Oxidation of Pure Metals Classification of Metals Nobel Metals - - PowerPoint PPT Presentation

Oxidation of Pure Metals

Classification of Metals

• Nobel Metals – Gold, Platinum, Palladium
• Transition Metals – Relatively moderate Oxidation- Fe,

Ni, Co etc.

• Alkali/Alkali earth metals – High oxidation

Based upon their Oxidation Resistance

• Al, Cu, – Low temperature
• Iron, Nickel, Cobalt - Intermediate Temperatures
• Pt, Pd, Gold and of course Ta, W, Re – Very high

Temperatures ( Vacuum or inert environment)

Based upon their Temperature of application

• Transitions Elements – used as structural elements (

negligible oxygen dissolution)

• Elements such as Ti, Ta, Nb, Zr and Hf ( High Oxygen

dissolution)

Based Upon the Dissolution of Oxygen during Oxidation

• Ni – Single layer oxide Formation
• Co – Double layer Oxide Formation
• Fe – Multi-layer Oxide Formation

Based upon the Complexity of Oxide Formation

Oxidation of Nickel

Simplest Element – forms a single oxide well adherent Oxidation very much dependent on impurity content. A Ni with purity more than 0.002% shows a well adherent single NiO layer which grows by outward diffusion of Ni++ ions. Impure Ni ( mainly C impurity) shows two oxide layer – an outer compact layer formed by outward diffusion of Ni++, and an inner loose oxide formed by inward diffusion of O--. Parabolic Oxidation rate of Ni at 600oC is about half of that at 1000oC. This is mainly due to that domination of grain boundary diffusion at lower temperature compared to manly volume diffusion at 1000oC. This discrepancy can be removed if the Wagners’ theory is modified by including GB diffusion

Oxidation of Cobalt

Cobalt is another very important metal for high temperature application. Depending upon the temperature and the partial pressure of oxygen, it oxidizes to form either CoO or a two layered scale, consisting of CoO and Co3O4. CoO has a NaCl type structure and is a p-type with cation vacancies. Markers tests confirmed the outward diffusion of cobalt ions. Oxidation constant varies as a function of pressure between 940 to 1300oC at oxygen pressure between 10-4 to 0.7 atm. Oxidation of cobalt to CoO at high temperatures is a good example of the applicability of the ideal Wagner model. This could be due to the large concentration of point defects, which suppress the importance of impurity effects, relative importance of grain boundary diffusion and hence makes it comparatively easy to study the defect dependent properties of CoO. Co3O4 is a spinel type oxide and in a two layer scale it is always at the oxide gas interface. Also, the thickness of this oxide is very small compared to CoO. Because of its lower thickness the overall oxidation may still be governed by diffusion through the CoO layer

Oxidation of Cobalt

750oC in Oxygen for 10h CoO Co3O4 Parabolic rate constants as a function of pressure

Oxidation of Iron

Iron on oxidation forms a scale, which is a mixture of three oxides, wustite, FeO; magnetite, Fe3O4 and haematite, Fe2O3. The composition of the scale varies with temperature and with the oxygen partial pressure as shown in Fe-O phase diagram. FeO is formed only above 570oC. Below which only magnetite and haematite are formed. Wustite, FeO is a p-type oxide, with metal vacancies. It exists over a wide range of stoichiometry, from Fe0.95O to Fe0.88O at 1000oC. With such high cation vacancies, the mobility of cations and electrons via metal vacancies and electron holes is extremely high. The magnetite, Fe3O4, has inverse spinel structure, that is all the Fe+2 ions and half of thetrivalent Fe+3 ions are occupied by octahedral sites, and the other half of the trivalent ions Fe+3, occupy tetrahedral sites. Defects occur on both the sites and consequently, iron ions Haematite, Fe2O3, exists in the a and ?forms, which are rhombohedral and cubic in structure

• respectively. Generally, at T > 400oC, Fe3O4 is oxidized to α-Fe2O3. In the rhombohedral form, the
• xygen ions exist on a close packed hexagonal arrangement with iron ions in interstices.

In general, Fe2O3 behaves as an n-type semiconductor, but evidence on the possible migration of cations is also there.

Phase Diagram of Iron with oxygen

Mechanism of Iron Oxidation

Multi Layer Oxidation

Growth of both the layers in a two layered scale is diffusion controlled. The growth occurs by outward movement of cations. The flux of cations in each oxide is assumed to be independent of distance; and Each oxide exhibits predominantly electronic conductivity. Local equilibrium exists at the phase boundaries.

A metal M oxidising to form two oxides of the type MaO and MbO Let molar volumes of these oxides are Va and Vb and thicknesses xa and xb.and Ka and Kb their individual parabolic rate constants and and K is the overall parabolic rate constant of the metal then the rates of oxidation of individual layer formation and that of total oxidation reaction are given by: