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A series of processing experiments and tests were conducted to develop the optimum processing conditions. The first group of tests evaluated the use of single (mono) layer and multi-layer coatings. Adhesion tests showed the monolayer coatings had superior adhesion and laboratory erosion tests confirmed dramatic increases in erosion resistance compared to the bare metal and malcomized substrates. The single (mono) layer nanocoatings had little to no mass loss compared to the multilayer coatings and the uncoated substrate materials. The tests also revealed that the coating performed better on materials that had been malcomized, thus utilizing a nanocoating over a malcomized surface was not only possible, but preferred for this application. X-ray diffraction was used to evaluate grain size and confirm that the structure of the coating was less than 100nm in size. The thickness of the deposited coating was approximately 20m.
A second group of tests were conducted with higher deposition rates, but coating microstructure, adherence and erosion resistance was not as good as
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EPRI and its member utilities currently are conducting field trials to assess in-plant performance. Four applications have been identified and trials are underway or will begin in early 2012:
Utility A – Speed matching valve stem
Valve background data
The utility is experiencing significant erosion in its speed matching control valve stems. The stem is approximately 23? in length, 3.1? in diameter and made from Inconel® 901, which is nitrided (malcomized) after machining. The erosion is localized to a < 2? region, and erosion has been severe enough that component replacement is expected every 12-24 months. To combat the erosion, the utility modified a new stem by machining the area of anticipated erosion damage back approximately 0.20? and filled the region with a Stellite 6 hardfacing weld overlay. However, subsequent service revealed this area was undercut by erosion, compromising the hardfacing alloy and causing failure of the stem. Therefore, the utility was interested in applying a thin nanocoating on top of the nitrided stem as a layer of additional protection.
Nanocoating application
A new valve stem was manufactured out of Inconel® 901 and given a final surface malcomizing heat-treatment. Prior to testing, the stem was dimensionally inspected by the utility’s metrology department to verify dimensions and clearances. After inspection, the valve stem was sent to SwRI for coating. To ensure even coating on the region of interest, stainless steel foil was used to mask-off regions where the coating was not needed. After coating, the stem was returned to the utility for dimensional inspection. The utility found the heat generated during the nanocoating process and the addition of an approximately 20m thick nanocoating did not appreciably change any dimensions, and the stem was within all of the required dimensional and tolerance limits.
Field test status
In the fourth quarter of 2011, the valve stem with the nanocoating was installed in Utility A’s unit and is now in operation.
Utility B – Interceptor valve stem
Valve background data
The utility has been experiencing varying degrees of wear in its Westinghouse BB44 style interceptor valve stems on multiple units. The source of the wear is the rotation of the plug on the stem. The utility believes that during operation the plug rattles and vibrates until the beveled region wears far enough that the plug pulls free from the anti-rotation pin, leading to the plug spinning around the stem and causing wear damage. In one case, the failure of the stem, subsequent unit trips and valve tests resulted in damage to the anti-rotation pins. Since the EPRI-developed nanocoatings have excellent erosion and wear characteristics, the wear damage was restricted to a small (3″-4″) region on this style of valve stem, and the valve stem material was an Inconel® 901 alloy, the utility was interested in applying the nanocoating to improve the performance of the valve stem. The utility thinks the nanocoating will act as a sacrificial ‘retaining nut’ via the increased hardness to prevent rotation and increase the lifespan of the stem.
Nanocoating application
The application of the nanocoating was in progress at press time. The utility has provided 4 stems for coating.
Field test status
Field installation is planned for first quarter of 2012.
Utility C – Drain valve stem
Valve background data
Utility C has been experiencing erosion damage in its drain valve stems. Erosion is taking place during startup when the drains are open and the oxide from the boiler is blown through the valve and drain line. The EPRI-developed nanocoatings appear to be a good option for extending the life of these components.
Nanocoating application
The stem had not been received for coating at press time.
Utility D – Main steam stop valve stem
Valve background data
The utility has been experiencing erosion in its main steam stop valve stem. The stem diameter has been reduced by 4.3 mm. The material is Inconel 901 and the utility is interested in applying a nanocoating to reduce erosion and increase the service life of this component.
Nanocoating application
Similar to Utility A, a targeted application in the region of interest. The stem had not been received for coating at press time.
Summary
EPRI research shows nanocoatings produced by PEMS technology can give dramatic improvements in resistance solid particle erosion and liquid droplet erosion. Research to apply this technology to steam turbine valve stems for erosion protection has found the optimized process parameters for the application and shown it is possible to coat the malcomized surface of both nickel-based and stainless steel valve stems. Other components that might benefit from this nanocoating technology include high-erosion and -wear components such as steam turbine nozzles, piping/tubing elbow, fan/agitators, bowl and ball mills, pulverizers and coal belts/escalators. Research on hot corrosion also is ongoing for boiler components.
