Patent Description:
Some turbine airfoils comprise internal cooling passages. Turbines have severe internal operating conditions and it is desirable to inspect turbine airfoils that have been in service for damage and corrosion. Currently available methods of inspecting for corrosion and repairing corrosion associated damage need improvement and greater sensitivity - particularly methods for inspecting and repairing the internal passages of the airfoils.

Disclosed is a method of detecting mixed mode corrosion of a turbine airfoil having internal passages including heat treating the turbine airfoil at a temperature of <NUM> to <NUM>°F (<NUM> to <NUM>) in a Ni/Co oxide reducing atmosphere; scanning an external surface of the heat treated turbine airfoil with a magnetometer to determine the presence of mixed mode corrosion on the internal passages; and comparing the results of the scanning to a threshold value, wherein the turbine airfoil comprises a nickel superalloy.

In addition to the features described above, the heat treating can occur for <NUM> to <NUM> hours.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, the Ni/Co oxide reducing atmosphere can include one or more noble gases, hydrogen, hydrogen fluoride, or a combination thereof.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, the Ni/Co oxide reducing atmosphere may have a partial pressure of oxygen less than or equal to <NUM>-<NUM> atmospheres (atm) at <NUM>°F (<NUM>).

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, the turbine airfoil has an existing thermal barrier coating system and the heat treating occurs after removing the existing thermal barrier coating system. Alternatively, the heat treating can occur with the existing thermal barrier coating system in place. In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, the scanning is performed parallel to an internal passage.

A detailed description of one or more embodiments of the methods are presented herein by way of exemplification and not limitation with reference to the Figures.

The high pressure turbine <NUM> comprise air foils (blades and vanes) having internal passages for cooling. Some low pressure turbines <NUM> also comprise air foils (blades and vanes) having internal passages for cooling. Once a turbine airfoil has been in use the airfoil must be periodically removed and inspected. While the external surface of the airfoil can be readily inspected using currently available equipment and techniques, nondestructive inspection of the internal passages is more of a challenge due to the restricted access.

The turbine airfoils are made of nickel superalloys which can undergo corrosion. Normal oxidation creates protective oxides such as aluminum oxide and chromium oxide. Mixed-mode corrosion attacks the nickel superalloy and the surface of the alloy grows nickel oxide and cobalt oxide at relatively fast rates and depletes protective elements such as aluminum, chromium, and tantalum in the nickel superalloy. This leaves a matrix of nickel and cobalt which is low in alloying elements and is also ferromagnetic. Ferromagnetic materials can be detected with a magnetometer. <FIG> shows mixed mode corrosion of an internal passage of a nickel superalloy turbine airfoil. Prior efforts to use magnetometry to detect the nickel/cobalt matrix experienced limited success due to the fact that the magnetic readings were low and difficult to detect, particularly when trying to inspect the internal passages of the turbine airfoil.

Disclosed herein is a method to improve the detection of the mixed-mode corrosion. Subjecting the turbine airfoil to a heat treatment having a temperature of <NUM> to <NUM>°F (<NUM> to <NUM>) under a nickel oxide/cobalt oxide (Ni/Co oxide) reducing environment converts nickel oxide to nickel, cobalt oxide to cobalt or both nickel oxide and cobalt oxide to nickel and cobalt respectively and enhances detection of the mixed-mode corrosion using magnetometry.

The method of detecting mixed mode corrosion of a turbine may be performed while the airfoils are located in an assembled engine (on wing or off wing), at the module level, or at the parts level.

A Ni/Co reducing atmosphere is any atmosphere in which nickel oxide can be reduced to nickel, cobalt oxide can be reduced to cobalt, or both nickel oxide and cobalt oxide can be reduced to nickel and cobalt respectively at temperatures of <NUM> to <NUM> °F (<NUM> to <NUM>). Exemplary reducing environments include a vacuum. Exemplary reducing environments also include one or more noble gases, hydrogen, hydrogen fluoride or a combination thereof. The reducing environment may have a low partial pressure of oxygen. A low partial pressure of oxygen is defined as less than or equal to <NUM>-<NUM> atm, or less than or equal to <NUM>-<NUM> atm at <NUM>°F (<NUM>). As understood by one of ordinary skill in the art, pressure is dependent on temperature. While a low partial pressure of oxygen is defined at <NUM>°F (<NUM>) a person of skill in the art would be able to determine the equivalent partial pressure of oxygen at another temperature. Such equivalents are included in the definition of low partial pressure of oxygen.

The heat treatment temperature, as disclosed above, is <NUM> to <NUM>°F (<NUM> to <NUM>). Within this range the heat treatment temperature may be <NUM> to <NUM>°F (<NUM> to <NUM>). The turbine airfoil may be subjected to the heat treatment for <NUM> to <NUM> hours. In some embodiments the turbine airfoil is subjected to the heat treatment for <NUM> to <NUM> hours. When cooling the airfoil it is may be desirable to cool at a rate of approximately <NUM>°F (<NUM>) per minute. It is also contemplated that the airfoil may not be cooled if the airfoil is being subjected to a repair process.

The heat treatment may be performed with the existing thermal barrier coating system in place or after removing the existing thermal barrier coating system. A thermal barrier coating system is defined as comprising a bond coat and a thermal barrier coating. It may be desirable to perform the heat treatment prior to applying any new thermal barrier coating. In some embodiments the heat treatment is performed twice, once before the existing thermal barrier coating system is removed and again after the thermal barrier coating is removed.

After the heat treatment an external surface of the turbine air foil is scanned with a magnetometer. Useful magnetometers include any magnetometer that can detect relative permeabilities of <NUM> to <NUM>. Scanning may be performed parallel to the internal passages. Scanning is performed at one or more positions on the external surface that correspond to an internal passage of the airfoil. The scanning results are compared to a threshold value (an abandonment threshold). If the scanning result is greater than the threshold value then the mixed mode corrosion on the internal passages exceeds the amount that can be repaired in a cost effective manner. If the scanning result is less than the threshold value then the mixed mode corrosion on the internal passages can be repaired in a cost effective manner.

The threshold value can be determined by correlating magnetometer readings to wall thickness where the wall thickness is determined by destructive evaluation. Stated another way, a statistically determined number of airfoils are heat treated, scanned, measured, and sectioned to determine the relationship between the magnetometry results and the amount of remaining wall material. Airfoils can have a range of designs and so the threshold value may be design specific, location specific or both. It is also contemplated that an airfoil of a specified design may have different threshold values at different locations due to the potential complexity of the repair. When there are multiple threshold values per airfoil the scanning result for each location must be less than the threshold value for that location in order for the turbine airfoil to be successfully repaired.

The effectiveness of heat treatment was examined by scanning turbine blades as received at a repair facility with a magnetometer to obtain an average magnetic value across a range of locations. Scanning was on an external surface of the turbine blades at locations corresponding to the location of internal passages as described above. The blades (before or after thermal barrier coating removal) were subjected to a heat treatment of <NUM>°F (<NUM>) under vacuum for <NUM> hours. The heat treated blades were scanned with a magnetometer in the same manner as before heat treatment and there was an average of an <NUM>% increase in mixed corrosion detection capability.

Claim 1:
A method of detecting mixed mode corrosion of a turbine airfoil having internal passages comprising:
heat treating the turbine airfoil at a temperature of <NUM> to <NUM>°F (<NUM> to <NUM>) in a Ni/Co oxide reducing atmosphere;
scanning an external surface of the heat treated turbine airfoil with a magnetometer to determine the presence of mixed mode corrosion on the internal passages; and
comparing the results of the scanning to a threshold value, wherein the turbine airfoil comprises a nickel superalloy.