Spraytime

Cold spray (CS) is a materials consolidation process whereby micron-sized particles of a metal, ceramic, and/or polymer are accelerated through a spray gun fitted with a de Laval rocket nozzle to form a coating or a near-net-shaped part by means of ballistic impingement (Refs. 1 ,2). The feedstock powder particles are carried within a heated high-pressure gas (i.e., air, nitrogen, helium) such that they exit at supersonic velocities and consolidate upon impacting a suitable surface. The cold spray process has been developed to deposit a wide variety of engineering materials, including metals, steels (carbon and stainless), titanium, aluminum, magnesium, nickel alloys, zinc, tin, copper, tantalum, niobium, monel, brasses, and bronzes. Even gold and silver have been used in the cold spray process. Cermets, carbides, polymers, and/or combinations of these materials are routinely cold sprayed, including CrC-Ni, WC-Co, and many more with near theoretical density. It is important to note that CS is considered an additive manufacturing (AM) process and has been adapted to produce 3D-printed parts as well as coatings (Refs. 3–5). 

Background on Repair and Maintenance Needs at Hydropower Facilities

Stabilizing the grid using pumped hydroelectric stations enhances the efficiency of baseload power plants (coal, natural gas, and, to a lesser extent, nuclear), enables the storage of energy from renewable technologies (wind, solar) for future use, and allows the utility to better match storage and generation with off-peak and peak energy demand. When there are many large structural components moving around in a feedwater environment, wear and corrosion can occur, requiring constant monitoring and frequent maintenance. Pumped hydropower plants generally have additional problems due to the bidirectional water flow through the facility, requiring complex components like reversible turbine/impellers with vibration modes and bidirectional valves that require frequent actuation. Maintenance outages of a pumped hydropower plant not only eliminate the utility’s revenue from on-peak/off-peak energy trading; they also reduce the utility’s ability to capture excess baseload, wind, and solar generation in its footprint, resulting in energy wastage and potentially massive losses. Prompt remediation of damage is critical to maintaining the operational capacity of the pumped hydropower facility and, therefore, minimizing the cost to the generating utility and, eventually, the rate payer.

A complicating issue is the overall size of the components involved and the massive scale of generation, which often requires these components to be remediated in the powerhouse. Owing to its highly portable nature, cold spray technology can be used to repair these components in the field and provide the utility with a fast, expeditionary repair solution.

Case Study: Rocky Mountain Hydroelectric Primary Valve Repair

The Rocky Mountain Hydroelectric Plant is a 1095 MW pumped hydroelectric storage facility located in Northwestern Georgia and operated by Oglethorpe Power. The facility supports three reversible generator/turbine units, each with its own supporting systems, including primary valves and other equipment. Wear on the primary valve seat from frequent operation can cause leaks and, based on the movable seat design, could lead to self-oscillation of the seat, which accelerates the wear. To mitigate this issue, traditionally, weld overlays of aluminum bronze were used to rebuild the seat and seals of the valve assembly. While aluminum bronze has great corrosion resistance and metal-on-metal bearing wear resistance, its resistance to abrasion from particulate and abrasive materials, such as high-strength seals and packing materials, is less than optimum, and the use of a higher-performance hardfacing coating in this area would be beneficial.

Cold spray coatings can be tailored to have the unique property of high compressive surface stresses, dramatically increasing the hardness of the coating even when the coating material is ductile and highly corrosion resistant. Additionally, cold spray metal powders with different properties (hardness, strength, corrosion resistance, galling resistance) can be blended, agglomerated, and processed such that a mixed surface functionality is obtained in the coating, resulting in a hard, wear-resistant, galling-resistant, and corrosion-resistant overlay.

Coating Qualification

Cold spray was used to repair a sliding seat surface of a primary shutoff valve at the Rocky Mountain Hydroelectric facility. The valve is the largest of its kind on the East Coast. In collaboration with Pacific Northwest National Laboratory (PNNL), Richland, Wash.; Voith, York Springs, Pa.; and Oglethorpe Power Co., Tucker, Ga., VRC Metal Systems developed, optimized, and qualified a cold spray coating using nitrogen process gas to replace an aluminum bronze overlay. The development resulted in an aluminum bronze coating with a small amount of free (unalloyed) tin and zinc to act as a metal-on-metal lubricant, preventing galling damage. The coating was also embedded with a significant metal-carbide concentration to provide good wear and abrasion resistance. The coating density was excellent, with an average porosity of < 0.50% and hardness of approximately 250 HV, which is roughly double the hardness of the aluminum bronze weld overlay. A bond layer was used and provided very high adhesion strength to 1018 steel, with all ASTM C633 bond strength test results > 10,500 lb/in.² (glue failures). Wear resistance of the cold spray coating and the aluminum bronze material were evaluated side by side in the lab using ASTM G77 block-on-ring testing with an SAE 4620 steel ring in distilled water following the testing standards. The cold spray coating showed 99.7% less wear than the aluminum bronze, almost completely eliminating any measurable wear. The coatings also met all machinability and inspection metrics set forth in the qualification criteria and were approved for application in the powerhouse.

Field Repair

The qualified coating was selected for application at the Rocky Mountain facility on a valve component that could be removed from the valve assembly. The valve component was a seat retention ring, 11 ft. in diameter and roughly 4 in. tall, that experienced wear from the seat seal. The valve component was placed on a turntable, and a spray area was set up around the valve component, including a portable frame and tarp assembly, a portable robotic arm, portable dust collection equipment, and the VRC Raptor portable cold spray system. The component was rotated while the spray nozzle was robotically controlled to apply a uniform coating across the wear surface to a thickness of 0.130 in. The field application process took 17 hours of spray time, which was accomplished over the course of two to three days. Compared to two to three weeks for a similar weld overlay, the CS application provided significant time and cost savings for the utility. The ring was postmachined and reinstalled on the primary valve assembly and has been operational since July 2024.

Conclusion

The valve repair project from conception to qualification to field repair was completed in only one month, which demonstrates the ability for high-performance CS coatings to be rapidly fielded to solve wear and corrosion issues in demanding applications.

References

1. Champagne, V., ed. 2007. The Cold Spray Materials Deposition Process: Fundamentals and Applications, p.57. Cambridge, England: Woodhead Publishing Ltd.

2. Champagne, V. K., Ozdemir, O., and Nardi, A., eds. 2021. Practical Cold Spray 1^st ed. Switzerland: Springer Nature.

3. Cadney, S., Brochu, M., Richer, P., and Jodoin, B. 2008. Cold gas dynamic spraying as a method for freeforming and joining materials. Surface and Coatings Technology 202(12): 2801–2806.

4. Pattison, J., Celotto, S., Morgan, R., Bray, M., and O’Neill, W. 2007. Cold gas dynamic manufacturing: A nonthermal approach to freeform fabrication. International Journal of Machine Tools and Manufacture 47(3–4): 627–634.

5. Champagne, V. Cold Spray Introduction. Cold Spray Action Team Meeting, 2012–2023, coldsprayteam.com.

VICTOR K. CHAMPAGNE JR. (vchampagne@wpi.edu) is with Cold Spray Innovations International, KYLE JOHNSON and AARON NARDI are with VRC Metal Systems, Box Elder, S.Dak., KEN ROSS is with Pacific Northwest National Labs, Richland, Wash., and SETH SMITH is with Voith, York Springs, Pa.

The authors would like to acknowledge the Pacific Northwest National Labs (PNNL), Voith, and Oglethorpe Power for their support in the planning and execution of this project.