25.1 Introduction

The earth’s atmosphere is oxidizing. We can get some idea of oxidation-resistance by using the earth as a laboratory, and looking for materials that survive well in its atmosphere. All around us we see ceramics: the earth’s crust (Chapter 2) is almost entirely made of oxides, silicates, aluminates, and other compounds of oxygen; and being oxides already, they are completely stable. Alkali halides, too, are stable: NaCl, KCl, NaBr—all are widely found in nature. By contrast, metals are not stable: only gold is found in “native” form under normal circumstances (it is completely resistant to oxidation at all temperatures); all the others in our data sheets will oxidize in contact with air.

Polymers are not stable either: most will burn if ignited, meaning that they oxidize readily. Coal and oil (the raw materials for polymers), it is true, are found in nature, but that is only because geological accidents have sealed them off from all contact with air. A few polymers, among them PTFE (a polymer based on —CF₂—), are so stable that they survive long periods at high temperatures, but they are the exceptions. And polymer-based composites, of course, are just the same: wood is not noted for its high-temperature oxidation resistance.

25.2 Energy of Oxidation

If this energy is positive, the material is stable; if negative, it will oxidize. The bar-chart of Figure 25.1 shows the energies of oxide formation for our four categories of materials; numerical values are given in Table 25.1.

Table 25.1

Energies of Formation of Oxides at 273 K

Material	Oxide	Energy (kJ mol-1 of Oxygen, O2)	Material	Oxide	Energy (kJ mol-1 of Oxygen, O2)
Beryllium	BeO	−1182	Woods, most		≈ −400
Magnesium	MgO	−1162	polymers, CFRP
Aluminum	Al2O3	−1045	Diamond, graphite	CO2	−389
Zirconium	ZrO2	−1028	Tungsten carbide	WO3+CO2	−349
Uranium	U3O8	≈ −1000	cermet (mainly WC)
Titanium	TiO	−848	Lead	Pb3O4	−309
Silicon	SiO2	−836	Copper	CuO	−254
Tantalum	Ta2O5	−764	GFRP		≈ −200
Niobium	Nb2O5	−757	Platinum	PtO2	≈ −160
Chromium	Cr2O3	−701	Silver	Ag2O	−5
Zinc	ZnO	−636	PTFE		≈ zero
Silicon nitride, Si3N4	3SiO2+2N2	≈ −629	Gold	Au2O3	+80
Silicon carbide, SiC	SiO2+CO2	≈ −580	Alkali halides		≈ +400 to ≈ +1400
Molybdenum	MoO2	−534
Tungsten	WO3	−510	Magnesia, MgO	Higher oxides	Large and positive
Iron	Fe3O4	−508	Silica, SiO2
Tin	SnO	−500	Alumina, Al2O3
Nickel	NiO	−439	Beryllia, BeO
Cobalt	CoO	−422

25.3 Rates of Oxidation

When designing with oxidation-prone materials, it is obviously vital to know how fast the oxidation process is going to be. Intuitively one might expect that, the larger the energy released in the oxidation process, the faster the rate of oxidation. For example, one might expect aluminum to oxidize 2.5 times faster than iron from the energy data in Figure 25.1. In fact, aluminum oxidizes much more slowly than iron. Why should this happen?

If you heat a piece of bright iron in a gas flame, the oxygen in the air reacts with the iron at the surface of the metal where the oxygen and iron atoms can contact, creating a thin layer of iron oxide on the surface, and making the iron turn black. The layer grows in thickness, quickly at first, and then more slowly because iron atoms now have to diffuse through the film before they make contact and react with oxygen. If you plunge the piece of hot iron into a dish of water the shock of the quenching breaks off the iron oxide layer, and you can see the pieces of layer in the dish. The iron surface now appears bright again, showing that the shock of the quenching has completely stripped the metal of the oxide layer which formed during the heating; if it were reheated, it would oxidize at the initial rate.

The important thing about the oxide film is that it acts as a barrier which keeps the oxygen and iron atoms apart and cuts down the rate at which these atoms react to form more iron oxide. Aluminum, and most other materials, form oxide barrier layers in just the same sort of way—but the oxide layer on aluminum is a much more effective barrier than the oxide film on iron is.

How do we measure rates of oxidation in practice? Because oxidation proceeds by the addition of oxygen atoms to the surface of the material, the weight of the material usually goes up in proportion to the amount of material that has become oxidized. This weight increase, Δm, can be monitored continuously with time t in the way illustrated in Figure 25.2. Two types of behavior are usually observed at high temperature. The first is linear oxidation, with

Δm=kLt