Novel Surface Solutions for Corrosion Protection in a sustainable building industry.

Fred van Rodijnen

Oerlikon Metco Europe GmbH, Kelsterbach, Germany

fred.vanrodijnen@oerlikon.com

Abstract Surface additions and alterations contribute to improve the sustainability from constructions. This presentation focusses on novel surface coating solutions that provide corrosion protection in green energy producing wind turbines. The coatings contribute to sustainability from the construction while providing longer life, leaner construction design and reducing corrosion induced friction. Thin Film-, Thermal Spray- and Laser Cladding Surface Solutions are described with practical applications

Key words

“Thermal spray”, “HVOF”, “Arc wire spray”, “Combustion wire spray”, “Plasma spray”, “Thin Film”, “Laser cladding”, “Balinit”, “Balitherm”, “BALITHERM IONIT”, “BALITHERM® IONIT OX”

1. Introduction Wind turbines are exposed to drastically varying operating conditions ranging from extremely hot to freezing temperatures. Moisture and condensation on components cause damaging wet corrosion that degrades these high investment and complex machines. Due to the presence of salt, such corrosion degradation will be even more pronounced for offshore turbines than onshore installations.

Figure 1 Coating solutions for wind turbine parts

Typical wind turbine components that are exposed corrosion are the foundation, the tower, the pitch gear/bearing, the jaw gear/bearing the hub, main shaft, machine frame and brake disk. Within the nowadays weight savings and savings in natural resources are demanding improved component surfaces, resisting corrosion and/or oxidation. See figure 1.

2. Surface protection solutions A variety of surface coating technologies is available for protecting wind turbine components against corrosion and wear. See figure 2. The following part will give brief descriptions about Nitriding, Thin Film Coating, Thermal Spray and Laser Cladding. Combinations from these technologies are also possible, it would take too far to describe the combinations in this paper. We encourage you to visit the respective web sites mentioned at the end of each brief description.

Figure 2 Coating technologies for wind turbine parts

Nitriding Technology

Plasma nitriding This is an industrial surface hardening treatment for all kinds of steel. Under vacuum and through the application of a bias voltage, the Nitrogen and Hydrogen gas molecules become ionized and begin to react with the iron of the part to develop a hard nitrided zone (visible as glowing). This process is performed over a broad temperature range (350°C – 600°C) and in a vacuum environment. Process treatment takes place by diffusion and not by coating.

Plasma nitrocarburizing Addition of carbon leads to the formation of hard, wear-resistant compound layers, as well as corrosion resistance even better than that of nitride steels. Nitrocarburizing also reduces reactivity with materials contacted by coated parts such, as lubricants (tribo-oxidation).

Nitriding Thin Film Coating

Thermal Spray Laser Cladding

For more details, please visit the following webpage: https://www.oerlikon.com/balzers/com/en/portfolio/surface-technologies/heat-treatment/

Thin Film Technology In the PVD process the high purity, solid coating material (metals such as titanium, chromium and aluminium) is either evaporated by heat or by bombardment with ions (sputtering). At the same time, a reactive gas (e.g. nitrogen or a gas containing carbon) is added; it forms a compound with the metal vapour which is deposited on the tools or components as a thin, highly adherent coating. A uniform coating thickness is obtained by rotating the parts at a constant speed about several axes. Coating properties (such as hardness, structure, chemical and temperature resistance, adhesion) can be precisely controlled. PVD processes include Arc evaporation, Sputtering, Ion plating, and Enhanced sputtering.

For more details, please visit the following webpage: https://www.oerlikon.com/balzers/com/en/markets/energy/power-generation/components/wind-turbines/

Thermal Spray Technology As shown in Figure 3, thermal spray coating involves the use of a torch to heat a material in powder or wire form to a molten or near-molten state, and the use of a gas to propel the material to the target substrate, creating a completely new surface. The coating material may be a single element, alloy or compound with unique physical properties that are, in most cases, achievable only through the thermal spray process. Thermal spray coatings are a highly cost-effective and straight-forward method for adding superior properties and performance qualities to a given engineered surface. The varieties of products and coatings that can be enhanced by thermal spray are virtually limitless, as are the types of coatings that can be produced. The coatings are usually metallic, ceramic, carbides, or a combination of these materials to meet a range of physical criteria.

Figure 3 Thermal Spray Principle

As a family of related technologies, each thermal spray process brings distinct advantages. This provides a high degree of flexibility to meet a wide array of application and production requirements. These processes include:

Atmospheric Plasma Spray Champro® Controlled Atmosphere Plasma Spray HVOF (High Velocity Oxy-Fuel) Spray, using either gas or liquid as the combustion fuel Combustion Powder Thermospray® Combustion Wire Spray Electric Arc Wire Spray

For more detailed information about the thermal spray processes visit the following site: https://www.oerlikon.com/metco/en/products-services/coating-equipment/thermal-spray/processes/

Laser Cladding Technology In Laser Cladding (LC), also known as LMD (Laser Metal Deposition) the Laser source is used as a local heat source. Defocusing the laser beam on the work piece enables a defined source of heat. The coating material, in the form of wire or powder, ca be carried by an inter gas stream through a nozzle and into the melt pool. The laser head, consisting of optics and nozzle, moves across the workpiece surface to deposit single tracks, complete layers or multi-layer high-volume build ups of simple or complex geometry. The schematic overview of the process is shown in figure 4.

Figure 4. Laser Cladding Process

For more details, please visit the following webpage: https://www.oerlikon.com/metco/en/products-services/coating-services/coating-services-laser-cladding/process/

3. Applications for wind turbines

Figure 5 Parts with coating solutions

Wind Turbine Tower A wind turbine tower is a hollow, cylindrical structure that supports a wind turbine. The tower consists of multiple cylindrical or conical tubes between 4.0 m and 2.0 m in diameter and lengths up to 45 m per element. The elements are bolted together with flanges. A complete tower can reach up to 160 m in height. A tower accounts for almost half of the construction costs of a wind turbine system. It is very important to give the tower a superb corrosion protection coating to avoid corrosion maintenance that would result in extremely high costs in remote areas. Wind turbine towers for offshore installation are thermal spray coated completely on the inside and outside with a galvanically active metallic coating. For onshore towers, the lower 2.5 m on the inside, as well as the connection flanges, are thermal spray coated. Thermal Spray Coatings can be applied with standard arc equipment. Follow the link for some examples: https://www.oerlikon.com/metco/en/products-services/coating-equipment/thermal-spray/systems/arc/ Pitch, jaw gear/bearing The bearing and gear wheel combination, called slew rings, position the wind turbine system into the wind (yaw control) and position from the blades on the hub into the wind (pitch control). Corrosion of the bearing system would prohibit correct positioning and thus influence the efficiency of the wind turbine system. Rusted slew rings that could not idle the blades during extremely strong winds, have been known to cause catastrophic wind turbine failure. For this reason, the majority of slew rings are thermally sprayed with zinc-aluminum or zinc. For large gears, such as those wind turbines, BALITHERM® IONIT nitriding is a powerful treatment for parts up to 3 meter diameter, 10 m length, and/or weights up to 40 tons. BALITHERM® IONIT plasma nitriding of a 2.5 meter diameter wind turbine ring gear. Accurate

process parameters allow for significantly less distortion than conventional gas nitriding processes. A BALITHERM® IONIT treated 2 meter diameter ring gear showed five times less roundness deviation and seven times better planarity than gas nitriding. Main shaft It is very important to give the main shaft a superb corrosion protection coating to avoid corrosion maintenance that would result in extremely high costs in remote areas. Main shafts for offshore installation are thermal spray coated on the non-bearing seats surface with a galvanically active metallic coating like Zinc or Zinc Aluminium 85/15 Machine frame and Hub The machine frame, the base for the generator and the hub as well as the hub, carrying the blades and transferring the energy to the generator, are exposed to extreme weather conditions, especially for offshore wind turbines. For a perfect corrosion protection in these extreme circumstances the parts are coated with a sacrificial Zn or ZnAl 85/15 layer and painted as top layer. Brake disc A brake disk is a critical part in a wind turbine system. During high wind load events and during maintenance, the blades are placed in an idle position and the wind turbine rotor system must be in a safe, fixed position. This position is held via the brake disk system. In order to avoid corrosion, the disc is coated with zinc or zinc aluminum, except for the braking surface. Corrosion of the brake disc could cause a brake malfunction possibly resulting in a catastrophically failure. Tailored coatings for the braking surface can achieve an increased corrosion and wear resistance. Reducing the fine dust generation at the same time results in an ecologically and economically optimized brake system.

4. Other industry applications Boiler Besides in wind turbines, thermal spray solutions are also used in many other industries for example boilers to protect against high temperature corrosion and erosion. Coal-fired and waste-

to-energy (WTE) boilers are exposed to complex corrosive and erosive surface mechanisms on the waterwalls, the superheaters and the flue gas desulfurization systems. In fact, boiler tubes are subject to some thirty different failure mechanisms 1, many of which are the result of some form of corrosion or erosion. These attacking mechanisms can lead to reduced operational efficiency and failures that result in unscheduled outages for component replacement. The industry has recorded more than 30,000 boiler tube failures, and 80 % of those failures have resulted in forced plant shutdowns. The material and installation costs, alone, to replace these components can cost more than one million U.S. dollars for a large boiler system. But the greater cost can be measured in terms of lost revenues to the operating utility or company for plant downtime, which can be millions of U.S. dollars per day. Surface protection for these boiler components can extend their useful service life, thereby reducing replacement costs and outage time. Figure 6 On site, manual electric

arc wire spraying of a coal-fired boiler waterwall

Recommended surface protection materials for boiler applications. The materials applied by electric arc wire spray for boiler applications are chemically the same as those used for weld overlay. These materials are proven performers for erosion and corrosion resistance.

Steam Turbine Improve efficiency and reduce emissions with abradable coatings for steam turbines. With steam turbines being the largest power generation type worldwide, both steam turbine designers and operators find they are under tremendous pressure to reduce emissions that contribute to global warming. This, combined with rising fuel prices and the need to deliver affordable power drives the on-going need for greater efficiency. One area where such efficiency gains can be achieved is through reduction in the clearances between rotating components (such as blades and shafts) and stationary components through the introduction of abradable seals. The steam turbine industry is in a process of adopting abradable sealing technology which has long been used in other turbomachinery applications, in particular aero and industrial gas turbine engines. The weight and size of steam turbine components, particularly for the largest plants, precludes the possibility of transporting these components off-site for processing. Furthermore, extended overhaul outage time can cost millions of dollars per week in lost revenues. Therefore, the industry needs services that can apply these sealing technologies at the manufacturing or final assembly site, or on-site during turbine overhaul. Oerlikon Metco, the leading supplier of abradable solutions for IGT and aero applications, has developed a portfolio of abradable materials for long term service in high pressure steam environments. Applying abradable sealing technology to steam turbines results in increased power output and efficiency, or reduced fuel consumption. Both scenarios result in reduced CO2 emissions, and in the case of coal-fired and oil-fired plants, reduced SO2 emissions. Just a 0.5 % improvement in steam turbine efficiency is worth over US$ 1 million per year for a 1000 MW unit. Within a typical 5-year overhaul period, the investment in steam turbine abradables pays back 13 times. When all relevant sections of

the steam turbine are fitted with Oerlikon Metco’s abradable technologies, significant efficiency gains can be achieved. In addition, Oerlikon Metco Coating Services have developed specialty skills to apply abradable seals to very large steam turbine components as large as 2 m (6 ft) in diameter and 4.5 m (15 ft) in length and weighing more than 10 tons. And these coatings can be applied either in their spray shops or at the site of final turbine assembly or overhaul to ease logistics and reduce outage time. Leading turbine OEM’s have already incorporated Oerlikon Metco’s abradable technology with very successful results.

Figure 7 A) Abradable coating applied to the outer steam path seals on the casing. B) Abradable coating applied to the inner steam path seals on the shrouds of the stationary blades.

Hydro Turbine Robust coating solutions for hydropower turbines extend operating life and maintain efficiency. The wear mechanisms in hydropower turbines vary considerably, depending on the service conditions that the individual components are exposed. Turbines operating in high head conditions, such as Pelton turbines, may be exposed to greater wear from water erosion. On the other hand, turbines exposed to low head conditions, such as Kaplan turbines, may be exposed to greater wear from entrained debris in the water. The type of entrained debris in the water must also be considered. Wear effects will be significantly different depending on whether the entrained debris is silt, sand or gravel, and differences will also be evident depending on the amount of debris. Finally, different head waters will have different corrosion potentials, further complicating the means to mitigate the wear effects. Some components may be exposed to sliding wear primarily caused by the dynamic interaction between mating components. Here, wear coatings can also prove to be an effective deterrent. While no coating will completely stop these effects, coatings are an effective means of greatly prolonging service life and efficiency. But for the coating to be optimally effective, all factors must be considered.

Proven Oerlikon Metco hydropower coating solutions, Coatings applied to hydropower turbine components can extend service life, thereby maintaining turbine efficiency and decreasing maintenance costs. Oerlikon Metco has decades of experience with proven and highly effective coating solutions for hydropower turbine components. Some commonly applied Oerlikon Metco coatings are listed below:

Figure 10 Eroded guide vanes on a Francis turbine

Figure 8 Cavitation damage on a Francis turbine

Figure 9 Worn buckets on a Pelton turbine

5. Summary Protective coatings can greatly extend the service life of power generation components. However, the coating material, application equipment and parameters must be carefully chosen for optimal coating performance. Seemingly similar materials and application technologies can perform very differently in service. Oerlikon Surface Solutions has extensive experience in coatings for power generation applications. Our broad portfolio of materials, equipment, application know-how and our testing capabilities allow us to provide both standard and application-specific coating solutions that are optimized for conventional and green power generation components.