Corrosion

Corrosion for the generator, as with the entire device, is an acute problem for marine energy converters. Each part of a device will either be constantly submerged, above sea level, or periodically submerged, the latter of which will suffer the most.

There is great opportunity for technology transfer from the oil and gas industries, both of which have experience in the manufacture of long lasting offshore structures. Selection of materials that have well understood behaviour in the offshore environment, for example steel and concrete, will simplify protection strategies. However, the two generators considered here contain two slightly more exotic materials: copper and Nd-Fe-B. The machine may hence be considered either as a whole, or with an alternative strategy for each component.

When two different metals are in electrical contact, the less noble of the two corrodes more than if they were isolated. This paves the way for attaching sacrificial components, anodes, to the outer surface of the generator, which would have to be periodically replenished but provide constant protection to the entire device.

The rare earth magnetic material is likely to suffer the most from corrosion due to its strongly negative electrochemical potential. The standard coating for Nd-Fe-B magnets is zinc based, which was thought to be suitable. Fundamental tests conducted prove this not to be the case, Figure 6.4.

Figure 6.4: Standard coating for magnets (A) new, (B) after 6 weeks and (C) 2 years submersion in seawater.

Figure 6.4: Standard coating for magnets (A) new, (B) after 6 weeks and (C) 2 years submersion in seawater.

The supply industry claims to offer suitable corrosion protection in the form of electro-painting, whereby a 15^m layer is applied to the material, giving it salt solution resistance up to 130°C[85], Figure 6.5B(i). Alternatively a 5^m aluminium yellow chromate coating may be applied, Figure 6.5A(i). Again fundamental tests demonstrated these coatings to be unsuitable, Figure 6.5A&B(ii).

Figure 6.5: Alternative coatings for magnets, (i) as new, (ii) after 2 years submerged in seawater

Ceramic coatings are used by the hydraulics industry and have a proven reputation in the marine environment. For example, a 300^m layer of the commercial material

Ceramax [86] would effectively seal the device from the surrounding environment.

This material, on the market for over 10 years as a coating for offshore cylinder rods

[87], is very hard (1000 Vickers) thus durable and abrasion proof. It is a non conducting inert material and at this thickness could be used to coat even the small airgap of the electrical machine. Unfortunately, there are concerns about its application to non-smooth surfaces [88] questioning its suitability for use on the VHM translator.

For the coils, insulation breakdown in a flooded environment is clearly a problem and may need separate investigation.

Figure 6.6: Coal Tar epoxy 6 weeks after submersion

Coal tar epoxy is used throughout the marine industry to protect large steel structures, such as docking gates. A small VHM stator coil was coated and submerged in seawater to investigate its behaviour for this application.

Figure 6.7: Coal Tar epoxy coating of magnets after 2 years submersion

Figure 6.6 shows sections of the stator shortly after it was submerged. The coils and surrounding cloth-tape were not covered as the tar does not adhere to a non-rigid surface. In Figure 6.6A, brown deposits can be seen covering the outside of the tar layer. This is likely to be deposits from the unprotected coils and magnet shown at the top of the picture. A close up of the magnet region in Figure 6.6B, however, shows significant corrosion. After two years submersion the corrosion in this area predictably deteriorated, Figure 6.7A, and the coating could be removed from the magnets with soft abrasion Figure 6.7B. Furthermore, after this period most parts of the stator suffered slight corrosion Figure 6.8.

Figure 6.8: Coal tar epoxy coating of steel section after2 years submersion

From these brief experiments of general observation, it is concluded that there is scope for more research into corrosion protection for all aspects of the generator design. Suggested coatings appear to be ineffective, whereas application of ceramic coatings is said to be troublesome. The issue is unresolved.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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