A company that doesn't have a well controlled stripping process will not be able to salvage as many components, and the result will be a higher scrap rate, thus, defeating the purpose of refurbishing.
Advances in stripping and coating processes
There have been significant advances in coatings over the years, but most of the important advancements related to coatings have been in the application processes. New application methods allow for more even distribution of coatings around the profile of the part and better control of the overall coating process. One of the advancements in the past 15 years has been the development of the "vapor" coating process.
Paul MacGregor, Sr. Coating Engineer for Walbar Metals explains that a key advantage to the the vapor coating process is that the coatings are not diffused into the base metals as deep as the pack coatings, in order to achieve the same results. This means, that when parts are eventually stripped for re-coat, there is a reduced loss of base metal during the stripping process.
According to Palatine, A high percentage of our repaired components are "older generation" components that are still coated with "pack cementation" coatings involving traditional technology to deposit an aluminum coating on the base metal. We are slowly implementing the vapor deposited aluminide coatings onto refurbished components where feasible. The vapor coatings are more commonly specified as OEM "new part" requirements.
Aluminide coatings can be modified with the addition of platinum to enhance the oxidation and hot corrosion protection of the base metal from the different atmospheric conditions the engine performs in. The "aluminide" portion of the process can be performed by the conventional pack cementation or the vapor processes.
The coating must protect the part from the environmental conditions that the component will be exposed to during operation. However, the cost for that must also be considered. Economically speaking, it doesn''t always make sense to use a coating that lasts for 3,000 hours, but the base metal may only last 1,000 hours; it would be more economically feasible to use a coating that has a life expectancy consistent with the base metal."
Palatine explains, "The key is to pick a coating that is compatible with the expected life of the component. The coating applied to a part is designed to be "sacrificial" versus deteriorating the properties of the base alloys. The coating specified for a given part is usually designed to meet the engine and environmental conditions that the part will be exposed to. Base metal properties will be inspected during and after engine service to ensure component integrity (for example, creep properties, erosion, sulfidation etc.). These inspection checks will ensure the serviceability of the part and the effectiveness of the coating.
A case in point is that Walbar applied a modified aluminide (RB505) to a LP blade. The turbine blade base metal fatigued before the coating failed. A different coating was specified for future hardware which was more compatible with the operating conditions of the part. Palatine says, "There are some instances where the selection of modified and improved coatings really make sense, however. The RB505, for example, is an aluminide coating that is offered as an option (it is approved on the Pratt & Whitney PT6) to CPW333, which is defined as a simple aluminide coating. Another option that is available is our Aluminum-Silicon coating, a modified aluminide through the addition of silicon." Silicon can be co-deposited with the aluminum during the coating thermal cycle. Silicon is added to improve hot corrosion and sulfidation protection of alloy without the added cost of platinum.
Although noble metals such as platinum have been introduced to improve turbine hot section coatings, many of the same existing variations of aluminides continue to be the most popular in the aviation industry.
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