High-Temperature Sulfur Corrosion of Iron-Chromium Alloys電子書

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High-Temperature Sulfur Corrosion of
Iron-Chromium Alloys
S. Mrowec,* T. Walec,* and T. Werber*
Received April 28, 1969
The kinetics and the mechan&m of sulfurization of Fe-Cr alloys containing
from 0.35 to 74 at. % Cr were studied in the temperature range 700-1000~
The sulfurization was conducted in sulfur vapor at atmospheric pressure.
The reaction rate was determined by the continuous gravimetrie method.
The scale composition was studied by X-ray and electron microprobe methods.
The contribution of the reactants to the process of material transport through
the scale was examined with both the marker method and the autoradiographic
method using the radioactive isotope asS. It has been found
that, irrespective of alloy composition and temperature, the sulfurization
follows a parabolic rate law. On the dilute alloys (up to about 2 at. % Cr)
the scales formed are monophase consisting of Fel_xS only. In the range
2-40 at. % Cr the scale is a heterophase mixture of Fe~_xS and the mixed
spinel FeFe2_xCrxS~. On the chromium-rich alloys the scale is monophase
and is built of a solid solution of FeS in Cr2S3. The scale growth on the
alloys under examination proceeds without the participation of the inward
diffusion of sulfur. The mechanism of the scale formation has been proposed.
INTRODUCTION
In recent years the problem of the resistance of metals and alloys to the
aggressive action of sulfur and its compounds has drawn much attention.
The mechanism of sulfur corrosion of metallic materials is much less
understood than the mechanism of oxygen corrosion due to a number of
experimental and theoretical difficulties encountered in studies of the
kinetics and mechanism of sulfur corrosion. 1-3 Systematic investigations
*Institute of Solid State Chemistry, School of Mining and Metallurgy, Cracow, Poland.
93
94 Mrowec, Walec, and Werber
of the sulfurization of pure metals and selected binary alloys provide much
valuable information of general importance concerning the mechanism of
high-temperature corrosion of metallic materials. 4-1~
In a previous paper, a we presented the results of studies on the
kinetics and mechanism of sulfunzation of Ni-Cr alloys, which made
possible the formulation of some interesting conclusions pertaining to the
mechanism of formation of the heterophase scales and which allowed the
proposal of a new model of the defect structure in the crystalline lattice of
Cr2S3. 5.11 The present paper reports the results of analogous studies
obtained on the Fe-Cr-S system.
EXPERIMENTAL: MATERIALS, APPARATUS, AND METHODS
The alloys under examination were prepared from pure metals.
The contents of the impurities present are given in Table I. Before preparation
of the alloys, pure iron was fired in hydrogen at 900~ The alloys
were prepared in an induction vacuum furnace under a pressure of
10 -3 mm Hg. The alloys were cast in an atmosphere of pure argon under a
pressure of 300 mm Hg in a round cast-iron mold coated with aluminum
oxide. The chromium content in the alloys is given in Table II.
In order to homogenize the alloy structure and to remove mechanical
stresses, the ingots were annealed for 10 hr in an atmosphere of pure argon
at 1200~ The ingots were then cooled down to room temperature, abraded,
and cut into samples 2.8 cm in diameter and 0.2 cm thick. The surfaces of
the samples were pohshed with emery paper through 4/0 grade and then
washed successively with water, ethanol, and acetone.
Sulfurization was conducted in sulfur vapor under a pressure of
1 atm in the temperature range 700-1000~ in the apparatus shown sche-

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