ABSTRACT:

The hot corrosion resistance of nickel- base superalloys ChS91VI and EP708 for stator parts of marine gas turbine engines is investigated. The resistance to hot corrosion of superalloys ChS91VI and EP708 at 700oC are determined mainly by the microstructure of grain boundaries. At 800oC there are catastrophic corrosion damage of these alloys.

INTRODUCTION

The major difficulty of high-temperature nickel alloys development for marine lies in achieving a balance between hot salt corrosion resistance, high-temperature oxidation resistance, phase stability, structure stability and mechanical properties. Hot (hot salt) corrosion is defined as an accelerated, often catastrophic surface attack of superalloy hot-gas-path components, containing ?orrosion active admixtures (sulfur, sodium, vanadium, etc.) or in marine conditions1, e.t.c.This form of attack is especially severe in temperature range 760-1000 o C (high-temperature corrosion - HTC) and in range 649-760o C (low-temperature corrosion - LTC) (2-8). Investigators observed low temperature corrosion in the pitting form for IN939, Rene 80, ChS70VI, ZMI-3. Above the terminal temperature (above the dew point of the salt or salt mixture) the corrosion rate is decreasing and test is predominantly oxidizing.

EXPERIMENTAL PROCEDURE

We studied the corrosion resistance to the hot (salt) corrosion of high-temperature nickel-base alloys,that are used, for example, for hot part elements of marine gas turbines: - CHS91VI (Ni-16%Cr- 15%Fe- 2,0%Ti- 1,0%Al- 5,0%W- 3,0%Mo) and - EP708 (Ni-19%Cr- 0,4%Fe- 1,0%Ti- 2,0%Al- 6,0%W- 5,0%Mo). The corrosion resistance of the alloys to the hot (salt) corrosion was determined by the method of semi submersion of the samples (dimensions: diameter- 8 mm, length- 30mm) into crucibles with molten salts (90% Na2SO4 + 10% NaCl) at 700 and 800° C. The nickel - base alloys for the study were obtained by induction melting in vacuum furnaces at a pressure of 5,3 - 1,4 Pa in crucibles with basic lining. The molten alloys were cast under vacuum into heated ceramic molds. We used the methods of light microscopy, x - ray diffraction, and microscopic x - ray spectral analysis

RESULTS

The magnitude and nature of the corrosion losses are directly dependent on test temperature, chemical composition and microstructure of the alloys. Into crucibles with molten salts (90% Na2SO4 + 10% NaCl) at 700 ° C the corrosion resistance of the alloy CHS91VI much higher. At 800°C alloys have comparable corrosion rate with a slight advantage CHS91VI. Feature of the corrosion of alloys CHS91VI and EP708 at 700°C in molten salts, in the first place, is the destruction of the grain boundaries (fig.1). However, metallographic and microscopic x - ray spectral analysis of the surface did not reveal a significant decrease in the alloying elements. The metal and scale have clear boundaries. This character of corrosion damage (along the grain boundaries) promotes to the separation of significant amount of metal in scale. The hot corrosion resistance of superalloys ChS91VI and EP708 are defined at 700oC in the main by there grain boundaries structure. Rapid advancement of corrosion front at 800 ° C leads to significant depletion of the alloying elements of the surface layers.

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