Volatile metal halides

Other routes include the high-temperature halogenation of metal oxides, sometimes in the presence of carbon, to assist removal of oxygen the source of halogen can be X2, a volatile metal halide CX4 or another organic halide. A few examples of the many reactions that have been used industrially or for laboratory scale preparations are ... 

Another problem in high-temperature corrosion can be the effect of the formation of volatile metallic halides which can, in turn, disrupt the integrity of a protective surface oxide. Figure 7.73 shows that in the Ti-O-Cl system at very low oxygen potentials, volatile TiClj can be formed directly from TiO and Ti, whereas from Fig. 7.74 it is clear that in the system U-O-Cl at 450°C the volatile chloride cannot be formed directly from the oxides. 

From a mechanistic perspective, one of the most important aspects of the results of these experiments is the further confirmation of the formation of volatile metal halide species by a reaction pathway, as illustrated by Scheme [6], that does not include the formation of HX. 

Nitromethane CH3NO2 Volatile metal halides TiCL ... 

It has been shown 162) that volatile metal halides with relatively weak Me-X bonds are extremely effective flame retardants. Di- or trivalent iron halides in very low... 

A mechanisri1 based on the formation of volatile metal halides is presented. The corrosiomprocess proceeds in three stages ... 

Volatile metal halides, usually chlorides and fluorides, also form the heart of several processes used to produce surface layers, rich in aluminium, chromium, or silicon, or combinations of these. In these processes, the workpiece to be coated is buried in a powder bed and heated to reaction temperature. The bed consists of a mixture of inert alumina filler, a master alloy powder that contains the aluminium, etc., and an activator such as ammonium chloride. Basically, at about 630°C, the activator volatilizes and the aluminium chloride vapour reacts with the master alloy to produce a volatile aluminium chloride, which then reacts with the workpiece surface to deposit aluminium. The deposited aluminium proceeds to diffuse into the surface layers of the workpiece to produce a diffusion coating. The process is driven basically by the difference in aluminium activity between the master alloy and the worlqtiece. These processes are well documented in principle, but their execution to provide reproducible and reliable results still involves considerable experience, or rule of thumb. These processes will be described in detail in Chapter 10. Finally, a chlorination treatment is used to remove tin from tin-plated steel. This uses a normally deleterious reaction to advantage and profit in the recovery of both tin and steel for recycling. Fluorination is used in the manufacture of polymers and fluorocarbon consequently, materials suitable for construction of these plants must be resistant to fluorine attack. 

Heumann, T., and Venker, H. (1968). Formation of Alloys Between Low and High Melting Metals by the Reduction of Volatile Metal Halides and Clarification of the Composition of Alloys Obtained by This Process. Communaut6 Europ6en de L En-ergie Atomique (Euratom), 76 pp. 

It has been known for a long time that the efficiency of the halogenated flame retardants can be much increased by the use of certain additives, usually known as synergists. Most of these synergists are particulate inorganic fillers. Their main mode of action is believed to be the result of the formation of volatile metal halides. 

The tin additives exert their fire-retardant action in both the condensed and vapour phases, by promoting the formation of a thermally stable carbonaceous char and (in halogen-containing polymer formulations) by generating volatile metal halide species which assist in free radical scavenging reactions in the flame. 

C, which is considered typical of the metal temperature that components would experience in gas turbine service. Hot eor-rosion tests for up to 100 h at 700 °C, involving oxidation in air of samples coated with sea salt, indicated an increase in metal consumption rate by a factor of up to 20 compared with air oxidation. The mechanism of hot corrosion described by Nicholls et al. (1997) involved interactions among the deposited salt, the predominantly Ti02 scale and the alloy substrate to form volatile metallic halides, which were subsequently oxidized (pyrohydrolyzed) to form non-protective oxides on the outer surface of the scale. In addition, the salt deposit became enriched in chlorides so that its melting temperature was lowered, allowing it spread laterally over the surface and increase the area susceptible to attack. 

---