In this article we will discuss about:- 1. Meaning of Oxidation 2. Mechanism of Oxidation 3. Kinetics.
Meaning of Oxidation:
Oxidation is a type of corrosion involving the reaction between a metal and air or oxygen at high temperatures in the absence of water or an aqueous phase. It is also called dry-corrosion. At normal temperatures, the oxides of the metals (except gold) are more stable than the metals. Metals being in the metastable state are bound to form oxides. The rate of oxidation of a metal at high temperatures depends on the nature of the oxide layer that forms on the surface of the metal.
In general, the oxides can be categorised in three groups:
1. The oxide is unstable, i.e., oxide docs not form i.e., oxidation does not occur. For example, gold is free of tarnishing as gold oxide does not form.
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2. Some metals form molten or volatile oxides, such as V2O5 on vanadium and MoO3 on molybdenum: or the oxygen dissolves in the metal itself, such as in titanium. The oxidation then occurs catastrophically, as fast as the oxygen gas is supplied.
3. More commonly a thin film or thick scale of oxide or oxides form on the surface, which slows down further oxidation. This aspect is more common and is dealt here.
Thin surface oxide layers formed up to a thickness of about 3000 A° are called films, but are called scales if greater than this, and are clearly visible. Very thin oxide films, such as 20 A° thick produced on aluminium at room temperature, are invisible, but can be seen when about 100-1000 A° thick from the interference colours produced by light rays.
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The process of tempering of quenched steel is sometimes controlled by observing the temper colours produced on the clean steel surface. These colours range from light straw (220°C) to light blue (3I0°C) as the oxide film increases in thickness as a function of time and temperature.
Thin films invariably decrease the rate of additional oxidation and may, as in the case of aluminium, be almost completely protective. The oxide film completely isolates the metal from the air. The tendency for an oxide film (or scale) to be protective can be approximately evaluated to act as a guide by a parameter called the Pilling-Bedworth ratio. This is the ratio of oxide volume to the volume of metal from which the oxide has formed. For the following oxidation reaction,
a M + (b / 2) O2 → Ma Ob … (14.2)
The Pilling-Bedwortli ratio is-
where, W is the formula weight of the oxide, w is the atomic weight of the metal, ρm and ρox are the densities of metal and the oxide, respectively. If this ratio is less than I, there is insufficient oxide to cover the metal. Tensile stresses develop in the oxide film, causing the film to become porous, and unprotective. Oxidation continues rapidly as happens for a metal like magnesium, Fig. 14.1 (a). If the ratio is equal to, or slightly greater than unity, an adherent, non-porous, protective oxide film forms such as typical on aluminium, Fig. 14.1 (b).
If the ratio is greater than two (to three), i.e., volume of the oxide is greater than that of the metal. Initially, the thin oxide film acts as a protective layer. However, as the thickness of the scale increases, high compressive stresses develop in the oxide. The brittle oxide scale cracks and spalls or flakes off the metal surface and normally a breakaway oxidation occur such as for iron, Fig. 14.1 (c).
Table 14.1 illustrates the P-B ratios for some metals. This ratio does not accurately predict oxidation resistance in all cases. This ratio acts as an empirical criterion and does not include other properties more important in determining oxidation resistance, such as good adherence, high melting point, a low vapour pressure, good high-temperature plasticity to resist fracture and low electrical conductivity, or low diffusion coefficients for metal ions and oxygen, similar coefficient of expansion as the metal itself for cyclic temperatures (alternate heating and cooling cycles in service can cause the oxide layer to crack).
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As oxidation proceeds more rapidly with the rise of temperature, even an adherent oxide film may not afford sufficient protection at elevated temperatures.
Mechanism of Oxidation:
When a fresh surface of metal like gold comes in contact with oxygen, the latter forms a loosely bound layer on the surface of gold due to secondary van der Waals attractive forces arising between the atoms of the metal surface and the oxygen molecules. This layer is said to be adsorbed oxygen layer, and no further chemical bonding occurs. This layer is easily removed by applying ultra-high vacuum.
In most other metals, the first oxygen molecules coming in contact with the clean surface of metal, dissociate into oxygen atoms and then, these oxygen atoms bond chemically with the atoms of the metal surface.
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This monolayer of oxygen atoms which forms rapidly over the whole surface of the clean metal is said to be chemisorbed. This process, involving dissociation and ionisation of oxygen molecules is known as chemisorption. Additional oxygen may be physically adsorbed on such a layer.
In the chemisorbed layer, oxide is nucleated at favourable sites on the surface such as ends of dislocations, steps in surface, impurity atoms, etc. This nucleation occurs slowly at low temperatures, but soon the whole surface is covered with a thin film of oxide, which continues to thicken, particularly at high temperatures.
The initial oxidation represents direct-chemical reaction between the metal and the oxygen. If the initial oxide layer on the surface has pores, or fissures in it (P-B ratio is less than 1), it is non-protective oxide. Here, molecular oxygen diffuses through the pores and reacts with the metal to form more oxide at the metal-oxide interface as illustrated in Fig. 14.2. The alkali and alkaline-earth metals such as calcium, magnesium Fig. 14.1 shows such behaviour. These metals follow linear-oxidation-rate law. Even metal like molybdenum forming volatile MoO3 follows the linear relationship.
When the initial oxide film on the exposed metallic surface is protective in nature, further oxidation is electro-chemical in nature.
The reactions are:
Metal ions (M2+) form at metal-oxide interlace-
M → M2+ + 2e̅ …(14.4)
Electrons produced (reaction 14.4) are conducted through the oxide layer to meet the oxygen atoms at oxygen-oxide interlace to result in-
1/2 O2 + 2 e̅ → O2- …(14.5)
Once, these ions have formed, these react to produce metal oxide by the reaction-
M2+ + O2- → MO …(14.6)
When a very thin film of oxide (monolayer, or so) has formed on the surface, the oxygen atoms on the outer surface extract electrons from the metal underneath this thin film to become negative ions by the reaction (14.5). These negative ions exert an electrostatic attraction on the positive metal ions in the metal.
Initial film being very thin and the force of electrostatic attraction being very strong, the metal ions (normally smaller than anions) are pulled through the film to produce metal oxide by reaction (14.6). As the oxide film thickens, the force becomes weaker since the distance between the metal-cation and anion-oxygen increases.
Further oxidation requires the normal diffusion of ions and the conduction of electrons through the oxide layer. The electronic conductivities of oxides are usually several orders of magnitude greater than their ionic conductivities. The rate of diffusion of ions through the oxide layer is a slow process, and hence the oxidation drastically slows down.
The oxidation now has a logarithmic type of growth. The rate of diffusion of ions becomes the controlling step for oxidation.
The oxidation reaction M2+ + O2-→ MO takes place either at the oxygen-oxide interface by the diffusion of metal ions and conduction of electrons through the oxide layer to this interlace, or at the oxide-metal interface by conduction of electrons to oxide-oxygen interface, and by the diffusion of oxygen ions through the oxide layer to oxide-metal interface. The place of oxide formation depends on the type of defect structure of the oxide lattice formed on the metal surface.
Defect Structures of Oxides:
Metal oxides, actually formed on metal surfaces do not have perfect crystal lattices. These crystal lattices of oxides, like metals, contain point defects at temperatures above absolute zero. The defects could be anion vacancies or interstitials and cation vacancies or interstitials. When a large concentration of defects is present, a change in composition of the oxide occurs, i.e., oxides are non-stoichiometric, that means, deviate from their ideal molecular formulas.
For example, cuprous oxide Cu2O is found by analysis actually to be Cu1.8O.
The deviations could be-
(i) Excess metal due to anion vacancies,
(ii) Excess metal due to interstitial cations,
(iii) Excess oxygen due to interstitial anions,
(iv) Excess oxygen due to cation vacancies.
Based on defect structures, oxides can be broadly divided into two groups:
1. Metal cation-deficient p-type semi-conducting oxides, such as NiO.
2. Metal cation-excess n-type semi-conducting oxides, such as ZnO, CdO, PbO2, ZrO2.
NiO, FeO and Cr2O3 are cation deficient oxides as these have cation (positive) vacant lattice sites. Fig. 14.4 illustrates the growth of NiO on the surface of nickel. NiO layer has vacant nickel ionic sites. As the oxide as a whole must be electrically-neutral, the presence of vacant sites of Ni2+ (positive charge) necessitates some electrons to be missing.
In NiO, to balance one vacant Ni2+ site, two electrons must be correspondingly missing, which is achieved by the removal of one electron each from two nearby Ni2+ ions (as illustrated in Fig. 14.4), by converting them to Ni3+ by the reaction:
Ni2+ ⟺ Ni3+ + e̅ …(14.7)
i.e., to make two electrons missing, two Ni3+ ions are present in the lattice instead of two Ni2+ ions. The defect oxide layer thus, consists of a number of vacant Ni2+ sites (two such sites are shown in Fig. 14.4), and double this number of Ni3+ ions are shown. These missing electrons in the oxide layer make it to be a p-type (positive) intrinsic semi-conductor because the missing electrons cause positive holes to be present in the lattice.
The growth of NiO layer occurs by the diffusion of Ni2+ ions (as Ni2+ vacant sites are present) and electron conduction through the oxide layer to the oxygen-oxide interface to form NiO there. The electrons are conducted through the oxide layer by the transfer of one electron from Ni2+ to the neighbouring Ni3+ ion changing the latter to Ni2+, but the former becomes Ni’,+ by reaction 14.7.
Thus, electrons are conducted from one site to the other, and finally to the oxygen-oxide interface for the reaction 14.5 to take place to produce negative oxygen ions. Ni2+ ions diffuse to the oxygen-oxide interface by exchanging places with the vacant ionic sites (the usual vacancy mechanism), making the vacant sites to move inside (pores have been seen inside the oxide layer) and the Ni2+ ions to go to the oxygen-oxide interface to produce NiO there.
In cation-deficient oxides, cations as well as electrons diffuse through the oxide film to the oxygen-oxide interface. The rate of oxidation depends on the conductivity of the oxide layer and the increasing deviation of NiO from the exact NiO formula.
The ionic lattice of ZnO contains more zinc ions (at elevated temperatures) than the formula ZnO requires. This is an anion-deficient structure and is called n-type intrinsic semi-conductor (extra electrons, i.e. negative charges). The extra Zn2+ ions as well as loosely attached electrons (to maintain neutrality) are accommodated interstitially.
The excess zinc ions and the electrons diffuse through interstitial sites to the oxygen-oxide interface to cause oxidation there, by reactions shown in Fig. 14.5. The oxides Fe2O3, ZrO2 and TiO2 are also n-type semi-conductors, but here some oxygen ion lattice sites are vacant, and the metal ions, though present at normal lattice sites, are excess of oxygen ions.
These are also anion-deficient oxides. The excess electrons (to maintain the neutrality) are conducted through the oxide layer to the oxygen-oxide interface to form the oxygen ions there by the reaction 14.5. These oxygen ions diffuse through the oxide layer (as it has oxygen ion vacant lattice sites) to the metal-metal oxide interface by vacancy mechanism to form the metal Reaction at metal-metal oxide Reaction at the oxygen-oxide oxide there, i.e. oxide grows downward in the metal as illustrated in simple Fig. 14.6.
The formation of oxide at metal-metal oxide interface tends to produce compressive stresses in the oxide layer. In the initial stages, these compressive stresses are helpful a bit as the stresses close the pores in the film to produce dense film, but as the thickness increases, the oxide film may break to relieve these stresses, and it may cause break-away-type of oxidation.
Another possible mechanism of oxidation is shown in Fig. 14.7, where metal cations diffuse outwards and the oxygen anions diffuse inwards. The metal oxide is formed anywhere in the oxide layer where these ions react.
Some metals simultaneously form more than one stable binary oxide. Iron, above about 570°C shows a phase sequence as Fe/FeO/Fe3O4/Fe2O3/O2 (copper forms CU2O and CuO), with most oxygen-rich-oxide at scale-oxygen interface. The relative thickness is determined by the rate of ionic diffusion through that oxide. FeO and Fe3O4 grow by diffusion of metal cations through the oxide layer, as these oxides contain cation vacancies.
But the growth of Fe2O3 occurs by the diffusion of oxygen anions inward from oxygen surface, as it has anion vacancies. Below 570°C, FeO layer is missing and the scale is more adherent and harder.
Kinetics of Oxidation:
An engineer is interested in knowing the rate of (oxidation) reaction. As in most cases, the oxide formed remains on the surface of metal, the rate of oxidation can be expressed as weight gain per unit area, or how the thickness of the oxide film increases with time. Oxidation usually develops with time ‘t’ according to one of the curves illustrated in Fig. 14.8 at a constant temperature, or more curves in different temperature ranges. These oxidation for various metals under various conditions.
Linear Rate Law:
This law states that oxidation takes place at a more or less constant rate, i.e., thickness, or weight gain of the oxide layer increases with time uniformly.
W = At + D …(14.8)
where, W is weight gain per unit area, t is time, A and D are constants dependent on temperature. The linear law applies to the initial stages of most oxidation before the film is thick enough to be protective, and also wherever the protection no longer increases as oxidation continues such as, Mo at temperatures where MoO3 is volatile. Linear relationship is characteristic of metals which form a porous or cracked scale. Sodium, potassium, magnesium (with P-B ratio less than 1) also oxidise with linear law, and tantalum, columbium too with P.B. ratio of about 2.5.
Logarithmic Rate Law:
Logarithmic oxidation is generally observed with thin highly protective oxide films (< 1000 A°) at low temperatures in Cu, Fe, Zn, Ni, Pb, Al, Cr, Mn, Ti, etc. Fig. 14.8 indicates that W, weight gained per unit area in a logarithmic way with time, t as:
W = E log (Ft + G) …(14.9)
where, E, F and G are constants for a particular temperature, environment and composition. The oxidation occurs very slowly to become almost constant after sometime (i.e., oxidation almost does not occur). This law mainly applies to highly protective thin films such as chromium oxide film, aluminium and beryllium oxides, iron below 200°C.
Such a rate of oxidation, probably, results from the effect of electrostatic forces between ions within very thin oxide film in assisting the transport of ions across the film. At this stage, forces become weaker between ions due to increased distance between them. Oxidation slows down to give logarithmic rate.
Parabolic Rate Law:
Wagner showed that when diffusion of ions occurs through the defect-structure oxide layer, the oxidation of pure metals should ideally follow the parabolic rate law:
W2 = At + B …(14.10)
Where, A and B are constants. Thus, it is a diffusion-controlled mechanism, and in most cases, the scale is protective (PB is > 1), though not as protective as logarithmic type. Here, the thickness of oxide increases at a decreasing rate. The scale grows mainly at oxide-oxygen interface, i.e., diffusion of metal-cations occurs. Oxidation of many metals, Cu (at 800°C), Fe(200°-900°C), Ni. Cr, Co at elevated temperatures obeys parabolic law.
Cubic Rate Law:
Some metals under specific conditions, particularly to short exposure periods follow the cubic law:
W3 = k t + C …(14.11)
where k and C are constants. Zirconium and hafnium oxidise with cubic law.
Fig. 14.8 illustrates a break-away type of oxidation. Here, the oxidation of metal initially follows the parabolic law, and the oxide is protective, but then the film breaks after acquiring a critical thickness, perhaps due to strains set-up during its formation. Direct access to oxygen of fresh metal surface takes place to let it grow parabolically again.
As the thickness grows, it breaks again. This repeated break-away on a fine scale can prevent the protective part of the film from increasing beyond a certain thickness, ultimately resulting in an overall linear oxidation rate (shown dolled). Breakaway parabolic is sometimes called paralinear.
Probably the plasticity of the oxide at high temperatures is important to prevent the cracking, and the breakaway. Copper oxide on copper at 800°C is plastic, and follows parabolic law, but when formed at 500°C, the oxide is brittle to result in break-away type of oxidation. Fe2O3 formed on iron gives a break-away type of oxidation.
It is common that some metals oxidise by a combination of rate laws, or the oxidation occurs by two simultaneous mechanisms. Many times, one mechanism predominates during initial and other at later stages.
However, parabolic law does not apply from zero time. Linear oxidation rate is least desirable, and parabolic as well as logarithmic oxidation rates are more desirable for alloys used in high temperature applications. Aluminium has high atmospheric oxidation resistance at normal temperatures as it oxidizes according to logarithmic rate law.
In equation (14.10), based on diffusion of ions, the rate constant A = A0 exp [-Q/RT], where Q is the activation energy for the rate controlling diffusion process. The growth rate law changes with temperature. For example, oxidation of titanium changes at about 360°C from logarithmic to parabolic, and to linear as temperature is 800°C or more.
Control of Oxidation:
High resistance to further oxidation is possible if the initial oxide film has Pilling-Bedworth ratio slightly greater than one so that it is continuous, compact and adhering. The film should have high resistance to the conduction of ions and/or electrons, and be almost stoichiometric i.e., has low diffusivity to reduce the oxidation rate effectively.
A high free energy of oxidation results in a dense, almost stoichiometric, chemically stable oxide with excellent protective properties like Cr2O3, AI2O3, BeO. The film of alumina on aluminium has few lattice defects and thus resists further oxidation, but above 400°C, concentration of defects in alumina increases, and growth becomes parabolic. Cr2O3 film on chromium or on chromium steels has poor ionic conductance and thus, prevents further oxidation.
The oxide film is protective if it has good adherence, is impervious, non-volatile, non-reactive with atmosphere, has similar co- efficient of expansion as the metal (if temperature variations are cyclic). The nature of oxide film and its rate of growth varies with the temperature and environment, and needs actual experimental testing.
The oxidation resistance of a metal surface thus can also be increased if a protective oxide film can be provided by some other ways. A suitable alloying element can be added to the metal which preferentially reacts with oxygen and forms the protective oxide film. Hauffe’s valency rule, though not absolutely reliable, is often very useful.
According to this rule, if the oxide film of the base metal is a cation-deficient structure, then the addition of an alloying element of lower valency, reduces the cation vacancies in the oxide formed. Addition of nickel to pure chromium, thus, decreases the oxidation rate but the reverse addition increases the oxidation rate.
If the base-metal oxide structure has excess cations and electrons in the interstitial positions, such as in ZnO, the reverse is true. Addition of aluminium to zinc metal lowers the oxidation rate of zinc, but addition of lithium, of lower valency increases the oxidation rate. Aluminium addition to Zn decreases electron concentration, decreases interstitial metal cations, whereas addition of lithium increases electron concentration and increases interstitial metal cations.
Selective oxidation can be used to an advantage, where one component or microstructural constituent of an alloy oxidizes more readily than the others. A more reactive metal must be chosen to be alloyed with the base metal, which has more affinity for oxygen then does the base metal.
Such an alloying element, if present, diffuses to the surface of the alloy to be oxidised there. If this oxidised film is stable, adherent, continuous, and has poor electrical conductivity with complex crystal structure to hinder the diffusion of ions, it acts as a protective film. The fact that oxides such as AI2O3, Cr2O3, BeO, SiO2, TiO2 formed on the respective metals, are dense, almost stoichiometric, chemically stable with good protective properties, is made use of in producing sterling silver (92.5% Ag, 6.5% Cu, 1% Al), which has AI2O3 formed on it due to aluminium addition, and to produce tarnish resistant silver.
Even beryllium does not allow tarnishing of silver. Silicon as SiO2 in cast iron improves its oxidation resistance. Steel requires larger additions of aluminium and silicon to produce very protective oxide films, but the amounts needed simultaneously embrittle the steel (thus, avoided).
Chromium additions are generally made to get protective oxide-film of Cr2O3 such as in chromium steels, stainless steels as illustrated in Fig. 14.9. A freshly cut surface of stainless steel immediately forms a layer of protective Cr2O3. The outstanding protective properties of Cr2O3 makes the presence of chromium in many high temperature alloys an essential alloy addition as it ensures high resistance to oxidation.
A good electric heater wire is made of nichrome (80% Ni-20% Cr). Inconel (76% Ni 16% Cr – 7% Fe) has good oxidation resistance. Kanthal (24% Cr-5.5% Al -1% Co and Fe) is used for furnace windings used up to 1300°C. If the oxide layer has more than a single phase, the properties of the oxide phase which is continuous, become the controlling factor of oxidation resistance. In passes, CU2O layer is continuous, and thus, the diffusion through this film governs the oxidation process.