two or three stages; spraying is preceded by grit blasting and may be followed by sealing of the deposit. This simplicity makes quality control relatively easy and offers fewer stages for errors to occur.
  5. Sprayed metal coatings may be handled immediately after treatment. There are no protracted drying times and factory floor space can be more efficiently utilised.
  6. Properly applied sprayed metal coatings are more robust than paint systems and are consequently able to withstand rougher usage.
  7. Sprayed zinc, aluminium and associated alloy coatings give long lives in most naturally occurring environments. Ten years to first maintenance is common and over twenty years may be readily achieved with the appropriate system.
  8. There is no distinct limit to the thickness of sprayed coatings. Zinc may be sprayed to over 3mm and unlike galvanising, thickness may be varied from place to place to provide protection in critical areas.
  9. Zinc, aluminium and associated alloy spraying wires are of consistent quality and purity. Properly stored, shelf life is indefinite; there are no settlement problems as may be experienced with powder spraying materials and no mixing as required with paints.
  10. Even if a sprayed deposit is locally damaged, the sacrificial action particularly of zinc prevents corrosion from edges and discontinuities. It may also delay the onset of rusting of structures which have been neglected.
  11. Although bare metal sprayed coatings give long lives, they may be sealed to extend the life time or enhance the visual appearance.
  12. The surface being sprayed remains cool. Consequently, there is no risk of heat distortion or metallurgical degradation of load bearing steel structures. Sealed tubular or hollow sections can be coated externally without danger.
  13. The sprayed metal surface maintains the efficiency of friction grip areas and ensures their effectiveness throughout the life of the structure
  14. Thick anti-corrosive paint systems are generally unnecessary but the texture of “bare” sprayed coatings provides an excellent “key” for subsequent paint treatment. In most cases where the sprayed metal is properly sealed, these need only be applied for decorative purposes.
  15. The nature of the equipment makes it ideal for either factory or site application and coatings can be deposited in ambient conditions totally unsuited to other methods of protective treatment.

Properties Of Sprayed Coatings Efficiency And Coverage

Efficiency is used as a test to assist in establishing the optimum economic and technical deposition parameters. In general, conditions giving high deposition efficiencies are close to those for optimum fuel utilisation, they are also close to those for maximum integrity.

Factors Affecting Efficiency

Efficiency will be affected by:

  • The shape and size of the component   
  • The basis material and its preparation
  • The spraying parameters

Measured efficiency will be reduced when spraying onto small components which are not completely within the spray stream. Even with large components, overspray at edges will reduce efficiencies. Spraying at angles other than normal to the surface will reduce efficiency. Normally, efficiencies will be higher when spraying onto similar materials and onto properly gritblasted surfaces.

Deviation from the recommended spraying parameters will reduce efficiencies. This will be particularly noticeable if atomising pressures and spraying rates are increased, when the deposit volatilises easily or forms a volatile oxide. It is important that deposition rate and feed rate are not confused. By increasing the fuel consumption, it is possible to spray slightly faster with most materials. In combustion gas spraying particularly, the resultant reduction in an efficiency together with increased fuel consumption renders the practice extremely uneconomic.

Measurement of Efficiency

A known weight of material is sprayed under closely controlled conditions on to a suitably prepared flat plate, round bar or tube. The efficiency is calculated as the weight gain of the sample per 100gm of material sprayed.

Significance of Efficiency

Because large areas are rare and because spraying is continued beyond the area being coated, perceived efficiencies may be lower than those quoted. Also deposition efficiencies will vary between companies and between operators; better trained and more highly skilled operators will usually achieve greater efficiencies and hence waste less material.

Thermal Spray versus Other Coating Processes

Thermal spray processes differ from other coating processes in that they are non-atomistic; that is, they do not deposit material onto surfaces as individual ions, atoms, or molecules. Instead, relatively massive particulates are deposited onto a surface in the form of liquid droplets or semimolten or solid particles. Coating feedstock materials usually come in the form of powder, wires, or rods. Feedstocks are generally heated to their melting point by a plasma jet, electric arc, or flame. The molten material is then atomized and propelled toward the substrate by process gases or atomizing jets formed through nozzles. Thermal spray is also a “line of- sight” process, where the projected stream of droplets deposits only onto surfaces that are directly in line with the spray stream. Because thermal spray processes are high-enthalpy (high energy density) processes, they are characterized as having high coating rates relative to other coating processes—for example, chemical vapor deposition (CVD), physical vapor deposition (PVD), and electroplating.

Additionally, thermal spray processes are capable of operating over a broad range of temperature, velocity, and atmospheric conditions, enabling them to apply the greatest variety of materials. Other advantages of thermal spray processes include a simplified waste-disposal stream and the ability to deposit thick coating sections. Thermal spray coatings are considered to be “overlay” coatings, which can be defined as materials added to an original surface (called the substrate) where there is little or no mixing or dilution between the coating and the substrate, thus preserving the composition of the base material. Some forms of surface treatments are entirely diffusional in nature. In these surface treatments, elemental materials are added to a base material through diffusional processes, which occur below the substrate surface and cause no thickness build-up. Alloying may occur with the base materials to form new protective compounds or phases. Diffusion and alloying often occur with thermal spray coatings, but the reaction zone is extremely narrow due to the extremely rapid cooling rates of the individual molten droplets impacting the relatively massive and cold substrate.