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Rust

December 19, 2014

When I was a child in the 1960s, there were clear gender roles, and no one was ranting about gender inequality. It was rare that a woman would work, so they were mostly happy mothers and homemakers. The men provided the household income, and their time away from home allowed their bonding with other males in the traditional tribal ways.

At that time, women in the Soviet Union needed to work, so they would leave their children at state-run daycare centers. Everyone in the US thought that this was cruel for both the mothers and children, and now we've defaulted to the same thing.

In those days, there was even a definite gender difference in the idea of rust. One type of rust is a fungus that attacks flowering plants, such as geraniums, carnations, asters, pansies and sunflowers.That was the rust that was on my mother's mind as she tended her garden. When my father heard the word, "rust," he thought instead of the oxidation product of iron.

Nowadays, it's rare to hear anyone talk about garden flowers, let alone rust fungus. Everyone thinks of red iron oxide (hematite, Fe2O3) when they hear "rust." Iron has been a large part of our culture since the Industrial Revolution, and the rust that accompanies it is ubiquitous. When the US gave up on manufacturing, following instead the lure of a false finance culture, the remains of our manufacturing infrastructure were aptly named the Rust Belt.

As long as your iron doesn't have too much surface area, as it would have in a fine powder or nanoparticle form, rusting happens at a very slow rate. The rust reaction,

Formation reaction of Fe2O3

is spontaneous at all temperatures above room temperature, as can be seen from the negative Gibbs Free Energy of its formation (see graph). This reaction is a good example that reactions found to be spontaneous by a free energy calculation do not necessarily occur at a rapid rate. In rusting, a slow rate of reaction is good, but when you're producing some chemical compound, a slow reaction rate might be a problem.

Free energy of formation of Fe2O3

Negative free energy of reaction indicates a spontaneous reaction. However, "spontaneous" is not synonymous with "fast."

(Data plotted using Gnumeric.)


Red hematite isn't the only iron oxide. Magnetite (Fe3O4), black iron oxide, is a common mineral. It's a magnetic material, and it's sometimes found in a naturally magnetized state as lodestone. Lodestone enabled early forms of the compass. Hematite forms from magnetite by the following oxidation reaction,

Formation of Fe2O3 from Fe3O4

Since the Industrial Revolution started about 250 years ago, and human discovery of iron was about seven millennia before that, you would think that we would know everything about the oxides of iron. Well, that doesn't seem to be the case, as scientists from the Vienna University of Technology (Vienna, Austria) and the University of Erlangen-Nürnberg (Erlangen, Germany) have found that the arrangement of atoms on the surface of magnetite is not what everyone had thought it to be.[2-3]

Figure caption

A ball-and-stick model.

Ball-and-stick modeling was made famous by Watson and Crick, but it's used by inorganic chemists and materials scientists as well.

(TU Wien image.)


The atomic-scale structure at surfaces is important, since the surface atoms are the first to participate in any reaction.[2] The surface state of magnetite is important, since it's used as a catalyst, in electronic devices and in medical applications.[3] One unusual property of the magnetite surface is that atoms of gold or palladium, deposited on its surface, stick where they attach and do not migrate to form nanoparticles. This is the reason why magnetite is an efficient catalyst, but the reason for this surface behavior was unknown.[3]

Such a phenomenon would happen when there are oxygen vacancies at the surface. What was unexpected was that the surface properties of magnetite arise from missing iron atoms, instead.[3] Says Ulrike Diebold, a professor at the Vienna University of Technology (also known as TU Wien) and the head of its metal-oxide-research group
"Because materials interact with their environment through the surface, it's really important to understand the structure of the surface and why it forms... It turns out that the surface of Fe3O4 is not Fe3O4 at all, but rather Fe11O16."[3]
The magnetite structure isn't an array of iron atoms with inserted oxygen; rather, its an array of oxygen atoms with inserted iron. There are missing iron atoms in the layer just below the surface.[3] The positions above these missing iron atoms are where other metal atoms attach. Not only that, but these positions for attachment are regularly-spaced, and the distance between such sites is large enough to prevent the attached surface atoms from attracting each other.[3]

Quantitative low-energy electron diffraction, scanning tunneling microscopy, and density functional theory calculations verified the structure.[2] The implication is that this phenomenon is not restricted to just magnetite, but it might occur in other oxides, such as the oxides of cobalt, manganese and nickel.[2] The Vienna University of Technology has a web site, "Solids4fun," that promotes interdisciplinary collaboration of materials and surface science.[4]

Figure caption

Particle physicists aren't the only ones who usew large, expensive machines in their experiments. Here's the magnetite research team around some impressive vacuum equipment.

(TU Wien image.)[3)]


References:

  1. Free energy data from L. B. Pankratz, "Thermodynamic Properties of Elements and Oxides," U. S. Bureau of Mines Bulletin 672, U. S. Government Printing Office (1982).
  2. R. Bliem, E. McDermott, P. Ferstl, M. Setvin, O. Gamba, J. Pavelec, M. A. Schneider, M. Schmid, U. Diebold, P. Blaha, L. Hammer, and G. S. Parkinson, "Subsurface cation vacancy stabilization of the magnetite (001) surface," Science, vol. 346 no. 6214 (December 5, 2014), pp. 1215-1218, DOI: 10.1126/science.1260556.
  3. Florian Aigner, "The finer details of rust," Vienna University of Technology Press Release, December 4, 2014.
  4. Solids4fun Web Site at the Vienna University of Technology.

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