How humidity controls rusting
The question
How does relative humidity affect the rate at which iron corrodes, and what humidity should iron meteorites be stored at?
Collectors are advised to keep iron meteorites in very dry air, because rusting speeds up sharply when the air is damp. Saturated salt solutions hold the air in a sealed box at a fixed humidity, so you can test this.
Equipment
- Fine steel wool pads (degreased with soapy water, rinsed and dried) or identical small steel washers
- Four airtight plastic boxes
- Silica gel (very dry), saturated salt solution (about 75% humidity), saturated potassium chloride solution (about 85%) and plain water (close to 100%)
- A balance reading to 0.01 g, and a cheap digital humidity meter if possible
Method
- Put a small open dish of each humidity control in its own box, with a weighed sample on a stand above it, not touching the liquid.
- Seal the boxes and keep them at the same temperature.
- Weigh each sample every few days for two to four weeks; rust adds mass as iron combines with oxygen and water.
- Photograph the samples each time.
- What to measure
- Percentage mass change against time for each humidity, and the humidity readings.
- How to present it
- Graphs of mass change against time, a conclusion about the humidity at which rusting becomes rapid, and storage advice for an iron meteorite.
- Take it further
- Add a sample sprinkled with a little salt to model the chlorine that makes some iron meteorites weep, and compare.
Safety
Steel wool is flammable: keep it well away from flames, batteries and electrical contacts. Wear gloves to avoid splinters. Do not taste any solution, and dispose of solutions as your teacher directs.