Numerous nickel-containing alloys possess a desirable combination of properties vital to long term dependability within petrochemical and refinery plants. Critical to many operations is the requirement for elevated temperature sulfidation resistance under either reducing or oxidizing environments. This paper surveys the role of materials, environmental factors, alloying elements and the formation of protective scales on the performance of selected nickel-containing alloys.
The literature defines sulfidation as the corrosive attack of a metal or alloy when exposed to a sulfur-bearing atmosphere at elevated temperature leading to the formation of metal sulfide (s) corrosion products.(1) Since virtually all physical sources of energy (including their waste products) contain sulfur in various forms, the need for alloys resistant to sulfidation is obvious. Sulfidation within the petrochemical and refining industry has been a matter of increasing concern in recent years as the demand for improved, more efficient processes expand to meet ever changing patterns of available energy sources and stricter end-product and plant emission specifications. (2-5)Since sulfidation resistance is ultimately related to protective scale formation, it is the purpose of this paper to examine the role that scales play in the process of sulfidation and to gain some degree of insight into the contribution that typical alloying elements make to scale characteristics. Without a working knowledge of scale behavior, proper alloy selection is difficult and improved alloy development efforts will be hit or miss at best. Some guidelines for both alloy selection and development will be presented.
Sulfidation behavior of metals and alloys is very similar to that of oxidation but with several notable differences. Perhaps the most important of these differences is the rate of reaction as compared to that of oxidation as shown in Figure 1.(6) The rate of sulfidation is commonly many orders of magnitude faster than oxidation and the corrosion products much more complex, thereby implying a degree of mechanistic behavior even more difficult to unravel than that associated with oxidation, Contrary to general oxidation rates which are ameliorated by low oxygen potentials, the rate of sulfidation becomes increasingly more severe as the oxygen partial pressure is reduced. Incidently, it is in these high sulfur/low oxygen potential environments that enhanced corrosion resistant alloys are needed. Unlike oxide scales which have melting points higher than the melting temperature of the alloy on which they form, sulfide scales have comparatively lower melting points than their base alloy. Figure 2 shows the influence of underlying nickel-iron-chromium composition on the melting point of the sulfide scale formed. Increasing iron and chromium contents raise the melting point of the sulfide scale for any given nickel level. Higher melting point scales should be expected to have lower self-diffusion rates at typical service temperatures. It should be mentioned that the self-diffusion coefficients for sulfides are substantially greater than those of their corresponding oxides. While information on oxidation mechanisms and oxidation products are relatively abundant, the paucity of fundamental information on sulfidation rates.
