The rate of absorption of hydrogen into iron and of nitrogen into Fe-Cr and Fe-Ni-Cr alloys containing sulfur
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molecule on the surface. Also, the combined effects of oxygen and sulfur on the rate of nitrogen absorption were determined. Pehlke and Elliott2 investigated the nitrogen absorption rate for Fe-Cr alloys and found that it was controlled by liquid-phase mass transfer because the activity of oxygen was low. However, sulfur present in stainless steels may retard the rate, and the present work will be primarily concerned with the effect of sulfur. Boorstien and Pehlke3 investigated the rate of hydrogen solution in iron alloys. They found that most alloying elements did not affect the rate, which was controlled by liquid-phase mass transfer. They did find that large quantities of sulfur decreased the rate slightly; 2 wt pct sulfur decreased the rate to half that for pure iron. However, they used pure hydrogen as the reaction gas, which could lead to uncertainties. As discussed later in detail, because of the formation of H2S the activity of sulfur on the surface could be poorly defined. In the present work the effect of sulfur on the rate was measured in such a way that the activity of sulfur on the surface was well defined. EXPERIMENTAL The experimental apparatus and techniques are essentially the same as described previously. 1 Briefly, a constant-volume Sieverts apparatus employing a sensitive pressure transducer was used to measure the gas-metal reaction rate. The melt weighed about 60 g and was contained in an alumina crucible with an internal diameter of 31 mm. The volume of the apparatus was adjusted such that the total pressure change would be small enough (~0.03 atm) that the experiments were done at essentially constant pressure, yet the pressure change would be large enough that the ab-
ISSN 0360-2141/ 81/0611-0379500.75/0 9 1981AMERICAN SOCIETYFOR METALSAND THE METALLURGICALSOCIETYOF AIME
VOLUME 12B,JUNE 1981--379
sorption rate could be measured accurately. All the experiments were made at 1600 __+2 ~ and the melts were chemically analyzed for oxygen, sulfur, and chromium at the end of the experiment. The technique for determining the rate of the hydrogen reaction was similar to that used for nitrogen. The rate was measured for low-sulfur iron with pure hydrogen. However, for the alloys containing sulfur, pure hydrogen could not be used because the following reaction would occur at the surface: H2(g) + S (in Fe) + H2S(g)
[1]
This reaction would not affect the measurement of the hydrogen reaction rate because one mole of hydrogen is used to form one mole of H~S. However, it would significantly affect the activity of sulfur at the surface; the activity would be poorly defined and could be significantly less than in the bulk metal. To avoid this problem, the rate was measured in an H2-H2S gas mixture tht would be in equilibrium with the Fe-S alloy. The amount of H2S in the gas was small; for example, for an alloy containing 0.38 pct sulfur the equilibrium gas phase 4 contains only 0.10 pct H2S. The sulfur content of the melt was first estimated from the relative amounts of the master alloy and pure iron u
