The Zone of Metal Phase Consumption in Gas-Metal Reactions

Abstract

It is shown by an analysis of the scale growth process that the scale of even approximately uniform composition formed in the reaction between gas and metal must consist of at least two layers. One layer is formed by the outward migration of metal ions and electrons through the scale; the other forms under the scale in the zone of consumption of the metal phase. Both layers form under completely different conditions. The inner layer is vital for the maintenance of metal transport to the bulk of the scale and grows at a gas pressure of the order of the dissociation pressure of the scale substance. It is demonstrated that the inner layer will develop cavities. A third layer may be formed in some secondary process, for example, that of recrystallization. It is therefore evident that the concept of a uniform process of scale growth is an oversimplification and fundamentally incorrect. This conclusion is supported by an investigation on copper iodide scale. The protective film formation on aluminum and the accelerated growth of oxide films on metals at the beginning‐of oxidation are explained on the basis of the considerations presented above.

Laser Clad and HVOF-Sprayed Stellite 6 Coating in Chlorine-Rich Environment with KCl at 700 °C

“…Figure 10 also depicts the presence of voids at the coating-substrate interface and underneath the voids the substrate microstructure was attacked up to a thickness of *280 lm. The cavity formation has also been observed earlier by Evans [23] and Dravnieks and McDonald [24] that the internal oxidation occurred under the scale in the zone of consumption of the metal phase (also known as MCZ). EDX analysis performed in the 150 9 150 lm large area of the exposed coating showed only 1.0 wt% chromium, pointing to its severe depletion during the exposure.…”

Laboratory scale investigation into the corrosion of copper in a sulphur-containing environment

“…A slower corrosion rate can also be attributed to the porous sulphide layer that formed on the metal-scale interface, which poorly adhered to the metal. Poorly adhered corrosion scale can retard the diffusion of copper species to the surface for the reaction to proceed, leading to slower corrosion rates [12,26]. Dravnieks et al [26] explained that when a metal is covered with an oxide layer which is in contact with an oxidising gas, the oxidation of the metal will proceed by diffusion of metal through the oxide layer.…”

“…Poorly adhered corrosion scale can retard the diffusion of copper species to the surface for the reaction to proceed, leading to slower corrosion rates [12,26]. Dravnieks et al [26] explained that when a metal is covered with an oxide layer which is in contact with an oxidising gas, the oxidation of the metal will proceed by diffusion of metal through the oxide layer. With decreasing direct contact between the metal and oxide the supply of metal to the bulk of the scale is slowed down, and corrosion rates decrease.…”

A chemical diffusion model for partitioning of transition elements in oxide scales on alloys

“…It is here proposed that the mechanism of outward transport in the oxide is by lattice diffusion of cations. Possible mechanisms for transport of oxygen through the inner layer have been discussed by Dravnieks and McDonald (1948) but, since these are not rate-controlling, they need not be considered further for the present analysis. …”

“…For transition metal oxides the energy term E, may be related to the difference in crystal field stabilization energy of the two types of interstice. The difference between the octahedral and tetrahedral crystal field stabilization energies for oxides, termed the crystal field preference energy (CFPE) in this paper, has been calculated from spectroscopic data by Dunitz and Orgel (1957) and is given in table 3. In order of increasing crystal field preference energy, the series may thus be written : for trivalent ions Fe3+ : Ti3+ : V3+ : Mn3+ : a s + , and for divalent ions Mn2+ : Fez+ : Cog+ : Cu2+ : Ni2+.…”