Wind turbines and their components are exposed to a number of challenging environmental conditions such as corrosion, salt spray and immersion, dust, moisture, ultraviolet radiation, and temperature variations that drive condensation and evaporation. Like gasoline to a fire, corrosion can act as an accelerant, resulting in a component’s catastrophic failure.
To protect parts from corrosion and other environmental conditions, a multi-functional coating was recently introduced for use on wind turbine components and other applications where long-term reliability is crucial. Intended to replace older surface treatments such as heavy zinc phosphate, organic coatings, and electroplating, the electroceramic coating combines chemical, corrosion, temperature, and abrasion resistance many times greater than traditional coatings.
Intended to work on surfaces of most light metals, the coating forms a protective layer of titanium-dioxide ceramic that resists corrosion, increases wear resistance, and reduces surface friction on the finished coated surface. In addition, the electroceramic coating enhances aesthetics of finished goods, significantly increasing the components’ service life. The coating can reduce part and processing costs by protecting lower cost substrate materials.
The electroceramic coating, applied in a “green” coating process, extends the life of onshore and offshore wind parts, delivering ceramic toughness that is flexible and durable. These coatings have become widely used in the marine vehicles, automotive, appliance, and cookware industries where they protect parts constantly exposed to moisture, fuel, oil, salt, and other environmental contaminants. The coating is RoHS, WEEE, and ELV compliant because no heavy metals are used or deposited on the coated part.
An electroceramic overview
Introduced in 2007 by Henkel Corp. as Alodine EC2, the electroceramic coating works on the surfaces of most light metals such as aluminum, aluminum alloys, titanium, titanium alloys, and aluminized, aluminum-plated, and Ion Vapor Deposition (IVD) aluminum substrates. The coating is also suitable for aluminized ferrous materials.
Based on a titanium analog of electrodeposited oxides, the electroceramic coating forms a protective layer of titanium oxide ceramic that resists corrosion, increases wear resistance, and reduces surface friction of the finished coated surface. The ceramic layer provides a smooth finish, so parts generally have a soft feel similar to that of a finished ground surface.
The coating can provide long-lasting protection for external and internal wind-turbine components such as blade tips, ladders, platforms, instrumentation, brackets, and connectors. Naturally flexible and tough, the electroceramic coating provides a protective barrier that resists chips and flakes. Gravelometer testing shows the new coating provides protection superior to that of e-coat and paint. The finish appears as a light metallic grey and requires no post-application chemical, thermal, or infrared cure.
This electroceramic coating is compatible with most typical paint finishes such as e-coat, powder, and liquid paint systems, and provides an excellent base for paints, adhesives, sealants, and thermal spray coatings. The material significantly improves the retention of oils, lubricants, and thread lockers.
The electroceramic-coating process is much less complex than traditional surface treatment chemistries. The process requires fewer steps, allows for faster process speeds, and reduces processing costs.
The technical side
An electrolytic process deposits the specially formulated ceramic layer onto the surface of a metal substrate.
The ceramic layer ranges from 3 to15-microns thick with transition metal oxide using the titanium analog as a focus. That means the coating uses titanium as the metal part of the metal oxide. The coating has a hardness of 637 to 800 Vickers, as tested with a nano-indenter, yet it is extremely flexible. The coating’s roughness is less than 0.07 microns with a friction coefficient of 0.2 dry, resulting in a smooth finished surface.
After coating a metal substrate with the electroceramic material, the surface resists abrasion and provides increased wear resistance. Tested to 10,000 Taber cycles, CS-10 Wheel, with a Tabor Ware Index of 1.5. This low-wear index outperforms more traditional coatings such as electroless nickel by at least three to one. Unlike traditional finish coatings, electroceramic coatings exhibit a pass rating for flexibility of 1-2 T bend, per ASTM D 4145.
With proper current flow, the electroceramic material will coat the metal substrate in any wet area. This means internal passageways, crevasses, and holes can benefit from the coating technology, not just part exteriors.
Electroceramic coating is stable at high temperatures, to 900°C, well beyond the melting temperature of aluminum. In applications where thermal resistance is critical, electroceramic coating will remain intact even when substrates fail, situations that can be experienced in pumps, compressors, and engines.
The mechanical, chemical, and life-cycle requirements of finished parts dictate the substrates selected for use in an assembly. It is no small task to select appropriate substrates for anticipated loads and end-use environments.
A material’s cost and appropriateness for the application factor into substrate selection. With the electroceramic coating, engineers can consider potentially cost-saving alternative substrates for components found in turbines. These could be ladders, clamps, platforms, instrumentation, blade tips, and parts within the nacelle.
By coating substitute materials with electroceramic material, less-expensive substrates can perform at levels meeting or exceeding the performance of the most expensive materials. In fact, on smaller, non-utility turbines, even aluminum blades and spinners are plausible if treated with electroceramic coatings.
Less-dense materials make it possible to reduce component weights. Through material substitution, manufacturers can reduce overall material costs without sacrificing performance or working part life, evident by coated plates in impact tests.
The electroceramic coating is easily repaired by putting the coated part through the application process one additional time, so the electroceramic coating can deposit onto the damaged area.
Unlike many traditional coatings, the electroceramic coating does not have to be stripped off or sanded prior to reprocessing. When parts undergo a subsequent application, the new coating builds on the bare area and evens itself, out to a layer uniform with the original deposited coating.
Electroceramic coatings provide a surface that is ready for paint, bonding, and finish coats. The final surface is thousands of times harder than paint, yet, as flexible as paint.
Parts treated with electroceramic coatings require no additional primers prior to bonding with most adhesives. After applying the electroceramic coating to a substrate, the part can be treated with Teflon, Kynar, and other thermal spray coatings as needed.
In the Wet Real World
Volvo Penta engineers nonwind experience with the coating can be useful to turbine OEMs and their suppliers, especially if their equipment is headed offshore.
Volvo Penta, a supplier of engines and complete power systems for marine and industrial use, recently redesigned one of their engine exhaust systems and stern drives used on 20 to 35-ft pleasure craft. The redesign goal was to make the equipment more durable in saltwater and reduce the weight of assemblies. Reduced weight lets boats accelerate more rapidly, plane out faster, and travel at higher top-end speeds using less fuel than previously possible.
Intended for use with select gasoline and diesel engines, the Volvo Penta Ocean X stern drive and engine exhaust are of higher quality, environmentally-friendly components that provide pleasure-craft owners enhanced performance, lower operating expenses, and fewer warranty issues than other designs.
Although continuously exposed to seawater, the stern drive and prop-assembly housings are made completely of aluminum, which is susceptible to corrosion. To protect the material, for the last 30 years the company has treated the aluminum with chromate coating.
Chromate has long been the industry standard to retard corrosion, but it contains an extremely hazardous, EPA-regulated carcinogen that can contribute to medical problems such as various cancers and skin irritations. Also, the process of applying chromate coatings generates hazardous waste-water effluent that requires a complex on-site water treatment facility.
During the redesign, the company had two goals: Better protect the stern-drive assembly from corrosion, and eliminate the dangerous, expensive, and environmentally hazardous chromate coating process.
As of January 2010, the EPA introduced new regulations requiring catalytic converters to improve exhaust emissions. Anticipating this deadline, company engineers began to redesign the exhaust manifold two years earlier to meet the pending federal requirements. Adding a catalytic converter, however, would add to the overall size of the engine exhaust system and greatly increase its weight.
Manifold exhaust stream gets as hot as 850ºC. Exhaust gases must pass through aluminum and rubber components before they are expelled into the water. To ensure components do not melt from exposure to super-heated gases, seawater passes over the manifold, cooling the exhaust. Because seawater is highly corrosive, few substrate materials withstand continuous exposure.
The existing exhaust system was made of heavy cast iron and painted for corrosion resistance. To reduce the weight of the newly-designed, larger engine exhaust system, engineers wanted to use a lighter substrate material, such as aluminum, but were challenged to find a material that could withstand the corrosive seawater and temperature extremes.
Representatives from the company encountered Alodine EC2 electroceramic coating technology at a conference. This environmentally safe coating comes with a less-complicated coating process that generates no effluent that would require water treatment, as did the previous chromate coating. The new coating lets engineers substitute aluminum as the substrate of choice for the manifold redesign. They say the lighter-coated substrate exceeded their expectations. Because aluminum is much less dense than cast iron, the company reduced the weight of the exhaust manifold, improving both fuel economy and overall performance.
The electroceramic coating is compatible with most paint finishes such as e-coat, powder, and liquid paint systems. Hence, the coating easily accepted the paints used to make the Ocean X stern drive and engine exhaust more aesthetically pleasing.
Aluminum components coated with the electroceramic have now been used at the company since 2008. Corrosion resistance is significantly improved when compared to chromate coated parts. In tests, electroceramic coated parts were scratched and then subjected to salt water immersion. Scratched surfaces coated with EC2 easily resist corrosion creep damage, which is a deepening and widening of the damaged area.
Filed Under: O&M