Oxidation mechanisms 391

17.3 Oxidation mechanisms 391

Oxide MO grows

O + 2e O V −

at surface

Metal Oxide MO grows

M M V ++ + 2e−

(a) at interface (b)

Figure 17.3 Oxidation mechanisms. (a) Growth by metal diffusion and electron conduction

(b) Growth by diffusion of oxygen and holes.

The weight gain shown in Figure 17.2 reveals two different types of behav- ior. For some metals the weight gain is linear, and this implies that the oxida- tion is progressing at a constant rate:

giving ∆ m ⫽ kt ℓ (17.2)

where k ᐍ is the linear kinetic constant. This is because the oxide film cracks (and, when thick, spalls off) and does not protect the underlying metal. Some metals behave better than this. The film that develops on their surfaces is compact, coherent and strongly bonded to the metal. For these the weight gain is para- bolic, slowing up with time, and this implies an oxidation rate with the form

d( ∆ m )

giving ∆ m 2 ⫽ kt p (17.3)

where k p is the parabolic kinetic constant. The film, once formed, separates the metal from the oxygen. To react further, either oxygen atoms must diffuse inward through the film to reach the metal or metal atoms must diffuse outward through the film to reach the oxygen. The driving force is the free energy of oxida- tion, but the rate of oxidation is limited by the rate of diffusion, and the thicker the film, the longer this takes.

Figure 17.3 shows a growing oxide film. The reaction creating the oxide MO

M ⫹ O ⫽ MO

goes in two steps. The metal first forms an ion, M 2⫹ say , releasing electrons:

M⫽M 2⫹ ⫹ 2e

392 Chapter 17 Durability: oxidation, corrosion and degradation

The electrons are then absorbed by oxygen to give an oxygen ion:

O ⫹ 2e ⫽ O 2⫺

The problem is that the first of these reactions occurs at the metal side of the oxide, whereas the oxygen is on the other side. Either the metal ions and the electrons must diffuse out to meet the oxygen or the oxygen and electron holes (described in Chapter 14) must diffuse in to find the metal. If the film is an elec- trical insulator, as many oxides are, electrons cannot move through it, so it is the oxygen that must diffuse in. The concentration gradient of oxygen is that in the gas C o divided by the film thickness, x. The rate of growth of the film is pro- portional to the flux of atoms diffusing through the film, giving

⬀ D ⫽ K o ⎜⎜ exp

d ⫺ o  (17.4)

RT  ⎠ x where D is the diffusion coefficient, K o is a kinetic constant and Q d is the acti-

vation energy for oxygen diffusion. Integrating gives

x 2 ⫽k p t

This has the same form as equation (17.3), with

This explains why oxidation rates rise steeply with rising temperature, and why the growth is parabolic. The most protective films are those with low diffusion

coefficients, and this means high melting points. This is why the Al 2 O 3 oxide film on aluminum, the Cr 2 O 3 film on stainless steel and chrome plate, and the

SiO 2 film on high silicon cast iron are so protective. Not all oxides grow with parabolic kinetics, as we have seen. Those that show linear weight gain do so because the oxide that forms is not compact, but has cracks or spalls off because of excessive volume change, leaving the fresh sur- face continually exposed to oxygen. Linear weight loss, the other behavior, occurs when the oxide is volatile, and simply evaporates as it forms.