Thermal Spray Coating Applications for Boiler Tubes

Summary

Thermal spray coating applications for boiler tubes represent a critically important engineering solution. This article examines damage mechanisms occurring in boiler applications, which incur particularly high maintenance and renewal costs, and comparatively presents materials and thermal spray coating methods applied to boiler tubes.

1. Introduction

Erosive wear under high-temperature conditions constitutes a serious engineering problem, particularly for boiler systems. Pipe lines carrying hot fluids, tubes, hot vessel tanks and many other connecting equipment are adversely affected by this metallurgical damage mechanism, resulting in crack formation, tearing, leakage and consequently downtime and maintenance requirements. Solid particles released by combustion reactions reach temperatures far exceeding boiler operating temperatures. These particles strike all metallic surfaces in the connection system at high velocity and temperature, causing wear over time. In coal-fired boilers (particularly when low-quality coal is used), high sulfur content emerges from combustion reactions, causing serious corrosive effects. Additionally, water vapor formed in connecting pipes produces corrosive and erosive effects. In response to this situation, boiler maintenance requirements increase, service periods shorten, and component repair or renewal becomes necessary. During this process, maintenance, repair and renewal costs rise significantly. The boiler operating environment is highly aggressive, with corrosive and erosive effects threatening the lifespan of metallic components at elevated temperatures. Accordingly, there is a need for alloys resistant to wear and corrosion at high temperatures, or protective surface coating types and methods. Figure 2 displays tubes removed from a long-used, damaged boiler system. As evident, these tubes (Cr-Mo steel) have been subjected to intense corrosion and erosion. Measurements revealed significant reduction in wall thickness. The presence of oxide layers, spalling, surface cracks and cross-sectional changes are distinctly apparent on surfaces where hot corrosion is active and steam passes through, and on the inner surfaces of the tubes.

2. Damage Mechanisms in Boiler Tubes

a. Corrosion fatigue in the combustion zone: generally in coal-fired boilers, cracks forming on the outer surfaces of tubes due to thermal stresses develop and progress more rapidly under corrosive effects. b. Fly ash erosion: erosive wear caused by fly ash impacting tube walls results in significant reduction in wall thickness. c. Corrosion in water flow zones: occurring mostly in the lower sections of boilers. As a result of corrosion, cross-sectional changes occur in tubes due to corrosion. d. Thermal fatigue: particularly in superheaters, cyclic stresses develop under variable thermal gradients. This results in high stresses and crack formation. Over time, these cracks develop causing damage. e. Erosive corrosion: metal surface corrosion is further increased by contact with an erosive fluid. During circulation, corrosive fluid (such as water or water vapor) reaches high velocities, creating an abrasive effect. This results in erosive corrosion. f. Overheating: short-term or long-term overheating can cause high thermal stresses in tubes, resulting in damage. g. Chemical cleaning damage: improper chemical solution cleaning of tubes, extended waiting periods for rust and scale dissolution can result in additional corrosion. h. Pressurized air/steam effects: maintaining pressurized air or steam to ensure pipe flow can cause internal solid particles to adhere to surfaces, create impact, and lead to crack formation. As evident, numerous damage mechanisms affect the boiler system and its components. Engineering solutions in response typically focus on protective surface coatings: welded repair and build-up applications or thermal spray coating methods are applied. In welded build-up applications, heat-resistant steel material can be significantly affected by heat during welding.

This condition weakens the weld zone in terms of corrosive and erosive wear. For this reason, thermal spray coatings stand out both in terms of application advantages and material diversity.

Without creating thermal heat load on the substrate material, coating material in powder or wire form can be melted with heat input (electric arc or flame energy) and sprayed onto the surface at high pressure—either on a local surface or over the entire surface—to obtain protective coating layers in lameller form (<2 mm thickness) (single-layer or multi-layer metallic or ceramic-based). Thermal spray coatings can be applied manually or robotically to surfaces. Thermal spray coating methods are a surface technology family encompassing numerous coating methods, coating materials and applications. In applications, coating material in wire or powder form is melted using flame, plasma or arc energy through an appropriate spray gun or nozzle and converted into droplets. Subsequently it is sprayed onto the surface at high velocity, rapidly solidified and accumulated on the surface as splats (Figure 3a). The lifespan and performance of coatings vary depending on coating material type and coating method.

3. Thermal Spray Coating Methods Used in Boiler Applications

Thermal spray coating methods can be used to achieve protective coatings, coatings aimed at performance and lifespan extension, or repair coatings. If coating spalling, surface separation or cracking occurs, the coating can be removed and reapplied within a short timeframe. The coating process consists of several stages: pre-surface preparation (surface cleaning), surface activation (grit blasting), coating operation (with appropriate method, parameters and suitable equipment), post-coating operations (machining or sealing). Coating applications can be performed on-site using mobile systems. When components are disassembled, they can be coated manually or robotically or in a controlled manner with the aid of a PLC-controlled system at homogeneous thickness (Figure 4). At this point, determining expected coating performance and properties is highly important. Additionally, attention must be paid to the surfaces where coating will be applied and application angles and geometries. Otherwise, failure to achieve uniform thickness will limit coating lifespan as a result of thermal stresses, erosive and corrosive effects. Thermal spray methods used in boiler tubes and pipes can be divided into 3 groups: low-velocity flame spray (powder or wire), high-velocity flame spray (powder), electric arc spray (wire) methods. These methods have relative advantages and application benefits. The most important differences between them are spraying capability, coating cost and coating quality. We can examine the coating methods in order:

1. Low-velocity flame powder/wire spray (FS):

These are the most basic and most economical coating methods in the thermal spray coating family. When powder is used it is called powder flame spray, and when wire is used it is called wire flame spray. Coating materials (metallic, ceramic or polymeric) fed to the flame energy obtained from the combustion of fuel and oxygen can be sprayed with pressurized air onto surfaces and practically accumulated. Because spray temperature and velocity are lower compared to other thermal spray coating methods, coating adhesion and microstructural properties are relatively weak. They are frequently preferred in repair and corrosion protection applications (Figure 5).

2. Electric arc spray (TWAS):

A method demonstrating superior performance compared to flame spray and enabling production of quality coatings. By using conductive metallic materials in wire form (such as Al, Cu, stainless steel, nickel alloys), an arc is created between 2 wires, and as this high-energy arc forms, the wires are fed to the nozzle tip via a drive mechanism, resulting in wire melting and atomization, then spraying at high velocities and accumulation on surfaces—a practical and economical coating solution. It is suitable for on-site applications. Below is a schematic representation of the electric arc spray system (Figure 6).

3. High-velocity flame spray (HVOF):

A flame spray method capable of producing high-performance coatings with high-velocity spraying capability. Coating durability and lifespan are considerably higher than arc spray. Coating application is performed with powder materials through a special spray gun, as in typical flame spray methods. Kerosene or hydrogen, propane gases can be used as fuel. Flame energy varies depending on the fuel used. Because it has spraying capability above sonic velocity, very dense and hard coatings (metallic, cermet) can be produced. The oxide and porosity ratio in coating structure is at the lowest levels compared to other coatings. Figure 7 presents HVOF process and application images.

4. Coating Materials Used in Thermal Spray Coating Methods for Boiler Applications

Generally, wire or powder form coating materials are used in thermal spray coating methods employed in boiler systems. These coatings can be metallic or metal-matrix composite. Depending on the method, coating material form and composition varies. Powders and wires with specific properties can be used. For each spray system, coating powders in specific size ranges and wire diameters (solid wire or cored wire) are available, and selecting material and method appropriate to application purpose is an important matter. Table 1 below presents coating materials and materials suitable for methods. Globally, intensive research, development and project activities are being conducted on surface technologies for boiler applications.

5. General Evaluation

The performance of materials used in boiler tubes varies depending on alloy composition and operating environment. For extended tube service life, reduced maintenance and renewal costs, protection with coating is absolutely recommended. In this context, thermal spray coatings are an advantageous surface coating technology enabling production of high-performance coatings in boiler applications, both in terms of application practicality and breadth of material options. In protecting tubes and extending service lifespan, in addition to appropriate method and material selection, pre-coating preparations and post-coating operations must be carefully followed during applications. With thermal spray coating technology, dense coatings resistant to oxidation, hot corrosion, thermal fatigue and erosive wear can be produced under high-temperature conditions. Particularly when high performance is expected, HVOF/HVAF method should be preferred. At high spraying velocities, coating bond strength is considerably higher compared to powder/wire flame spray and arc spray methods, and coating lifespan is longer (2-5X). The point to note here is that coating must be applied at uniform thickness, coating density must be high, coating defects and structural discontinuities must be minimized, and good adhesion to the substrate surface must be achieved. This is related to both the coating unit and equipment capacity and performance, as well as coating material suitability and spray parameter optimization during operation, and it is recommended to work with expert technical engineers and applicators in the field.

Otherwise, coating lifespan will be limited even when the most suitable system and material selection are applied.

For this reason, conducting testing, analysis and reporting work on coating quality control and reproducible quality coating production is recommended. Below, corrosion performance at high temperatures of different tube materials used in boilers (Fe and Ni-based alloys) and thermal spray coatings are compared (Figures 8 and 9). As evident, Figure 10 compares wall thickness changes in coated and uncoated boiler tubes. It can clearly be seen that coating protects tubes and extends their lifespan. Industrial consulting services are provided to all organizations and companies seeking support in this area.

References

1.Sharobem, T.T. (2016). Mitigation of High Temperature Corrosion in Waste-to-Energy Power Plants.

2. Galetz, M. C., Bauer, J. T., Schutze, M., Noguchi, M., & Cho, H. (2013). Resistance of Coatings for Boiler Components of Waste-to-Energy Plants to Salt Melts Containing Copper Compounds. Journal of Thermal Spray Technology, Volume 22(5), Page 828-837.

3. Kawahara, Y. (2006). Application of High Temperature Corrosion-Resistant Materials and Coatings Under Severe Corrosive Environment in Waste-to-Energy Boilers. Journal of Thermal Spray Technology. Volume 16(2). Page 202213.

4. Kawahara Y. (2016). An Overview on Corrosion-Resistant Coating Technologies in Biomass/Waste-to-Energy Plants in Recent Decades.

5. Epelbaum, G., Hanson, E., & Seitz, M. (2010). New Generation of Tube Surface Treatments Help Improve EfW Boiler Reliability. NAWTEC18.

6. Oksa, M., Tuuna, S. and Varis, T. (2012). Increased Lifetime for Biomass and Waste to Energy Power Plant Boilers

with HVOF Coatings- High Temperature Corrosion Testing Under Chlorine-Containing Molten Salt. Journal of Thermal Spray Technology, Volume 22(5), Page 783-796.

7. Krause, H.H., and Wright, I.G. (1996). Boiler Tube Failures in Municipal Waste-to-Energy Plants. Materials Performance, Volume 35, Number 1, Page 46-54.

8. Oksa, M., Tuuna, S. and Varis, T. (2012). Increased Lifetime for Biomass and Waste to Energy Power Plant Boilers with HVOF Coatings- High Temperature Corrosion Testing Under Chlorine-Containing Molten Salt. Journal of Thermal Spray Technology, Volume 22(5), Page 783-796.

9. Chatha, S. S., Sidhu, H. S., & Sidhu, B. (2012). HighTemperature Behavior of a NiCr-Coated T91 Boiler Steel in the Platen Superheater of Coal-Fired Boiler. Journal of Thermal Spray Technology, Volume 22(5), Page 838-847.

10. Guilemany, J.M., Torrell, M., & Miguel, J.R. (2007). Study of the HVOF Ni-Bases Coatings' Corrosion Resistance Applied on Municipal Solid-Waste Incinerators. Journal of Thermal Spray Technology. Volume 17(2). Page 254-262.

11. Sidhu, B.S., & Prakash, S. (2005). Nickel-Chromium Plasma Spray Coatings: A Way to Enhance Degradation Resistance of Boiler Tube Steels in Boiler Environment. Journal of Thermal Spray Technology, Volume 15(1), Page 131-140.

12. Chatha, S. S., Sidhu, H. S., & Sidhu, B. (2012). HighTemperature Behavior of a NiCr-Coated T91 Boiler Steel in the Platen Superheater of Coal-Fired Boiler. Journal of Thermal Spray Technology, Volume 22(5), Page 838-847.

13. Guilemany, J.M., Torrell, M., & Miguel, J.R. (2007). Study of the HVOF Ni-Bases Coatings' Corrosion Resistance Applied on Municipal Solid-Waste Incinerators. Journal of Thermal Spray Technology. Volume 17(2). Page 254-262.

14. Sidhu, B.S., & Prakash, S. (2005). Nickel-Chromium Plasma Spray Coatings: A Way to Enhance Degradation Resistance of Boiler Tube Steels in Boiler Environment. Journal of Thermal Spray Technology, Volume 15(1), Page 131-140.

15. Dianyi Yan, Application of Thermal Spray Techniques for Combatting High Temperature Corrosion of WTE Superheater Tubes, Department of Earth and Environmental Engineering, Fu Foundation School of Engineering & Applied Science, Columbia University, 2017.

16. www.oerlikon.com 17.www.kermetico.com 18. http://www.tmcpoland.com 19. https://www.gtv-mbh.com 20. http://www.thermalsprayworld.com/boiler-spraycoating

  Dr. Ekrem Altuncu European Thermal Spray Association - ETSA Sakarya University - Thermal Spray Technologies Research Application Laboratory - TESLAB Sakarya Applied Sciences University - Material and Production Technologies Research Application Center - SUMAR