Oxidation of metal

Diffusion of oxygen and/or metal atoms near the surface then occurs and the initial oxide layer is formed. The further growth of this layer involves two steps (a) reaction at the metal/oxide and oxide/oxygen interfaces, and (b) transport of material through the oxide layer. As the oxide layer gets thicker, the rate of growth is controlled by (b) which becomes the slower of the two steps. If the volume of the oxide produced is less than that of the parent metal, the simple rate law for an unimpeded reaction is observed 

Theories of oxidation have been developed by Wagner and by Mott ° . In general the logarithmic rate law applies to very thin oxide layers which form protective coatings and the parabolic rate law to thick oxide layers. More recent reviews of the subject have been given by Grimley , Kubaschewski and Hopkins and by Wyn Roberts .  

Weight changes, measured with a microbalance, are a convenient way of following the kinetics of oxidation. The use of this technique is discussed in Section 2.1.4. Many metals have been studied and differing behaviour is often observed in differing temperature ranges. Very high temperatures are required for 

Condit and Holt have reviewed the use of radioactive tracers in the study of oxidation. Radioactive platinum has been used to study the oxidation of cobalt and radioactive silver to study the oxidation of molybdenum. If the tracer element remains at the surface it indicates that the oxygen is diffusing into the metal, and if it is located some distance from the surface shows diffusion of the metal through the oxide. The tracer can be detected by autoradiography and its distribution studied by gradual removal of the oxide layer. Its distance from the surface can also be estimated by measurement of the energy of the emitted radiation at some point outside the surface .  

An important characteristic of radioactive decay is that the momentum of the emitted particle must be balanced by the momentum of the product nucleus. In suitable cases it should be possible to detect the recoil nucleus. Thorium-228, which decays to radium-224 having an energy of 97 keV, has been used to study the oxidation of a number of metals. Its advantages are its low volatility (as thorium oxide) and its relatively low rate of diffusion in lighter metals. The maximum range of recoils in solids is of the order of 300-500 A and for thinner oxide layers, its distance from the surface can be measured. One difSculty with quantitative work is that the radium undergoes a sequence of further decay, which complicates calculation of recoil ranges, and calibration may be necessary. 

The transformation of metal-electrode surfaces via electrooxidation to their metallooxides, solvated ions, and metal complexes is fundamental to most anodic electrochemical processes (batteries, electrorefining, anodic-stripping analysis, and reference electrodes). Although this is traditionally represented as the removal of one (or more) electrons from a metal atom at the electrode 

The structure and the physical properties of oxides determine the oxidation kinetics at high temperamre. Table 9.5 provides a seleetion of relevant data. 

Common metal oxides most often exhibit a eystal strueture that eoiresponds to a eompaet arrangement of oxygen anions. The metal eations are loeated at the tetrahedral and oetahedral sites of the anion lattice. On average, there are two tetrahedral and one oetahedral site per oxygen ion. 

In divalent oxides of type MO (e.g., MgO, CoO, NiO, FeO, TiO, NbO, VO), the eations generally oeeupy the octahedral sites. These oxides crystalhze in the NaCl strueture, although there are some exceptions to this rule. The oxides ZnO and BeO, for example, have the wurtzite structure in whieh the eations oeeupy half of the tetrahedral sites. 

A large number of trivalent oxides with the general formula M2O3 have the strueture of eorundum a-A 20. Examples inelude a-Fe203, Cr203, Ti203 and V2O3. The metal eations in these oxides occupy two thirds of the octahedral sites. 

Oxide Crystal structure Melting Boiling or point decomposition (°C) temperature (°C) Molar volume (cm mol ) PB Thermal expansion coefficient xlO Rqd 

Before the theory of metal oxidation had been formulated, a large number of experiments showed that, at sufficiently high temperatures, metals and alloys react with oxidizing gases and liquids by forming more or less adherent (protective) product lay- 

The rate of increase in sample thickness (A ) in terms of the current density / is 

If we express the driving force by the Gibbs energy of formation, that is, U= AGao/2-F, Eqns. (7.1) and (7.2) yield 

Some limiting cases are noteworthy. 1) The most frequent case is the oxidation of metals leading to semiconducting oxides with dense anion packing, that is, fel = 1 and D0 = 0. This gives 

AGa0 is the Gibbs energy of formation of AO from metal A and oxygen gas at ambient atmosphere with partial pressure p0j (AGao = AGao + (RT/2) ln (po/PoJ)-2) If products with predominantly ionic conduction are formed, the tarnishing layer is very thin in view of tel l. From Eqn. (7.3), one has with tkm = 1 

---

Aronoff Y G, Chen B, Lu G, Seto C, Schwartz J and Bernasek S L 1997 Stabilization of self-assembled monolayers of carboxylic acids on native oxides of metals J. Am. Chem. Soc. 119 259-62... 

Folkers J P, Gorman C B, Laibinis P E, Buchholz S and Whitesides G M 1995 Self-assembled monolayers of long-chain hydroxamic acids on the native oxides of metals Langmuir 813-24... 

N. Birks and G. H. Meir, Introduction to High Temperature Oxidation of Metals, E. Arnold, London, 1983. 

Basic oxides of metals such as Co, Mn, Fe, and Cu catalyze the decomposition of chlorate by lowering the decomposition temperature. Consequendy, less fuel is needed and the reaction continues at a lower temperature. Cobalt metal, which forms the basic oxide in situ, lowers the decomposition of pure sodium chlorate from 478 to 280°C while serving as fuel (6,7). Composition of a cobalt-fueled system, compared with an iron-fueled system, is 90 wt % NaClO, 4 wt % Co, and 6 wt % glass fiber vs 86% NaClO, 4% Fe, 6% glass fiber, and 4% BaO. Initiation of the former is at 270°C, compared to 370°C for the iron-fueled candle. Cobalt hydroxide produces a more pronounced lowering of the decomposition temperature than the metal alone, although the water produced by decomposition of the hydroxide to form the oxide is thought to increase chlorine contaminate levels. Alkaline earths and transition-metal ferrates also have catalytic activity and improve chlorine retention (8). 

This reaction is important in the manufacture of long-chain alcohols by means of hydrolysis of the aluminum alkoxide. Examples of oxidation of metal alkoxides (40,42) include ... 

Fluxes. Fluxes, composed mostly of salts or oxides of metals, serve to protect underlying metal from the air. This prevents the formation of surface oxides that impede fusion and the formation of a strong solder joint. Fluxes may also act to selectively leach elements from the surface of the underlying metal. The result is a surface free of obstacles to fusion, and of a composition readily wetted by the solder. 

Evans, U. R., The Corrosion and Oxidation of Metals, First Supplementary Volume, Edward Arnold Ltd., London, 1968. 

Oxidation of metals forming more than one oxide... 

P. Kofstad. High Temperature Oxidation of Metals. J. Wiley Sons. New York (1966) TA 462. K57. 

O. Kubaschewski and B. E. Hopkins, Oxidation of Metals and Alloys, 2nd edition, Butterworths, 1962. 

The principal constituents of the paniculate matter are lead/zinc and iron oxides, but oxides of metals such as arsenic, antimony, cadmium, copper, and mercury are also present, along with metallic sulfates. Dust from raw materials handling contains metals, mainly in sulfidic form, although chlorides, fluorides, and metals in other chemical forms may be present. Off-gases contain fine dust panicles and volatile impurities such as arsenic, fluorine, and mercury. 

Providing protective atmospheres to reduce the oxidization of metals... 

Although all the types of corrosion discussed in this section result in the oxidation of metal and some involve direct electron transfer, they can be understood without reference to electrochemistry. 

Benard, J., Adsorption of Oxidant and Oxide Nucleation , in Oxidation of Metals and Alloys, Seminar, 1970 American Society for Metals, Ohio, 1 (1971)... 

Kofsted, P., High Temperature Oxidation of Metals, John Wiley, New York 0965) Kubaschewski, O. and Hopkins, B. F., Oxidation of Metals and Alloys, Butterworths, London... 

---