Since the aiid 1960's, gas turbine engines have been used for ship propulsion. These engines, such as the 20,000 horsepower General Electric LM 2500, undergo a form of high temperature corrosion, called hot corrosion, when used in a marine environment. The parts most severely attacked are the metallic coatings used on the hot-section turbine blades. These coatings are often a mixture of cobalt, chronium, aluminum and yttrium and as a class are identified by the acronym CoCrAlY. The type of hot corrosion most frequently occurring in the Navy's use of these coatings,has been designated type 2 hot corrosion. The most destructive part of this corrosion (the propagation phase) takes place when the turbine blade tem peratures are in the range of 677° to 732°C (1250° to 1350°F). The macroscopic details of this corrosion process have been described as follows:

• Sulfur in the fuel forms sulfur dioxide and sulfur trioxide during combustion.

• Sulfur dioxide and/or sulfur trioxide reacts with the cobalt in the coating to form cobalt sulfate.

• Sodium sulfate from the sea salt ingested by the engine deposits in solid form at 704° (1300°F) on the turbine blades.

• The sodium sulfate and the cobalt sulfate form a low melting point mixed sulfate and in the presence of the sulfur trioxide acidically flux the coating from the turbine blade.

Unpublished data has shown that increasing the chromium content of the coating to 30 or 40 wt% from the originally used 20 wt% dramatically improved the hot corrosion resistance of these coatings. It has been speculated that in the 20 wt% chromium coatings the oxide scales that form on the coatings are a mixture of cobalt oxide, chromium oxide, and aluminum oxide, while in the 40 wt% chromium coatings, the oxide scale is predominantly chromium oxide. Thus, it had been hypothesized that in high chromium coatings the formation of cobalt sulfate is impeded by the continuous nature of the chromium oxide.

In order to study the validity of this hypothesis, the goal of this research effort was to examine the initial protective oxide scales that form on actual coatings. This was accomplished by using the surface sensitive analytical technique of x-ray photoelectron spectroscopy and will be described in detail later.

X-ray Photoelectron Spectroscopy

To study the nature of the initial oxides formed on production CoCrAlY coatings, their composition as a function of depth from the surface was studied using x-ray photoelectron spectroscopy (XPS). XPS involves exposing the surface of interest to x-rays of a discrete energy. In the Kratos model XSAM 800 surface analyzer used in these experiments Al Ka(1486.6eV) was the radiation source. The interaction of this radiation with the specimen causes the surface to emit electrons with energy characteristic of the atoms from which they were emitted. This process is diagrammatically represented in Figure 1. The XPS equipment has an electron energy analyzer which measures the kinetic energy of these emitted electrons. This kinetic energy can be related to the binding energy of the electrons by the relation

Ekin = kinetic energy of electrons as passed by spectrometer <j>a = work function of entrance to spectrometer lens Ejj = binding energy of emitted photo-electrons.

As kinetic energy of electrons are being measured, ultra-high vacuum (UHV) conditions are required in the sample analysis chamber. Without the UHV conditions the electrons would be scattered by gas molecules. The surface sensitivity of the XPS method arises from the fact that it measures the energy of emitted electrons. These electrons have a very short mean free path in solid matter. Typically, this distance is on the order of 5 to 10 angstroms (see Figure 2). Therefore, these emitted electrons represent elements present in the outer layer or several atomic layers below the surface.


Figure 2. Sampling depth in XPS as represented by a plot of escape depth against kinetic energy of the escaping electron (reference 1).


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