Internal sulphidation

Another factor that determines the long-term stability of the protective oxide layer is its ability to prevent sulphur penetration which would lead to the eventual formation of chromium sulphide beneath the external oxide layer. With most commercial nickel chromium alloys internal sulphidation... 

Figure 7.17 Oxygen diffuses through the internal oxide zone and reacts with sulphides in the front of the internal sulphide zone to form oxide and release sulphur that diffuses deeper into the alloy to form new sulphide.

When the concentration of B is so low that a protective scale of B O cannot form, a zone of internal oxidation of B O particles in a matrix of A will form. The surface of the alloy, effectively pure A, can now react with the complex atmosphere to form a scale of either A 0 or duplex A 0 and A, S. Where a duplex scale is formed, the metal-scale interface will be at equilibrium with A + A 0 + A S sulphur will dissolve in the metal and diffuse inwards through the internal oxidation zone to form internal B S particles. This forms as a second, inner, sulphide-based internal zone of precipitation below the outer internal oxidation zone. Since BpO is assumed to be substantially more stable than B S, sulphide formation is not expected to be seen in the outer internal oxidation zone. As oxygen continues to diffuse inwards it wiU react with the internal sulphide particles, forming oxide and releasing sulphur to diffuse further into the metal. This is shown in Figure 7.17. Thus, once the internal sulphide zone is established, it can be driven into the alloy by this cascading mechanism, effectively removing the metal B from solution in the alloy. 

Alloys that are exposed to aggressive atmospheres at high temperatures are usually designed to be heat resistant and one of the most serious problems is how exposure to a complex gas can cause the protective behaviour of scales to change. Usually, such alloys rely on scales based on chronua or alumina formation. Consequently, they rely on sufficiently high concentrations of chromium and aluminium being available in the alloy. This can be disrupted if internal sulphidation removes the chromium or aluminium from solution in the matrix. 

It is possible that the sulphide formed in this way may form a zone of internal sulphidation rather than a complete layer at the scale-metal interface. This would remove the chromium and aluminium from solution in the matrix and leave the system open for the formation of rapidly growing sulphides of metals such as iron,... 

Oxidation and hot corrosion in sulphate, chloride and vanadate environments of a cast nickel-based superalloy have been reported by Deb et al. (1996). Weight gain studies were carried out in air for uncoated samples and for samples coated with 100% Na2S04, 75% Na2S04 + 25% NaCl, and 60% Na2S04 -I- 30% NaV03 -i-10% NaCl. The presence of sulphur in the form of sulphates was reported to cause internal sulphidation of the alloy beneath the external oxide layer. Deb etal. observed the formation of volatile species by chlorides, which further led to formation of voids and pits at grain boundaries that reportedly provide an easy path for flow of corrodents. The presence of vanadate in conjunction with sulphate and chloride is proposed to provide... 

Continued exposure of the nickel-chromium alloy to more severely sulphurising and reducing atmospheres results in local depletion of chromium to such an extent that nickel sulphide and the eutectic are formed internally. The latter constituents are not often observed in service failures, but the relative instability of nickel sulphide in the presence of chromium sulphide can result in its reduction to nickel during slow cooling on shut down. That nickel sulphide is formed is suggested by the frequent occurrence of blisters, associated with the formation of molten eutectic on the surface of sulphur-attacked specimens . 

Cast iron may be used under similar circumstances, but has inferior mechanical properties. It has been used, although not in current practice, for internal cathodic protection, where it has been demonstrated that the presence of ferrous ions in water is of benefit in reducing sulphide-induced attack on Cu alloy tube plate and tubesWater treatment has now been found to be a more practical method. 

The pressed disc (or pellet) type of crystalline membrane electrode is illustrated by silver sulphide, in which substance silver ions can migrate. The pellet is sealed into the base of a plastic container as in the case of the lanthanum fluoride electrode, and contact is made by means of a silver wire with its lower end embedded in the pellet this wire establishes equilibrium with silver ions in the pellet and thus functions as an internal reference electrode. Placed in a solution containing silver ions the electrode acquires a potential which is dictated by the activity of the silver ions in the test solution. Placed in a solution containing sulphide ions, the electrode acquires a potential which is governed by the silver ion activity in the solution, and this is itself dictated by the activity of the sulphide ions in the test solution and the solubility product of silver sulphide — i.e. it is an electrode of the second kind (Section 15.1). 

The tube B is loosely packed with purified glass wool soaked in lead acetate solution (to remove hydrogen sulphide and trap acid spray), and C is a capillary tube (4 mm external and 0.5 mm internal diameter). Place 1.0 mL... 

Loss of catalytic activity resulting from internal displacements is not usually a serious problem below temperatures of about 100 C. However, highly active R-groups, such as benzyl, methyl and allyl, undergo internal displacement more readily, particularly in the presence of strong nucleopfiles. For instance, the presence phenolates and thiolates may lead to the formation of benzyl alcohol, ethers, or sulphides from benzyl-substituted quaternary ammonium salts. 

Studies of n4 compounds possessing three P-C bonds are presented below. In addition to a diazaphospho1e oxide,174 the internal molecular motions of triphenylphosphine oxide have been analysed,173 and the anomeric interaction of a diphenylphosphinoyl-1,3-dithiane estimated.174 The structures of two sulphides have been examined,... 

Electrodes responding to other halides, sulphide, cyanide, silver, lead, copper and cadmium are made using membranes fabricated from pure or doped silver sulphide (Ag2S). The membrane potential is affected by the movement of Ag+ ions between cationic lattice sites which in turn is determined by the activities of the Ag+ ion in the internal and sample solutions. As the activity of the former is fixed, that of the latter alone influences the membrane potential. The electrode will also respond to the presence of S2- ions because of their effect on the Ag+ ion activity via the solubility product expression ... 

Hvitved-Jacobsen, T., B. Jiitte, P. Halkjaer Nielsen, and N.Aa. Jensen (1988), Hydrogen sulphide control in municipal sewers. In H. H. Hahn and R. Klute (eds.), Pretreatment in Chemical Water and Wastewater Treatment, proceedings of the 3rd International Gothenburg Symposium, Gothenburg, Sweden, June 1—3, 1988, Springer-Verlag, New York/Berlin, pp. 239—247. 

Ekmekci, Z., Bradshaw, D.J., Harris, P.J., and Buswel, A.M., Interactive Effect of Milling Media and CuS04 Additions on the Flotation Performance of Sulphide Minerals from the Morensky ore, Part II Froth Stability, International Journal of Mineral Processing, Vol. T8, pp. 164-174, 2006. 

Adam, K., Natarajan, K. A. and Iwasaki, I., 1984. Grinding media wear and its effect on the flotation of sulphide minerals. International Journal of Mineral Processing, 12(1 - 3) 39 - 54 Ahlberg. E. and Broo. A. E., 1988. Proc. Inter. Symp. Electrochem. In Mineral and Metal Process. 

Chander, S. and Fuerstenau, D. W., 1975. Sulphide minerals with thiol collectors the chalcocite diethy dilhiophosphate system. 11th International Mineral Processing Congress, 1 583 - 603 Chander, S., Wie, J. M., Fuerstenau, D. W., 1975. On the native floatability and surface properties of naturally hydrophobic solids, hr P. Somasunfaran and R. G. Grieves(eds.), Advances in Interfacial Phenomena of Particulate/Solution/Gas Systems. AIME Symp., Ser., 150(71) 183-188... 

For a detailed study, see [421-424], In general, all-solid-state ISEs are used with an internal silver contact. Selectrodes (see p. 60) can also be prepared from a mixture of halide and silver sulphide [328,329]. 

An internal electrochemical mechanism was proposed long ago for deposition on certain metal substrates, since the rate of deposition sometimes depended on the nature of the substrate [11].) The standard potential of Reaction (5.3) is -l- 0.08 V, considerably more positive than the rednction potential of S to (-0.45 V). Free sulphide, if formed, would be in a very low concentration, since it will be removed continually by precipitation of PbS this will move the S rednction potential strongly positive according to the Nemst equation [Eq. (1.32)]. This positive shift will be even greater than normal because of the non-Nemstian behavior of the S /S couple when [S] > [S ] (at least in alkaline solntion) [12]. In opposition to this, the solubility of S in the (slightly acidic) aqneons solntions is very low, which will move the potential in the opposite direction. Add to this the very small concentration of S in acid solution [Eq. (1.15)], and it becomes clear that it is not trivial to estimate the feasibility of the formation of PbS by free snlphide. The non-Nemstian behavior of the sulphur-rich S /S couple and the lack of knowledge of the solnbility of free S in the deposition solntion are the two factors that complicate what would have been a tractable thermodynamic calcnlation. 

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