Method for cleaning cracks and surfaces of airfoils

The invention is a process for removing oxides, dirt, and organic impurities during repair of airfoils without damaging or effecting surface bond coats or base metal substrates of the airfoils. The process is especially suited for cleaning deep, tortuous cracks in the airfoils prior to brazing or alloying repair operations. The process entails using an autoclave with an organic caustic solution to fully remove the unwanted oxides, dirt, and organic impurities.

FIELD OF THE INVENTION 
This invention pertains to the cleaning of airfoils during repair 
operations. More particularly, the invention is related to the removal of 
oxides, organic impurities, and dirt on the surfaces of airfoils by 
processing airfoils in an autoclave with an organic caustic solution. 
BACKGROUND OF THE INVENTION 
During the service use of turbine engine parts or airfoils, various modes 
of damage can occur. The engine parts are highly susceptible to damage 
from erosion, oxidation, and attack from environmental contaminants. 
Examples of turbine parts that often need to be repaired due to damage are 
blades, buckets, nozzles, combustion chamber liners, vanes, and the like. 
As stated, one type of damage to engine parts occurs from the environmental 
contaminants. At temperatures of engine operation, adherence of these 
environmental contaminants on the engine part form molten compositions on 
the surfaces of the engine parts. As a result, chemical and mechanical 
interactions occur between the environmental contaminant compositions and 
the engine parts. The molten contaminant compositions can infiltrate pores 
and openings in the engine parts, that lead to cracks and possible part 
failure. 
Some environmental contaminant compositions that deposit on engine parts 
contain oxides of calcium, magnesium, aluminum, silicon, and mixtures 
thereof. These oxides combine to form contaminant compositions comprising 
calcium-magnesium-aluminum-silicon-oxide systems (Ca--Mg--Al--Si--O), 
herein referred to as CMAS. Damage occurs when the molten CMAS infiltrates 
engine parts. Still other contaminants may include iron and nickel oxides, 
sodium vanadates, sodium sulfates, sodium phosphates, and the like. Some 
of these contaminants originate from marine environments, such as engines 
operating at offshore sites. 
Repair of turbine engine parts involves cleaning cracks, crevices, and 
surfaces so that there is complete removal of CMAS and other oxides, 
organic and inorganic impurities, and dirt prior to alloy filling and 
brazing. Presently, fluoride ion cleaning (FIC) or etching is used to 
clean the surfaces of engine parts, including shallow cracks. Fluoride ion 
cleaning/etching techniques require high temperatures, about 1900.degree. 
F., and several hours for processing. In addition, hydrogen fluoride is 
used in fluoride ion cleaning and is listed as a hazardous air pollutant 
under the Clean Air Act, Section 112(b). As a result, processes using 
hydrogen fluoride may be subject to a Maximum Available Control Technology 
(MACT) standard. Another drawback of fluoride ion cleaning is that it is 
not effective for cleaning deep and tortuous cracks. In addition, a 
further deficiency of FIC is that on parts where an overlay layer is used, 
for example, a nickel-aluminide or platinum-aluminide, fluoride ion 
cleaning attacks the overlay layer that is deposited on the metal 
substrate. Thus, there is a need for a process that operates at lower 
temperatures in shorter time and utilizes chemicals that are compatible 
and safe with the environment without damaging the overlay layer or bond 
coat. Further, there is a need for a cleaning process of turbine engine 
parts that can effectively clean deep cracks and crevices. 
SUMMARY OF THE INVENTION 
This invention is directed to a wet chemical process for cleaning oxides 
from cracks and surfaces of airfoils before repairing the airfoils which 
comprises exposing surfaces and cracks of airfoils to an admixture of an 
organic solvent, a base, and water in an autoclave at a temperature and 
pressure sufficient to lower the surface tension of the admixture for a 
period of time to completely remove all oxides from the surfaces and 
cracks of the airfoils. The term "airfoil" herein means turbine parts such 
as blades, buckets, nozzles, combustion chamber liners, vanes, and the 
like. 
This invention also provides a method for removing surface oxides, dirt, 
and organic impurities from flat, contoured, and cracked surfaces of 
turbine parts comprising the step of: treating the turbine parts in an 
autoclave with an organic caustic solution at a temperature, a pressure, 
and a time sufficient to completely remove the surface oxides, dirt, and 
organic impurities from the turbine parts' surfaces and cracks without 
damaging an underlying metallic layer or a metallic substrate surface. 
During the process it is beneficial, but not necessary, if the organic 
component of the organic caustic solution acts as a supercritical fluid. 
By supercritical fluid it is meant that the liquid is above its critical 
temperature and pressure where the surface tension of the organic solution 
is near or about zero. 
Organic caustic solutions comprise chemical admixtures of an organic 
compound, such as an alcohol, a basic compound, such as an hydroxide base, 
and water. The ratio of base to water may be about one to one (1:1), or 
fifty weight percent base in water. The organic compound, generally a 
solvent to reduce surface tension of the solution, such as ethanol, must 
be present in a sufficient amount to cause all of the oxide to be removed 
from the treated part. 
An advantage of the invention is that the underlying metallic layer and the 
substrate of the airfoil are not damaged, which allows multiple repairs to 
the airfoil. This is a substantial savings in refurbishing time and costs. 
Another advantage of the invention is that compared to fluoride ion 
cleaning, the organic caustic treatment is conducted at lower 
temperatures, for instance 480.degree. F. versus 1900.degree. F. Also, the 
fluoride ion cleaning process uses and discards a hazardous chemical, 
hydrogen fluoride vapors, whereas, the organic caustic treatment of this 
invention is conducted in a closed system and the chemicals are 
recyclable. Still another advantage of this invention is that the organic 
caustic treatment cleans deep, tortuous cracks.

DESCRIPTION OF THE INVENTION 
The invention is directed towards a wet chemical process for removing 
oxides, dirt, and organic impurities during repair of airfoils without 
damaging or effecting surface bond coats or base metal substrates of the 
airfoils. The process is especially suited for cleaning deep, tortuous 
cracks in the airfoils prior to brazing or alloying repair operations. The 
process entails using an autoclave with an organic caustic solution to 
fully remove the unwanted oxides, dirt, and organic impurities. Dirt 
includes sand, volcanic ash, fly ash, cement, runway dust, substrate 
impurities, fuel and air sources, oxidation products from engine 
components, and the like. Oxides are also included as dirt, and may derive 
from environmental contaminants that adhere to the airfoil surface during 
service use, such as CMAS or calcium-magnesium-aluminum-silicon-oxide 
mixtures. In addition to CMAS, other oxides can also be removed from 
airfoil surfaces by the method of this invention. These oxides include, 
but are not limited to, oxides of magnesium, calcium, aluminum, silicon, 
chromium, iron, nickel, barium, titanium, alkali metals, and mixtures 
thereof. Other contaminants include vanadates, sulfates, and phosphates of 
alkali metals. 
Bond coats and overlay layers are usually metallic compositions, including 
platinum-aluminum, aluminum, aluminum-nickel, 
nickel-chromium-aluminum-yttrium, iron-chromium-aluminum-yttrium, 
cobalt-chromium-aluminum-yttrium, nickel-cobalt-chromium-aluminum-yttrium, 
and the like. 
Substrate materials often used in turbine parts or airfoils for aircraft 
engines and power generation equipment may include nickel, chromium, or 
iron based superalloys. The alloys may be cast or wrought superalloys. 
Examples of such substrates are GTD-111, GTD-222, Rene 80, Rene 41, Rene 
125, Rene 77, Rene 95, Inconel 706, Inconel 718, Inconel 625, cobalt-based 
HS188, cobalt-based L-605, and stainless steels. The process is suited for 
parts and hardware used in turbines or on airfoils. An example of a 
turbine part would be a turbine blade or vane. The term airfoil refers 
also to turbine parts, such as blades, vanes, buckets, nozzles, and the 
like. 
Additional substrate materials may be used in this invention. For instance, 
it is also contemplated that this invention may be utilized for removal of 
oxides, dirt, and organic and inorganic impurities in marine environments, 
electronic applications, and power generators, such as gas, steam, and 
nuclear, to mention a few. 
The autoclave reactor is a pressure vessel and is built to withstand high 
pressures at high temperatures. Pressure in the system is elevated by 
heating the contents (reaction mixture) in the autoclave or by using an 
external source of compressed gases to overpressurize the vessel. The 
autoclave may be operated in batch fashion; that is, the ingredients of 
the caustic organic solution are charged, the unit is closed, and the 
charge is brought to the desired conditions of temperature and pressure. 
Continuous or semicontinuous operation can be undertaken if one or more of 
the reactants are continuously fed and products withdrawn. 
In the autoclave, the temperature and pressure that is applied may cause 
the organic component of the organic caustic solution to become a 
supercritical fluid or have properties similar to that of a supercritical 
fluid. By supercritical fluid it is meant that the surface tension of the 
fluid is zero or approaches near zero which completely wets the surfaces 
in contact. The organic caustic solution does not have to be a 
supercritical fluid for the oxide or dirt to be removed. However, if the 
organic component of the organic caustic solution is near or approaches a 
supercritical state in the autoclave reactor during treatment of the 
airfoil, the surface tension is dramatically reduced thus enhancing the 
activity of the organic caustic solution and its wettability towards fine 
cracks and pores. 
The organic caustic solution is generally an admixture of an organic 
compound, a base, and water. Other admixtures may also be used, such as 
acetone, liquid ammonia, or liquid carbon dioxide, provided they 
dramatically lower the surface tension of the fluid during treatment of 
the airfoil in the autoclave. Examples of organic compounds are alcohols, 
such as methanol, ethanol, propanol, isopropyl alcohol, and acetone and 
liquid carbon dioxide, liquid ammonia, and mixtures thereof. Examples of 
caustic compounds are sodium hydroxide, potassium hydroxide, ammonium 
hydroxide, lithium hydroxide, triethylamine (TEA), tetramethylammonium 
hydroxide (TMAH), and mixtures thereof. Use of additives, such as 
surfactants and chelates, to further reduce the surface tension of the 
caustic solution can be beneficial. 
The caustic compound (the base) and water may be present in about a one to 
one ratio. The concentrations of the bases may range from very dilute, 
about one weight percent, to very concentrated, about sixty-five weight 
percent. The organic compound is usually present in a sufficient amount as 
a solvent media for the caustic solution to fully clean the alloy 
surfaces. The amount also depends on the size of the autoclave reactor and 
the size of the part being processed. Commonly known engineering 
principles can be used to calculate various amounts of the organic 
compound that is sufficient with the caustic and water to remove the oxide 
or dirt. Generally, the base is about 1-65 weight percent, the water is 
about 1-35 weight percent, and the organic compound is about 1-98 weight 
percent. A preferred weight percent for the caustic organic solution is 
about 6 weight percent base, 6 weight percent water, and about 88 weight 
percent organic compound. 
The temperature and pressure that is used during treatment can vary, 
depending on the amount and the type of oxides and dirt to be removed and 
the capabilities of the autoclave reactor. The organic caustic treatment 
can be performed at a range of temperatures, pressures, and reaction 
times. For example, the treatment may involve combinations of 
ultrasonication and boiling with autoclave treatment. The autoclave 
treatment can be conducted under several conditions. For instance, the 
pressure can range from about 100 pounds per square inch to about 3000 
pounds per square inch, and the temperature can range from about 
150.degree. C. to 250.degree. C. Higher pressures and temperatures can be 
applied to achieve shorter process times. Also, pressurization can be 
achieved at room temperature using compressed gases. Still yet, the 
process can start with zero pressure and by increasing the temperature of 
the reaction mixture, the autoclave pressure automatically rises resulting 
from the increase in the vapor pressure of the reaction mixture. The time 
to remove the oxides and dirt depends on the amount to be removed and the 
temperature and pressure conditions that are applied. Usually, the time is 
between about 0.1 to 8.0 hours. Also, it should be noted that using a 
mixer, such as a mechanical stirrer, a magnetic stirrer, or an 
ultrasonicator, at low pressures or high pressures may enhance the ability 
of the organic caustic solution to remove the oxides and dirt in 
torcherous locations and in shorter duration of time. 
An example of one of the organic caustic autoclave treatments for cleaning 
airfoil samples is now described for purposes of demonstrating the 
invention, and does not limit the invention to only this one treatment or 
set of conditions. 
EXAMPLES 
Example 1 
A CFM6-80C2 blade (airfoil) with desert dust, herein referred to as CMAS 
(calcium-magnesium-aluminum-silicon-oxides) was cleaned by the organic 
caustic autoclave method of this invention. The blade was placed in a 
Monel autoclave and submerged in a solution containing twenty grams of 
sodium hydroxide, twenty grams of water, and 330 milliliters of ethanol. 
After sealing the pressure vessel, the temperature was raised to 
250.degree. C. with a resultant increase in pressure to 2000 pounds per 
square inch. The temperature and pressure conditions were maintained for 
approximately one hour. After the autoclave was cooled, the sample was 
removed and cleaned (sonicated) in a three step process, including water 
cleaning, acid neutralization (5 weight percent hydrochloric acid 
solution) of base, followed by water cleaning. 
The CMAS was removed from the blade showing a weight loss of 389 
milligrams. Optical microscopy of the various sections of the CFM6-80C2 
blade indicated complete removal of oxides from the surface and in the 
cracks. Optical micrographs of the cross section of a leading edge of the 
blade with CMAS (FIG. 1a) and after CMAS removal (FIG. 1b) are shown in 
FIG. 1. 
Example 2 
A CFM6-6 twin blade with a thermal barrier coating (8 weight percent yttria 
stabilized 92 weight percent zirconia) and CMAS 
(calcium-magnesium-aluminum-silicon-oxides) was cleaned in addition to 
removing the thermal barrier coating by the organic caustic autoclave 
method of this invention. The blade was placed in a Monel autoclave and 
submerged in a solution containing twenty grams of sodium hydroxide, 
twenty grams of water, and 330 milliliters of ethanol. After sealing the 
pressure vessel, the temperature was raised to 250.degree. C. with a 
resultant increase in pressure to 2000 pounds per square inch. The 
temperature and pressure conditions were maintained for approximately one 
hour. After the autoclave was cooled, the sample was removed and cleaned 
(sonicated) in a three step process, including water cleaning, acid 
neutralization (5 weight percent hydrochloric acid solution) of base, 
followed by water cleaning. 
The TBC and CMAS were removed from the blade showing a weight loss of 989 
milligrams and 389 milligrams, respectively. Optical microscopy of the 
various sections of the CFM6-80C2 blade indicated complete removal of TBC 
and CMAS oxides from the surface and in the cracks.