Sulfide scale

Sulfide scales deposited on the casing wall were found. The minerals of the. scales are listed in Table 2.7 (Imai et al., 1988, 1996 Nitta et al., 1991). The chemical compositions of sphalerite, chalcopyrite, and tetrahedrite are given in Table 2.7. 

Criaud, A., C. Fouillac and B. Marty, 1989, Low enthalpy geothermal fluids from the Paris basin, 2 - oxidation-reduction state and consequences for the prediction of corrosion and sulfide scaling. Geothermics 18, 711-727. 

Fuel sulfur is reduced to H2S by Disulfovibrio. Hydrogen sulfide can attack iron to form an iron sulfide scale and result in loss of iron from a metal surface. In the presence of water, iron sulfide scale can act as a site for severe pitting corrosion. This type of corrosion is often uncontrollable. 

The principal anode performance problem of Soderberg cells is the low-baked anode carbon. This results in preferential attack on binder coke and creates some level of filler dust problem as a standard operating condition. For VS Soderberg anodes, there is additional performance loss for the lower-quality pinhole carbon, which fills the space created when pins are reset. This is due to porosity created when pinhole paste is baked in place by the existing excessive heatup rates, VS Soderberg anodes are also adversely affected if the carbon has a high sulfur content. Conductor pin tips will become coated with an iron sulfide scale, which interferes with electrical conduction in the anode. 

Sulnir Corrosion Chromium is the most important material in imparting resistance to sulfidation (formation of sulfidic scales similar to oxide scales). The austenitic alloys are generally used because of their superior mechanical properties and fabrication qualities, despite the fact that nickel in the alloy tends to lessen resistance to sulfidation somewhat. 

It was quite interesting to observe the sulfidation behaviour of TiAl. Narita et al. [60] found that the sulfidation of TiAl in a gaseous mixture of H2S and H2 results in the formation of titanium sulfides. As the sulfide scale grew by the outward diffusion of Ti. Al was enriched on the substrate surface as TiAl3 with a small amount of TiAl2 as shown in Fig. 10 [60]. After removing the sulfide scale they oxidised the specimen to show a very excellent oxidation resistance. A similar idea may apply to nitridation of TiAl, if preferential nitridation of Ti takes place. 

Even a small amount of water vapor suppresses the growth of sulfide scales described above. 

There is a need to develop the concept of stability diagrams to complex systems such as real alloys in concentrated acids or organic solvents. In such systems, it is critical to accurately represent the standard state properties as well as the activity coefficients. Recently, approaches have been developed and applied to a range of problems such as the formation of iron sulfide scales [7]. 

The mechanisms of CO2 corrosion are generally well defined however, the reality inside a pipeline becomes complicated when CO2 acts in combination with H2S, deposited solids, and other environments. H2S is highly corrosive, but can, in some cases, form a protective sulfide scale that prevents corrosion. 

Let s consider the case of components of an oil refining plant made of stainless steel that are sensitized under the operating conditions and are covered with sulfide scales, which have operated satisfactorily aud which presumably would have continued to operate in a satisfactory manner. In a shntdown event, these components might be subject to rapid failure due to SCC promoted by polythionic acid formed by the reaction of sulfide scales with oxygen and air humidity. 

Destructive examinations of the tube disassembled after 11 years of service revealed only a thin and fragile sulfide scale layer on the IS. It has been proved that sulfide corrosion of the steel was manifested by the thinning of the tube wall only to a small degree. However, under the influence of sulphur, a broad internal degradation of the chemistry and stmcture of the steel took... 

As with oxygen, sulfur-contaiiimg environments are quantified on the basis of their sulfur activity. Solid sulfide scales... 

Since essentially all corrosion product layers have some ion exchange properties, and since these result in an osmotic pressure gradient across the interphase, one can expect a structural relaxation following a potential perturbation. It has been shown that iron sulfide scale, for instance, has a different permeability for corrosion reactions depending on solution pH [39]. This structural relaxation due to surface pH changes is slow (much slower than interfacial capacitance charging) and makes it practically impossible to measure a representative polarization curve. 

Thickness of sulfide scale that forms on Pd and PdyoCuso, Pd4yCu53, and Pd4oCu6o alloys upon exposure to 1000 ppm H2S in H2 for five days as functions of temperature. 

The resistance of Pdg3Ag2Aui5 and Pd74Agi4Aui2 to bulk sulfide formation was more recently reported by Braun [21]. After exposure to 1000 ppm H2S/H2 for 30 h, neither ternary developed a bulk sulfide surface S (confined to 40 run) was detected only by XPS depth profile. At 400 °C, both alloys exhibited clean-H2 permeability similar to that of pure Pd. Upon exposure to 100 ppm H2S at 400 °C, both lost 70% of their pure H2 permeability, but recovered most of that when the H2S was removed [55]. Recovery is consistent with the absence of a bulk sulfide scale. Performance in H2S was in the range that might be expected for PdAu with Au from 4% to 26%. 

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. 

Figure 2-40. Multilayered sulfide scales on a 12Cr steel after 340 h isothermal sulfidation in Ar-H2-H2S at 600 °C (Schulte et al 1998).

Figure 2-42. Schematic diagram of the structure and chemical composition of the sulfide scale grown on 18Cr-lONi-Ti steel at 500 °C after 500 h (Schulte etal., 1998).

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