NDTsi 2001 Session: Major Component Inspection I-IV
NDE of Combustion Turbine Coatings
K. Krzywosz, EPRI, USA
ABSTRACT
EPRI has initiated a program to test, evaluate, and develop a field-deployable NDE system for life assessment of blade coatings for advanced gas turbines. Primary objectives include developing a suitable NDE technology for on-site testing and data acquisition, and providing NDE input into a predictive coating model to estimate the remaining coating life. Turbine blades often fail well before the typical 50,000-hour projected service life. To minimize catastrophic failure, the first stage blades are often replaced after 25,000 hours of operation at a high replacement cost of $2,000,000. To extend service life of the first stage blades, thermal barrier coatings (TBCs) are commonly used. A TBC is made up of at least two layers: the outer ceramic layer reduces the heat flow into the blade material, while the metallic bonding coat, a member of the MCrAlY coating family, protect the blade from the oxidizing and corrosive environment. Zirconia is typically used as the ceramic layer because of its similar thermal expansion coefficient with that of metallic blade materials. However, due to observed failures of TBCs due to spalling, cracking, and delamination, more advanced coatings that replace the ceramic coating with another layer of aluminum on top of the metallic-bonding coat are used in such GE Frame 7FA/9FA gas turbines. The coatings that are currently used for these advanced GE engines include CoCrAlY and NiCoCrAlY with a top aluminide coating, which are commonly referred to as GT 29+ and GT 33+ coatings, respectively. To assess and estimate service life of these blades, it is desirable to develop an in-situ NDE system to test, evaluate, and predict remaining coating life. The primary goal of the NDE program is to offer a retirement-for-cause method of repairing coatings based on actual findings rather than based on time-in-service approach. This paper reviews the latest state-of-the-art NDE technology evaluated and progress made to date for assessing the coating thickness and coating integrity of a high-temperature coating.
Publication Source: 3nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, November 14-16, 2001, Seville Spain. Publisher: Tecnatom s.a. - [Homepage]
