We continue our discussion from last week’s blog.

Logarithmic Rate Law

The logarithmic rate law (Equation 2) is an empirical relationship that has no fundamental underlying mechanism. This law is mainly applicable to thin oxide layers formed at relatively low temperatures and, therefore, is rarely applicable to high-temperature engineering problems.

(2)    x = ke log(ct +b)

where:

ke = rate constant

c and b = constants

Linear Rate Law

The linear rate law (Equation 3) is also an empirical relationship that is applicable to the formation and build-up of a nonprotective oxide layer:

(1)  x = kL ·t

where:

kL = rate constant

t = time

In general, the high-temperature oxidation rate decreases with time (parabolic behavior) due to an increasing oxide thickness acting as a diffusion barrier. Due to the formation of highly porous, poorly adherent or cracked nonprotective oxide layers, however, corrosion rates may remain linear.

Metals with linear oxidation kinetics at a certain temperature have a tendency to undergo so-called catastrophic oxidation (also referred to as breakaway corrosion) at higher temperatures. In this case, a rapid exothermic reaction occurs on the surface, which increases the surface temperature and the reaction rate even further. Examples of metals that may undergo extremely rapid catastrophic oxidation include molybdenum, tungsten and vanadium. In the case of magnesium, ignition of the metal may even occur. The formation of low melting-point oxidation products (eutectics) on the surface has also been associated with catastrophic oxidation.

In many materials, elements such as aluminum or chromium are added to improve their oxidation resistance. The reasons for this include the fact that the partial pressure (PO2) required for the oxidation of aluminum (Al) to aluminum oxide (Al2O3) and chromium (Cr) to chromium oxide (Cr2O3) is much lower than for iron (Fe) to iron oxide (FeO), nickel (Ni) to nickel oxide (NiO) or even cobalt (Co) to cobalt oxide (CoO). In addition, the oxide resistance is enhanced by the formation of a protective layer of aluminum oxide or chromium oxide on the surface.

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