March 2020

I ndustrial gas turbines rely on specialised coatings to deliver continued performance and reliability. Renewing these protection systems is an important part of the routine maintenance schedule and the quality of the new coatings is dependent on attention to detail and the expertise of the refurbishment team. However, finding the most appropriate coatings supplier requires the turbine operator to have a certain amount of knowledge about the process. By asking a number of insightful questions, it is possible to determine what level of expertise and quality controls are at the disposal of the potential vendor. Methodology Industrial gas turbine coatings require an array of application methods that involve specific processes and equipment. High velocity oxygen fuel (HVOF), plasma, arc wire, combustion, air spray and chemical vapor deposition (CVD) are all used in the refurbishment of gas turbine components. Different coatings have slightly varied bonding properties with different substrates, so it is essential to understand the conditions required to achieve a perfect bond. A coatings bond is one of the most critical aspects of its success in service. As such, it should be in focus during all processes associated with coating. Furthermore, the remaining range of properties of the finished coating must be sufficient for the application – the hardness value is an indicator of the proper application of wear coatings while the surface roughness will have a major impact on flow efficiency. By inspecting the microstructure and mechanical properties of the coating it is possible to verify that it was applied to required specifications and that it will provide all of the expected benefits in operation. In every refurbishment project, establishing the process foundation is essential to the long- term success and durability of the coating. This involves detailing the equipment and parameters as well as the properties required for the coating, such as its tensile strength, microstructure characteristics, hardness and surface roughness values. Together with a revision-controlled shop process scope, this 10 | Plant & Works Engineering www.pwemag.co.uk March 2020 Insight Knowledge is key Garret Haegelin, HICoat Division Superintendent at Sulzer, looks at the features and processes used when applying high-quality coatings and how attention to detail is essential for optimum performance. information forms the basis of a high-quality application. Qualifying and freezing all influencing parameters of the process for each layer and each component helps ensure the quality and consistency that is provided by the vendor. Preparation In many cases, coatings are applied as one of the final stages of a larger repair project. It is therefore important to first make sure all prerequisite steps have been taken to ensure the substrate is properly prepared for application. The criticality of preparation for coating is magnified when the repair and coating suppliers are not integrated. Without a mutual understanding of the importance of surface preparation, repair projects can be protracted and offer less than optimum results. A sound substrate is essential for optimum performance of the protection systems. Therefore, the component repair process is critical to the quality of the coating. Once the coating is applied, minimal process can be done without removing or damaging the coating. Pre-coating checks Most of the superalloys that are used in gas turbine components develop oxidation and corrosion while in operation. It is essential that any of these contaminants are removed completely, including remnants of the previous coating. The presence of any intermediate layer between the substrate and the new coating will likely cause issues with the bond between the two. However, care should be taken when grit blasting or blending, to minimise any removal of the original substrate. To identify any remaining areas of oxidation or residual coating, components are heat tinted. If contaminants remain, the process repeats until suitable results are achieved. Once any intermediate layers are removed, further processes may be required. In some cases, the component’s microstructure needs to be prepared in terms of any applicable heat treatments. These processes should be performed prior to application to ensure the coating is not subjected to anything outside of its previously qualified specifications. Similarly, the component may need to be dimensionally altered prior to coating. The thickness of the newly overlaid coating will affect the final dimensions of the component, so in many situations it will be necessary to remove some base material or adjust geometric profiles to accept the additional thickness. Final pre-coat quality control checks should be completed, including dimensions, flow checks and inspections for defects, using penetrant if necessary. Coatings will only bond properly if there are no gaps or cracks in the substrate; any such flaws will cause rapid deterioration of a new coating. Application process Having removed any debris, determined the part is crack free, dimensionally ready to accept the coating and all other repairs completed, the component is nearly ready for the coating application. Up to this point, the part likely came into contact with various contaminants, such as oil, machining fluid and non-destructive evaluation (NDE) penetrant fluid. These contaminants are removed via chemical or thermal means in the “degrease” process. From this point, extreme care is taken to ensure contaminants are not re-introduced to the substrate that would jeopardize the bonding of the coating. The next step is to prepare the surface to accept the new protection system using grit profiling. This process roughens the target surface of the component, creating an “anchor-tooth pattern” for the coating to mechanically bond to. This profile is attributed to the type and size of blast material used in this process, which will depend on both the substrate and the coating to be applied. However, in all cases care should be taken to use virgin grit as opposed to re-used grit to prevent contamination, which can result in a poor-quality bond or even diffusion of contaminants into the base material. At this point the equipment involved starts to become more complicated and for good reason. The application of both base and top coats requires considerable accuracy and precision to ensure the right amount of coating is applied to the correct areas and with the specified characteristics. Industrial

RkJQdWJsaXNoZXIy MjQ0NzM=