section epub:type=”chapter” role=”doc-chapter”> Halogens and many halogen compounds are very corrosive. In fact, iodine was discovered when, as an impurity in soda ash, it caused corrosion of copper vessels. Hence, materials for containing and handling halogens and corrosive halogen compounds must be selected with particular care. In this chapter, oxidation and corrosion theories are applied to the problems of destructive metal and alloy halogenation. Consideration of halogenation mechanisms is necessary because of the large number of variables involved, including temperature, flow rate, laminar or turbulent nature of flow, halogen partial pressure, diluent gases, active impurity gases such as oxygen, water and halide vapors, abrasion, thermal and mechanical stresses, metal impurities and microstructures, vibration, and radiation. In fact, the practical variations of alloy compositions and environmental conditions are virtually infinite, so that one cannot possibly evaluate all of them in laboratory or pilot‐plant studies. But from some basic laboratory data on reaction kinetics and observations of product morphologies and phases, coupled with an understanding of the scaling and vaporization processes involved, one can anticipate unforeseen problems, determine the cause of existing problems, and design better laboratory tests to evaluate materials for commercial service. Although the corrosion of metals and alloys by pure halogens receives predominant attention in this chapter, we also consider related problems of corrosion by interhalogens, gas mixtures, hydrogen halides, metal halides, and other halogenation agents. Metals may be passivated against halogen attack by deposition of solid reduction products on the metal surfaces: where MX4(s) further blocks rapid M–MX6(v) reaction. On the other hand, such product compounds could accelerate reaction rates. For example, if liquid or volatile oxyhalides or hydrates are formed, they may prevent passivation. The halogen‐metal reaction kinetics is largely influenced by a few pertinent properties of the metal halides, namely, the relative coefficients of thermal expansion for halide and parent metal, the thermodynamic stabilities of condensed and volatile halide species, and halide melting points, vapor pressures, plasticities, electrical conductivities, and ionic diffusion coefficients (Canterford and Colton 1968, 1969). The thermodynamic properties for the halogen–metal reaction can be determined from a knowledge of the standard free energies of formation such as those described in Chapter for several chemical reactions relevant to high temperature corrosion, using JANAF Thermochemical Tables and other sources (Kubaschewski et al. 1967; Reed 1971; Weast 1970). Concerning the melting temperature for the metal halides, one must use a binary phase diagram for ascertaining minimum liquidus temperatures when the reaction products comprise multiple scales, e.g. FeX2 and FeX3 upon the reaction of pure Fe on X2. Likewise, upon the reaction of a binary alloy in an X2 environment, the mixtures of product compounds for each component can establish a liquidus temperature far below those of the constituent compounds. Then, one must consult the pertinent ternary or quasi‐binary phase diagrams (see, for example, the phase diagram for ceramists published by the American Ceramic Society) to establish liquidus temperatures. Concerning the relative diffusion rates, at temperatures above one‐half the melting point, bulk diffusion of halogen and metal ions through the halide scale will be very significant. Using this 0.5 TM–diffusion coefficient relationship, at 150 °C one would expect much more rapid SnF2 (TM = 212 °C) scale formation on tin than MgF2 (TM = 1263 °C) scale formation on magnesium, providing that scale growth rates are diffusion limited in both cases. Concerning the vapor pressures, there are recommended Turkdogan’s and Kellogg’s methods (Turkdogan 1964; Kellogg 1966) for the analysis of equilibrium vapor pressures for vapor species that differ in composition from the condensed phase. In general, the methods used to study metal halogenation kinetics are the same as those used to study metal oxidation kinetics. These include a manometric or pressure‐drop method, several gravimetric methods, the photometric procedure, the electrical resistance method, and the quartz crystal microbalance. The Royal Society of Chemistry (RSC, UK) and the American Chemical Society (ACS, USA) provide good monographs on the topic, which also contain extensive bibliographies on chlorine and other halogens handling procedures and precautions. When a metal or alloy reacts with halogen gas to form a solid product, the product may physically separate the two reactants. This circumstance acts to slow the reaction rate if the product phase is compact and free of macroscopic cracks and voids. On the other hand, cracked or porous scales allow ingress of halogen molecules and reduce the degree of protection. One must also account for dissolution of the nonmetal into the metal and the evaporation of the scale. Thus, the rate of scale growth or metal recession depends greatly upon chemical, physical, and mechanical properties of the intervening product scale. The sealing behavior of pure metals and alloys in oxygen (and air) environments is available in Chapter , and this can be used to interpret scale growth in metal‐halogen systems. Theoretical and experimental studies described in the literature will help in the understanding and control of metal‐halogen reactions. As a base metal surface is exposed to an oxidant, an adsorbed film of a specific structure is set up. With a highly efficient capture of incident molecules, the adsorbed state progresses rapidly to a stable film (several angstroms thick), at which time the growth rate slows because either (i) cations, anions, or electrons are effectively immobile in the film or (ii) cations or anions are hindered at the metal/film or at the film/gas interface, respectively, from entering the scale. At the film/gas interface, dissociated oxidant molecules (i.e. atoms) are reduced when tunneling electrons are trapped to provide an excess of negative surface charge. The resulting electric field across the growing film provides (i) a reduction in the potential energy barriers required for the diffusion of ions or for the tunneling of electrons across the film and/or (ii) a reduction in potential energy barriers for the insertion of ions into the oxide. Various authors differ on the selection of the rate‐limiting step. If the rate of ionic or electronic diffusion or ionic insertion depends upon the electric field (dφ/dx) provided by adsorbed oxygen ions, then for a constant, equilibrium‐adsorbed state, the driving force (reduction in barriers) of the electric field decreases in importance as the film thickens to become a thin scale (from tens to hundreds of angstroms). But at the same time, the morphological restructuring of the scale is contributing in a similar manner. Theories based on these considerations lead either to a logarithmic rate equation [y = ke (te + at)] or to an inverse logarithmic kinetic expression [(l/y) = b − ki,l. ln t]. In the plotting of the kinetic data, one cannot usually distinguish between these rate “laws.” Data for halogenation of metals at low temperatures can frequently be fit to logarithmic expressions. Wagner (1973) has treated the more difficult cases of scale growth in the thin‐film range whereby an electrical charge localized to one interface is compensated by a diffuse charge distributed throughout the scales (as opposed to compensation by an opposing charge at the opposite interface). This treatment provides complicated expressions for the expected kinetics. During the early stages of scale formation and for intermediate temperatures such that T/TM ≤ 0.6, the incoherent crystalline boundaries and line defects set up short‐circuit diffusion of ions through the thin scale, and these can account for scaling rates that are higher by several orders of magnitude than those expected from the extrapolation of kinetic data from higher temperatures. Support for the importance of short‐circuit diffusion in the intermediate temperature range is gained from the response of scaling kinetics to vacuum “aging” of the scale, i.e. the interruption of the steady‐state, rapid scale growth by the removal of the oxidant to allow annealing and grain growth in the scale. When the predominant ionic defects in a compact scale are cation vacancies, the growth of a protective layer occurs by the outward diffusion of cations with a counterflow of vacancies and positive holes. Likewise, scale also forms at the scale/gas interface when the diffusion of interstitial cations and electrons predominate. If the predominant defects were anion interstitials (plus positive holes) or anion vacancies (plus electrons), a compact scale would grow at the metal/scale interface. At high temperatures of corrosion, Wagner (1933, 1936) has described the diffusion‐controlled parabolic growth of a compact one‐phase layered scale on a pure metal to obtain the result where dn/dt is the rate of growth of an MX scale in mol cm−2 s−1; ξ is the instantaneous scale thickness; zx is the valence of the anion; te, tM, and tX are the electrical transference numbers of electrons, cations, and anions; σ is the total electric conductivity of the scale formed; To explain the growth of a compact one‐phase layered scale that exhibits predominant electronic conduction, te > (tM + tX), Wagner (1951) has converted Eq. 11.2 as follows: where VMX is the molar volume of the product compound and through the relation where km represents the quantities in parentheses in either Eq. 11.2 or 11.3. While the validity of Wagner’s theory for the growth of oxide scales has been frequently checked by comparison of experimental values of kp with calculated values of km through Eqs. 11.2 and 11.5, comparable use of this possibility has not been made for the analysis of halogenation reactions. Obviously, these equations provide the basis to predict the dependence of kp on both temperature and Because a number of halides exhibit predominant ionic conduction (Rapp 1970), the phenomenon of local cell action can contribute to the consumption of a metal by halide scale formation as observed by Ilschner‐Gench and Wagner (1958). The established electronic short circuit leads to an overgrowth formation that should be anticipated whenever the corrosion product is a predominant ionic conductor. The relatively high vapor pressures of the halide compounds that form as surface products cause the loss of molecules from the scale to the vapor at the same time that the scale is growing by diffusional transport. Tedmon (1966) described the oxidation rates of pure chromium and Cr2O3− forming Fe–Cr alloys at temperatures greater than 1100 °C as the sum of a parabolic diffusion‐controlled scale thickening and a time‐dependent vaporization loss. The rate equation for the thickening of the scale is where ξ is the scale thickness, kp is the parabolic rate constant for the diffusion step, and kυ is the linear rate constant for the vaporization step. For short times or thin films, Eq. 11.6 predicts parabolic behavior; however, with continued growth, the two terms on the right‐hand side of Eq. 11.6 approach each other so that a steady‐state scale thickness, ξs.s., equal to kp/kυ, is approached. The integration of Eq. 11.6 for ξ = 0 at t = 0 yields the result
Chapter 11
Halogenation
11.1 Introduction
11.2 Metal–Halogen Reactions
and 
are the nonmetal activities at the metal/scale and scale/gas interfaces, respectively; and F and F′ are differing values for the Faraday constant having their appropriate units. Equation 11.2 shows that the growth of a compact scale can essentially be limited by the transport of electrons when (tM + tX) > te (when the scale is a solid electrolyte) or by the transport of ions when the scale exhibits predominant electronic conduction. For many halides, the partial electrical transference numbers have been established as a function of 
.
and
are the self‐diffusion coefficients for the cation and anion, respectively. These expressions for instantaneous scaling rate can be used for comparison with kp, the gross parabolic scaling rate constant, defined by
(Wagner 1936). For the formation of compact oxide scales on copper, nickel, cobalt, iron, and manganese, good agreement between kp and the calculated km has been found at sufficiently high temperatures, i.e. at about 1000 °C or T/TM (scale) > 0.75. Experimental activation energies and 
dependencies for parabolic oxidation also agree with the predicted values.
Halogenation
11.1
11.2
11.3
11.4
11.5
11.6
11.7