According to MacGregor, "Walbar has been using pack aluminides and chrome coatings for the last 25-plus years. These coatings continue to be very popular on many turbine engines."
Aluminide coatings can be applied using many different types of applications. The aluminide layer can be diffused into the base alloy through the use of slurries, paints, powders, or vapors, provided that the alloy is receptive to the deposition of the coating element.
A significant amount of these aluminides are still applied with a process known as Pack cementation. This is done by surrounding the component with an aluminum powder source, and subjecting it to a thermal cycle in an inert atmosphere.
Another popular type of coating is known as chromides. Chromides provide good corrosion protection in lower temperatures portions of the turbine hot section.
A more advanced coating system is the application of metallic and thermal barrier coatings. The metallic coatings are known as MCrAlY's and can be applied through various manufacturing methods such as HVOF, Low Pressure Plasma Systems, and electron beam physical vapor deposition. The manufacturing equipment is relatively expensive in relation to the equipment for aluminide coatings. The coating has significantly different properties than the aluminides but in many cases will out-perform the properties of the aluminide coatings. These metallic coatings are generally more expensive than aluminide coatings. Metallic coatings are used as a "stand alone" coating or as a bond coat for thermal barrier coatings. Thermal barrier coatings are typically ceramic coatings used to "insulate" the base alloy and dissipate the heat more uniformly from the part.
Palatine explains that with all the various types of coatings available to the turbine market, you need to understand the purpose and engine application for each before you make a decision. The alloy protection properties and the cost will vary.
Are your turbine components worth refurbishing?
According to Palatine, it is the customer's job to make certain determinations about whether or not to recoat their turbine components.
Turbine blades offer a particular challenge. For instance, with PT6 blades, it's the customer's job to measure the overall diameter of the blades and disc when they are installed in the wheel.
"We cannot measure this dimension for the customer, we can only measure certain dimensions on the blades, but not the installed dimensions. In the case of compressor turbine blades, there are some guidelines as to how much of the blade can be left, but the customer needs to determine whether or not there is enough material to build another wheel that will provide enough life and performance. It's a stack-up issue; it's something that the customer has to determine as to whether or not they want to do it. The engine manual provides the criteria for helping them do this. It provides OD dimensions that gives them a range that the blades must be within, and below that range, they are not repairable," Palatine says.
"We sometimes notice that the blades are worn on the low side and will alert the customer to this fact. It's up to them if they still want to proceed with the rebuild process. Remember that the shortest blade in the set is what you have to grind the wheel to when you reassemble the wheel — you need to take this into consideration."
Palatine adds, "Additionally, we perform a metallographic evaluation, which involves destructive testing of at least one blade from a set. We evaluate the base metal condition for over temperature operation, inter-granular corrosion, stretch, creep, and a number of other conditions. Blade retipping is a practice in the industry that is becoming more and more common."
"There are some tip repairs for different types of blades, particularly in the cold section," says MacGregor, "yet, for turbine blades that are worn beyond limits, Walbar chooses not to restore tips on these blades."
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