A recent heat-treat question about the oxidation of copper suggested that we all could benefit from a discussion of oxidation mechanisms.

The oxidation of copper follows the parabolic rate law (see below). For oxidation under normal pressures, at which only Cu2O is thermodynamically stable, a single Cu2O layer is formed. At oxygen pressures above the dissociation pressure of CuO, a CuO layer will be formed on the Cu2O layer.

 An Arrhenius plot (Fig. 1) of the parabolic rate constant kp reveals there are three primary regions for double-layer formation (CuO + Cu2O). These are: 

  • A low-temperature region, from 350-550°C (662-1022°F)
  • An intermediate-temperature region, from 600-850°C (1112-1562°F)
  • A high-temperature region, from 900-1050°C (1652-1922°F)

The Cu2O scale grows predominantly in the high-temperature range, the lattice diffusion in Cu2O being the rate determining step. In the intermediate-temperature range, a decrease in temperature dependence occurs. The Cu2O scale continues to grow predominantly, and the decrease in the temperature dependence has been attributable to the contribution of the grain-boundary diffusion in addition to the lattice diffusion.

The Theory[3]

Three basic laws have been used to characterize the oxidation rates of (pure) metals namely: (a) the parabolic rate law, (b) the logarithmic rate law and (c) the linear rate law. These laws can help predict the growth of an oxide layer (i.e. mass gain) over time.

The parabolic rate law (equation 1) assumes that the diffusion of metal cations (M+) or oxygen anions (O-) is the rate-controlling step and is derived from Fick's first law of diffusion. The concentrations of diffusing species at the oxide-metal and oxide-gas interfaces are assumed to be constant. The diffusivity of the oxide layer is also assumed to be invariant. This assumption implies that the oxide layer has to be uniform, continuous and of the single-phase type.

 

  1.               x2 = (kp ·t) + xo

where:

x = oxide film thickness (or mass gain due to oxidation, which is proportional to oxide film thickness)

t = time

kp = diffusion-rate constant (directly proportional to diffusivity of ionic

species that is the rate-controlling step)

xo = constant

 

The diffusion rate constant, kp, changes with temperature according to an Arrhenius-type relationship. We’ll look at the other two rate laws next week.

 

References

1. Zhu, Yongfu and Kouji Mimura, Jae-Won Lim, Minoru Isshiki and Qing Jiang, Brief Review of Oxidation Kinetics of Copper at 350C to 1050C, Metallurgica and Materials Transactions A, Volume 37A, April 2006, pp. 1231 – 1237.

2. Adegbuyi, P. A. O., and K. A. Adediji,. A. Adebosin and O. F. Alo, Effects of Temperature on the Oxidation Kinetics of Copper Alloys, The Pacific Journal of Science and Technology, Volume 10, Number 2, November 2009.

3. NACE International, High Temperature Corrosion Kinetics, (www.nace.org/library/corrosion/hotcorrosion/kinetics.asp)

4. Albina, Thesis, Chapter 4, Corrosion Kinetics

5. Zang, L., Kinetics of Oxidation, University of Utah, Lecture 32