An International project involving seven companies from four European countries resulted in Metallisation and their partners developing a new spraying process known as "PURECOAT™"

The new system minimises degradation in the sprayed material allowing it to behave more like the feedstock and improving the behaviour of the coating. The process is unique in providing near HVOF quality coatings at near Arcspray costs - satisfying both technicians and accountants. The heart of the system is a new spray head designed with the aid of a Cray Supercomputer which performed complex fluid dynamics modelling to optimise gas flows.

The head solves the problems previously encountered with attempts to shield the spray stream from oxygen when spraying in the open air. In previous systems, spray divergence caused the system to clog and turbulent recirculation sucked in the ambient air allowing particles to oxidise in flight. By clever design of the head geometry, these problems are overcome whilst using a relatively modest amount of shrouding gas. However the real value of the process lies in the coatings it produces and the applications that now become viable.

The fundamental virtue is that coatings are deposited with similar properties to the feedstock material. Normally this can only be achieved by some types of HVOF equipment and by very expensive vacuum processes. Basically this means that the coating material is not degraded. For example in potentiodynamic corrosion tests on conventionally sprayed Inconel 625 and those produced by the new process, the "PURECOAT™" coating is markedly superior.


Figure 1

The picture in Figure 1 shows a conventional coating in which dark layers of oxide are present around and between every sprayed particle. These oxides are principally composed of chromium oxide and the oxidation process 'sucks' chromium out of the metal alloy changing its' composition. In some cases as much as half the chromium can be removed from the material. Clearly such an inhomogeneous and damaged structure cannot be expected to perform.

The picture in Figure 2 shows a similar material sprayed by the new "PURECOAT™" process in which oxides are almost non-existent.


Figure 2
 

Having established that the coatings are a massive improvement on their conventional counterparts, what is the downside ? There is obviously a cost to be paid due to the use of shrouding nitrogen gas. However,looking at the costing of a typical sprayed coating of Inconel 625 it becomes obvious that the bulk of the cost is in the material. Our analysis of the costs shows that PURECOAT™ is approximately 10% more expensive than Arcspray and less than 50% the cost of HVOF.

Applications
Corrosion Resistant Alloys e.g. Inconel 625
These alloys provide corrosion protection by providing a resistant barrier layer. They offer protection by readily forming an adherent oxide film usually of chromium oxide, on their surface, which inhibits further corrosion. However this readiness to form oxides mean that normally coatings will contain oxidised particles, which are therefore depleted in chromium. Research has shown that these oxide layers around the particles can act as a path for further oxidation and penetration of the oxidising medium to the substrate. The lower the oxides content of the coating the better. Coating density is also very important in reducing the paths along which the corrosion medium can migrate.

High temperature oxidation and sulphidation.
The same factors arise in the protection of components against high temperature oxidation and sulphidation. In this case alloys such as Metallisation 78E and 88E are used to give ten times the resistance of normal steel. All of these materials rely on providing a dense resistant barrier of an alloy, which is not degraded by the spraying process. Normal coatings fail due to degradation of the alloy and penetration through the coating. The Purecoat process minimises these deleterious effects leading to better performance and longer life.

Electrical Alloys
The low level of oxides present in the coating ensures a much higher than normal electrical conductivity. A typical sprayed copper coating has an electrical conductivity around 40% of that of wrought copper. Using the new process electrical conductivity can approach 90%.

Typical Spraying Parameters
Metallisation Arc Spray Pistol base: 140 or 528E
Primary pressure: 3.0 BAR
Secondary pressure: 2.0 BAR
Tertiary pressure: 0.6 BAR
Current: Dependent on wire size. Must be limited to 300A to maintain high quality deposits.

Finishing
The coating may be finished in conventional ways. Improved surface finish may be expected due to the improved coating cohesion.

Precautions
Observe normal spraying practices, respiratory and hearing protection is advised. UV eye protection must be provided when arc spraying.

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