How to Calculate the Rate of Metal Corrosion
When most metals come into contact with certain substances in the air or water, they undergo a chemical change that decreases the integrity of the metal. This process is called corrosion. Oxygen, sulfur, salt, and other materials can all lead to various types of corrosion.
When a metal corrodes or deteriorates, it cannot hold the same loads as it did before corrosion began. At a certain point, corrosion can lead to dangerous conditions. The metals used in bridges, railroad tracks, and buildings are all subject to corrosion. Because of this, it is important to monitor and manage corrosion to avoid structural collapse.
The Rate of Corrosion
The rate of corrosion is the speed at which any given metal deteriorates in a specific environment. The rate, or speed, is dependent upon environmental conditions as well as the type, and condition, of the metal.
Corrosion rates in the U.S. are normally calculated using mpy (mils per year). In other words, the corrosion rate is based on the number of millimeters (thousandths of an inch) it penetrates each year.
In order to calculate the rate of corrosion, the following information must be collected:
- Weight loss (the decrease in metal weight during the reference time period)
- Density (density of the metal)
- Area (total initial surface area of the metal piece)
- Time (the length of the reference time period)
Online Resources for Calculating Corrosion Rates
Corrosionsource.com provides an online metal corrosion rate calculator for computing corrosion rates. Simply input the details and click "Calculate" to calculate corrosion rates in millimeters, inches, microns or millimeters per year, or inches per minute.
Converting Corrosion Rates
To convert the corrosion rate between the mils per year and the metric equivalent millimeter per year (mm/y), you can use the following equation:
1 mpy = 0.0254 mm/y = 25.4 microm/y
To calculate the corrosion rate from metal loss:
mm /y = 87.6 x (W / DAT)
W = weight loss in milligrams
D = metal density in g /cm3
A = area of sample in cm2
T = time of exposure of the metal sample in hours
Why Corrosion Rates Matter
Corrosion rates determine the lifespan of metal-based structures. This reality dictates the choice of metals used for different purposes, and in different environments. It also determines the maintenance requirements for structures: a metal structure in a wet environment may require more frequent maintenance than a similar structure in a drier location. Maintenance schedules are developed based on the types of calculations described above.
Corrosion engineering is a relatively new profession dedicated to slowing, reversing, preventing and avoiding the impact of corrosion on materials and structure. Corrosion engineers are responsible for developing coatings and treatments that can be used on metals to improve the metals' resistance to corrosion. They are also involved with the development of materials that are less vulnerable to corrosion. New non-corroding ceramics, for example, can sometimes be substituted for metals. In situations where corrosion is likely to cause hazardous or expensive situations, corrosion engineers can recommend and implement solutions.