Editorial Feature

Purecoat - An Optimized Thermal Spray Process

Thermal spraying is not a new method. Developed in 1912 in Switzerland, this method is well-established and extensively used. Coatings manufactured in this manner are being used in several different industries.

However, the prospects afforded by the process are so diverse that it is constantly evolving and expanding with new techniques and applications. One such development is the Purecoat process.

A little more than three years ago, a consortium of seven interested companies established a Brite project to discover ways of enhancing the coating quality of current methods, without considerably increasing the operating cost. The starting point for such an exercise is to examine the issues associated with prevailing technologies. To do that, one needs to have a fair understanding of the structure of a metal-sprayed coating.

Coating Structure and Buildup via Thermal Spraying

All types of thermal spraying require the projection of small heated particles of material at a prepared substrate. Upon impact, the particles disperse and freeze, and stick to asperities on the surface. As progressively more particles follow, they fall on each other and together form a continuous coating. The quality of the last coating depends on the properties of each of those particles.

For example, a cold particle will not deform correctly at the surface and will result in porosity around it and weak cohesive strength. In contrast, if the particle is very hot and exposed to oxygen, it will oxidize and the coating will have a large number of oxides, in addition to the desired metal. So, how to avoid such issues?

Ensuring Coating Materials are Molten

Arc spraying is a process that employs a wire feedstock melted by an arc. Due to this, it is not possible to create unmelted particles. This solves the initial issue of making sure that all the particles are melted. Now the question is how to prevent oxidation.

Thermal Spraying in Inert Atmospheres

The apparent answer is to use an inert gas to spray in an inert gas chamber. This has been done on numerous occasions since the 1950s, and it certainly works. Even titanium coatings can be created with virtually zero oxidation. But the use of a chamber means that the components are restricted in size and fixed in location, and one of the special advantages of thermal spraying may be lost.

How much better would it be to simply shroud the spray in inert gas so that an accurately portable inert environment for spraying can be obtained? Again, this is not a novel concept. Patents and papers for shrouding of plasma spray systems date back to the 1960s and different levels of success have been accomplished because it is not as easy as it would seem at first.

Thermal Spraying in Localized Inert Atmospheres

If the user merely puts the spray head into a plain tube and uses the inert gas as the propellant, then the divergence of the spray stream inside the tube is likely to coat the inside with molten material, which accumulates and ultimately hinders the spray.

Moreover, as the rapid moving jet of gas leaves the tube, it creates a region of low pressure and the surrounding air is absorbed into the inert gas, diluting it to the point of uselessness within a short distance. How can these glitches of spray divergence and oxygen entrainment be resolved?

A solution was discovered by modeling the spraying process by finite element methods using a Cray Supercomputer.

Modeling

The computer model created by ESIL in Dublin can predict the levels of oxygen in the gas stream at different distances downstream and radial distances from the spray centerline. At the labs of CISE in Milan, these outcomes were supported by physical measurements of the oxygen concentrations, and an extraordinary consistency of results was revealed.

Based on these outcomes and knowing the measurements of divergence, that nearly all the spray came under the low oxygen region, it was predicted that the coating material would not be extremely oxidized in flight.

Prototyping

Apart from the results from the modeling, prototypes were constructed to examine the divergence and the patterns of flow, and even the preliminary results were very promising. Initially, the goal was to beat the plasma specification of 8% oxide content in a nickel-chromium alloy but this eventually resulted in 3% spraying on tubes. In relation to porosity, the aim was to obtain 10% but less than 1% was achieved.

What are the practical advantages of this latest technology and what can be done that was not done before? To comprehend this, one needs to analyze the issues involved in a few real applications.

Wet Corrosion Resistance

Alloys such as Inconel 625 offer corrosion protection by providing a barrier layer impervious to an entire range of chemical attack. Such alloys provide protection by readily forming an adherent oxide film, generally chromium oxide, on their surface, which hinders further corrosion.

But this readiness to develop oxides means that commonly sprayed coatings will have oxidized particles, which are thus depleted in chromium. If the material is depleted in chromium, it is evidently not the anticipated material any longer, and cannot be predicted to function as expected.

The fact that a traditional coating contains depleted and non-depleted zones means that it is no longer uniform, and that internal electrolytic cells can be set up causing dissimilar metal corrosion.

Studies have also revealed that these oxide layers around the particles can serve as a path for additional oxidation and infiltration of the oxidizing medium to the substrate; thus, the lower the content of oxides in the coating the better. In addition, coating density is very critical in decreasing the paths along which the corrosive medium can migrate.

High-Temperature Oxidation and Sulfidation

Components such as incinerators, boiler firewalls, and the hot gas paths of gas turbines undergo corrosion at high temperatures. The same quality aspects arise when it comes to guarding the components against oxidation and sulfidation as with wet corrosion testing, but with various materials.

In this case, high chromium alloys or alloys with aluminum and chromium such as metallization 88E and 78E are used. The aluminum and chromium form an adherent oxide scale, which hinders more corrosion. Where the coating is already oxidized, the corrosive species move along the boundaries and enable the coating to be corroded, both internally and from the surface.

All of these materials depend on offering a dense resistant barrier of an alloy, which is not tarnished by the spraying process. Regular coatings fail because of degradation of the alloy and penetration through the coating. The Purecoat process reduces these toxic effects leading to improved performance and longer life.

Application Testing

Once the device was made and the microstructures were found to be promising, the coatings were tested in realistic situations. Three application areas were selected—high-temperature sulfidation as is observed in boilers burning low-grade coal and Orimulsion; wet corrosion as would be seen in seawater valves or chemical plant; and ductility as is mandatory in the production of inking rolls.

High-Temperature Sulfidation Testing

For the hot sulfidation tests, coatings were produced on cylindrical samples and exposed to hundreds of hours of testing under a regulated atmosphere in a particularly designed furnace. The pieces were tested at 250-hour intervals, and weighed to discover the weight gain because of the formation of sulfide. The ideal coatings did better than an HVOF coating costing five times as much to apply.

Thermal fatigue can be an issue in coated materials, and hence tubes coated on a single side only were thermally cycled for 1000 hours. There was no indication of spalling or lifting of the coating.

Wet Corrosion Testing

To evaluate the effects upon wet corrosion properties, one of the most extensively used corrosion-resistant alloys, Inconel 625 was chosen, and this was applied to carbon steel plates. These plates were tested using two approaches. Initially, electro-potential tests were carried out to compare the sprayed material itself with wrought Inconel 625.

Standard Inconel 625 freely passivates at a specific level. The behavior of the carbon steel plates will be more like the wrought material, based on how close to this level the sprayed coating passivates. Purecoat Inconel 625 acts a lot more like the real alloy than a traditionally sprayed sample.

The second test undertaken was a traditional salt spray test. This examines the resistance of the material itself as well as its permeability. If the salt spray enters the coating, then the underlying steel will be preferentially corroded and rust staining will show up on the surface. Testing revealed that the Purecoat sample considerably outpaced the regular spray.

Ductility Testing

Besides the advantages of corrosion behavior, electrical and mechanical properties can also be improved. Only by using methods like HVOF or Purecoat, can the surface finish be sufficiently good for use with the gravure printing process. In this process, a highly polished surface is etched with the image to be printed.

Likewise, the electrical properties are relatively better with the electrical conductivity nearing 90% of that of wrought copper compared to 40% for a traditional sprayed copper.

The transformation is not so surprising when the microstructures are equated. Even materials like 13% chromium steel—the most widely used material to be arc sprayed for engineering purposes—gains significantly from being sprayed in this way.

Costs

Usual operating costs are raised by the usage of 1.5 m3 of nitrogen per minute. Normally, this might cost 0.5 USD per m3 and thus 40 USD per hour or 0.75 USD per minute. If the material cost is 30 USD per kg and the spray rate is 10 kg per hour, the extra cost is 15% of the material cost and comes to approximately 10% of the entire cost.

Conclusions

  • General mechanical and electrical properties are enhanced.
  • The Purecoat spraying process is a functional shroud arc spraying system.
  • In salt spray corrosion tests, coatings display lesser penetration and hence relatively less corrosion of the substrate.
  • In hot corrosion and sulfidation, Purecoat-sprayed FeCrAl did at least as well as HVOF sprayed 50/50 NiCr without spalling under thermal cycling.
  • The coatings produced have significantly less oxidation and degradation.
  • In potentio-dynamic tests, Purecoat-sprayed coatings act more like the wrought material than do traditionally sprayed materials.
  • Usual coating costs are of the order of 10% higher than for conventional Arcspray.

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