High-temperature oxidation corrosion sulphidation

The reaction of metals with gas mixtures such as CO/CO2 and SO2/O2 can lead to products in which the reaction of the oxygen potential in the gas mixture to form tire metal oxides is accompanied by the formation of carbon solutions or carbides in tire hrst case, and sulphide or sulphates in the second mixture. Since the most importairt aspects of this subject relate to tire performairce of materials in high temperature service, tire reactions are refeiTed to as hot corrosion reactions. These reactions frequendy result in the formation of a liquid as an intermediate phase, but are included here because dre solid products are usually rate-determining in dre coiTosion reactions. 

Mrowec et examined the resistance to high-temperature corrosion of Fe alloys with Cr contents between 0.35 and 74 at% Cr in 101 kPa S vapour. They found that the corrosion was parabolic, irrespective of the temperature or alloy composition, and noted that sulphidation takes place at a rate five orders of magnitude greater than oxidation at equivalent temperatures. At less than 2% Cr, the alloys formed Fe, j.,S growing by outward diffusion of Fe ions, with traces of FeCr2S4 near the metal core. 

Corrosion refers to the loss or conversion into another insoluble compound of the surface layers of a solid in contact with a fluid. The solid is normally a metal, but the term corrosion is also used to refer to the dissolution of ionic crystals or semiconductors. In the majority of cases the fluid is water, but an important exception is the reaction of metallic surfaces with air at high temperature, leading to oxide formation, or, in industrial environments, to sulphides, etc. In the context of this book, corrosion of metals or semiconductors in contact with aqueous solution or humid air at normal temperatures is of predominant interest. 

The corrosion product, a mixture of oxide, sulphide at the metal interface and sulphate outside, has a weak adhesive bond to the metal surface and cannot support large deposit masses. It is therefore unusual to find excessive amounts of sintered ash deposits and fused slag in the exact localities where severe high temperature corrosion occurs. Conversely, a strongly adhering matrix of sintered ash deposit in the absence of sulphate, sulphide or chloride phases is not markedly corrosive. 

Sulphidation reactions follow a similar series of kinetic phenomena as has been observed for oxidation. Unfortunately, few studies have been made of the basic kinetic phenomena involved in sulphidation reactions at high temperature. Similarly, the volatile species in sulphate and carbonate systems are important in terms of evaporation/condensation phenomena involving these compounds on alloy or ceramic surfaces. Perhaps the best example of this behaviour is the rapid degradation of protective scales on many alloys, termed hot corrosion , which occurs when Na2S04 or other salt condenses on the alloy. 

Figure 8.40 Schematic diagram to show the relationship of the different hot corrosion mechanisms as a function of temperature and SO3 pressure, (i) Type II, gas-phase induced acidic fluxing, (ii) At high SO3 pressures (PSO3 > 10 atm) pronounced sulphide formation accompanied by oxidation of sulphides and fluxing reactions, (iii) Type I (alloy-induced acidic fluxing basic fluxing, sulphidation).

The two types of high temperature fuel cell are quite different from each other (Table 6). The molten carbonate fuel cell, which operates at 650°C, has a metal anode (nickel), a conducting oxide cathode (e.g. lithiated NiO) and a mixed Li2C03/K2C03 fused salt electrolyte. Sulphur attack of the anode, to form liquid nickel sulphide, is a severe problem and it is necessary to remove H2S from the fuel gas to <1 ppm or better. However, CO is not a poison. Other materials science problems include anode sintering and degradation, corrosion of cell components and evaporation of the electrolyte. Work continues on this fuel cell in U.S.A. and there is some optimism that the problem will be solved within 10 years. 

Hot corrosion is a complex phenomenon involving sulphidation, oxidation or both. Hot corrosion is a form of accelerated oxidation which affects the surfaces exposed to high-temperature gases contaminated with sulphur and alkali metal salts. These contaminants combine in the gas phase to form alkali metal sulphates. If the temperature of the alloy or coating surface is below the dew point of the alkali sulphate vapours and above the sulphate melting points, molten sulphate deposits are formed. Molten sodium sulphate is the principal agent in causing hot corrosion. 

High temperature (Type 1) hot corrosion (HTHC) is normally observed in the temperature range of about 825-950°C when the condensed phase is clearly liquid. The typical microstructure for HTHC shows the formation of sulphides and a corresponding depletion of the reactive components in the alloy substrate. The external corrosion products consist of oxide precipitates dispersed in the salt film. The presence of the pore, crevice or crack across a protective film can lead to the sulphidation of the alloy substrate. This results in a significant shift in the basicity of the salt film. Once the fused salt contacts the alloy substrate, the rate and duration of the rapid corrosion kinetics are decided by the magnitude and gradient of salt basicity relative to the local solubilities for the oxide scale phases (Rapp and Zhang, 1994). 

High temperature corrosion of Inconel-600 tube used as a furnace accessory has been reported by Krishna and Sidhu (2001). The corrosion of the tube was found to be due to severe oxidizing and sulphidizing atmospheres generated by interdiffusion of base metal constituents and sulphur through the microchannels. 

Habazaki H, Dabek J, Hashimoto K, Mrowec S, Danielewski M, The sulphidation and oxidation behaviour of sputter-deposited amorphous Al-Mo alloys at high temperatures Corrosion Sci, 1993 34 183-200. 

Niobium like tantalum relies for its corrosion resistance on a highly adherent passive oxide film it is however not as resistant as tantalum in the more aggressive media. In no case reported in the literature is niobium inert to corrosives that attack tantalum. Niobium has not therefore been used extensively for corrosion resistant applications and little information is available on its performance in service conditions. It is more susceptible than tantalum to embrittlement by hydrogen and to corrosion by many aqueous corrodants. Although it is possible to prevent hydrogen embrittlement of niobium under some conditions by contacting it with platinum the method does not seem to be broadly effective. Niobium is attacked at room temperature by hydrofluoric acid and at 100°C by concentrated hydrochloric, sulphuric and phosphoric acids. It is embrittled by sodium hydroxide presumably as the result of hydrogen absorption and it is not suited for use with sodium sulphide. 

Lee and Lin (1999) studied the oxidation, mixed oxidation-sulphidation and hot corrosion of ductile iron aluminide Fe3Al with Cr addition at temperatures of 605-800°C. They observed that hot corrosion of iron aluminide was significantly more severe than oxidation and mixed oxidation-sulphidation. According to Lee and Lin, this can be attributed to the formation of aluminium sulphide at the metal-salt interface as a result of high sulphur potential in the molten salt at the oxide-metal interface. 

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