Sulfide rates

Fig. 2 shows that the aluminium content of the alloy has only a slight influence on parabolic sulfidation rate. 

The influence of temperature on the sulfidation kinetics of Ni36Al at a constant sulfur partial pressure of 6.4X10-7 bar is shown in Fig. 3a. The sulfidation kinetic follows the parabolic rate law over the whole temperature range and the sulfidation rate increases with increasing temperature. From the Arrhenius plot of the parabolic rate constants for sulfidation of Ni36Al in Fig. 3b we calculate an apparent activation energy of 58kJ/mol. 

Variations in the Al-content of the alloy between 25 and 45 at.% A1 have nearly no effect on the parabolic sulfidation rate. 

As the example in this figure shows, there seems, however, to be some limited protective effect from such sulfide scales as the sulfidation rates are increased by thermocycling, where the scales crack due to the different coefficients of thermal expansion between sulfide and metal substrate. The transport processes responsible for growth of the sulfide layer in such a complex system are summarized schematically in Fig. 2-38. 

It is typical that a temperature increase of 55°C will double the sulfidation rate, whereas increasing the HjS concentration by a factor of 10 may be needed to double the sulfidation rate. Therefore, changes of H S concentration are generally less significant than temperature variations. 

In the second category, the presence of oxidizing gases such as HjO (steam) or CO slow the sulfidation rate below that expected if only the concentrations were considered. This can be important... 

Because the sulfidation rate is much higher than the oxidation rate, it results in less protective layers. A comparison between oxidation and sulfidation is rrrade in Table 15.1. Not only the defect concentration is responsible for this high rate. In fact, there are four reasons for this higher reaction rate. 

FIGURE 15.1. Dependence on the aluminum content of the sulfidation rate of ternary Fe-Cr-Al alloys containing 20% chromium. (Drawn from data of Mrowec, S. and Wedrychowska, M., OxidMet, 1979,13, 481. With permission.)... 

The high temperature sulfidation rate is mainly dependent on the content of active sulfur in crade oils, and also on the total sulfur content. When the temperature is increased, the reactions between active sulfides and metallic components are accelerated, while at the same time the decomposition of... 

However, despite great similarities in the growth mechanisms of oxide and sulfide scales, the sulfidation and oxidation rates of these alloys differ dramatically. This is illustrated in Fig. 20.15 by the dependence of oxidation and sulfidation rates of Fe-Cr alloys on their composition." As can be seen, the oxidation rate of chromia formers (about 40% Cr) is more than four orders of magnitude lower than the sulfidation rate of alloys, on the surface of which a homogeneous Cr2S3 scale is formed (about 60% Cr). These... 

Influence of aluminum content in Fe-Cr-AI alloys on their sulfidation rate. 

Influence of sulfur pressure on the sulfidation rate of iron and Fe-25Cr-10AI alloy. 

It has been found that there are a few metals highly resistant to sulfide corrosion, i.e. refractory metals and in particular molybdenum and niobium. The sulfidation rates of these metals are comparable with the oxidation rate of chromium, which is one of the most resistant metals to oxide corrosion. Taking into account this important observation, several years ago extensive research was undertaken in order to prove the possible role of refractory metals as alloying conponents in designing novel sulfidation-... 

It follows clearly from this figure that the sulfidation rate of these ternary alloys decreased dramatically with increasing aluminum content and reached virtually the rate of pure molybdenum sulfidation with only 5 wt% of this... 

Influence of aluminum on the sulfidation rate of Ni-30Mo alloy for several temperatures. 

Temperature dependence of the sulfidation rate of binary Ni-30Mo and ternary Ni-30Mo-xAI alloys compared to the sulfidation rates of nickel and molybdenum. 

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