Iron Corrosion as a Natural Clock

By Kevin Mellem

Iron corrosion is a naturally occurring process that takes place when iron metal reacts with environmental water to form hydrated iron (III) oxide. This process is commonly referred to as rusting, and, while it is generally something that is to be avoided, if at all possible, iron corrosion does have the unintended consequence of providing a method to keep time. This chronological record-keeping ability is made possible by fact that corrosion of iron occurs at a consistent, measurable rate under constant environmental conditions.

The chemistry behind iron corrosion can be characterized as a simple oxidation/reduction reaction where iron and water lose and gain electrons, respectively, and come together, along with environmental oxygen, to form iron (III) oxide. The transformation begins when water comes into contact with the iron metal, and iron becomes oxidized to iron (II), as diagramed in the following equation:

Fe → Fe2+ + 2e

The electrons released by the iron atoms further react with water and oxygen to produce hydroxide ions, as shown below:

4e + O2 + H2O → 4OH

Next, the iron cations and hydroxide anions join together and then further react with atmospheric oxygen to produce the final products, hydrated iron (III) oxide and water, as summarized by the following equations:

Fe2+ + 2OH → Fe(OH)2

4Fe(OH)2 + O2 → 2(Fe2O3xH2O) + 2H2O

As mentioned earlier, corrosion of iron can provide a method of both recording time and dating materials as a consequence of the fact that corrosion occurs at a constant rate under constant environmental conditions. Unfortunately, in any natural environment, conditions cannot be kept constant, and the rate of corrosion differs greatly with differing exposure to water and oxygen, as well as the pH, temperature, and electrolytic activity of the environment. This explains why salted roads produce rustier cars - higher salt content increases electrolytic activity of the water and increases the corrosion rate. As such, it would not be feasible to date any environmental iron object exactly, since the rates of corrosion would be nigh impossible to determine.

However, relative dating of similar objects in the natural environment would be feasible using iron corrosion. Take, for example, the previously mentioned rusting cars. When inspecting any vehicle, it is quite easy to observe any rust present on the vehicle's iron parts. In observing two different vehicles that have been driven in identical conditions, one could determine which car was older based upon which car had more rust present on its exposed components, since both cars would have had the same initial condition of having no rust present. Cars provide only a common example, and this relativistic dating of iron objects using the amount of rust present could be applied to any set of iron items that have similar starting conditions and have been exposed to the same environmental conditions.

Despite the difficulty in precisely dating iron objects in the natural environment, the corrosion of iron could still be used to accurately track time under carefully monitored laboratory conditions. Several initial experiments could be performed to determine the rate of corrosion under controlled amounts of water and oxygen. This experimentally determined rate could then be used to monitor the passage of time by measuring the depth of rust on iron pieces of various sizes that are exposed to the same conditions as the iron pieces in the initial experiment; dividing depth of rust on the metal by the experimentally determined rate would provide the amount of time that has passed since the exposure of the iron to the controlled conditions. The amount of time that could be recorded would be different with each size of iron piece. Thinner pieces could only record shorter amounts of time before they became completely corroded, while thicker pieces could record larger periods of time due to their increased depth.

Under these controlled conditions, iron corrosion would satisfy all of the conditions of a natural clock. The initial condition is easily identifiable as the pure iron that has been unexposed to the water and oxygen that initiates the corrosion, and the final condition is the observed depth of rust that is present on the piece of iron at any given point in time before total corrosion has occurred. The time frame allowed by the natural clock would vary with the size of the piece of iron that would be used in the experiment. As described earlier, slimmer pieces of iron would not keep track of as much time as thicker pieces would, due to the difference in depth available for corrosion to take place.

All things considered, though, it is apparent that iron corrosion would be a problematic natural clock. Since natural conditions do not permit the accurate dating of iron objects, only relative data could be gathered from common items, such as the estimation of a car's age due to the rust on their exteriors. Conversely, the monitoring of rust advancement in iron in a controlled laboratory environment would certainly be too inconvenient and cost ineffective to be of much use when compared to other chronological recorders. As such, while it provides an interesting scientific solution to keeping track of time, iron corrosion would not be a desirable natural clock when faced with better solutions.