Understanding what pitting corrosion is, how it starts and how to prevent it goes a long way to ensuring long, safe and useful service for metal assets exposed to the elements.
Pitting corrosion is believed to be the cause of a deadly bridge collapse in 1967. Forty-six people died when the U.S. Highway 35 between Point Pleasant, WV and Kanauga, OH fell into the Ohio River.
Investigators determined a small crack formed when one of the bridge’s eyebeams was cast decades before the collapse. The eyebar broke under the compounding stresses of a corrosive environment and heavier vehicles traversing the bridge.
What is pitting corrosion?
Pitting corrosion is a localized breakdown of metal manifesting in small cavities or “pits” visible on a metal surface. But there’s more to these pits than meets the eye. Far more damage is done beneath the metal surface because the corrosion bores inward.
Additionally, pitting corrosion is more difficult to detect than other forms of corrosion. It’s also harder to predict and to design against.
Pitting corrosion causes the loss of metal thickness. This loss of structural integrity in turn can lead to stress cracking due to metal fatigue.
For example, if a beam that bears a heavy load loses thickness and mass due to corrosion, there’s less beam available to support the weight. Over time, the fatigue this causes leads to the formation of cracks. Cracks can quickly lead to a failure of the beam, which could set off a catastrophic chain reaction as unplanned stresses multiply.
How does pitting corrosion start?
There are several potential causes for pitting corrosion, including the following:
- Localized mechanical or chemical damage to a metal’s protective oxide film
- Improper application of corrosion control products
- The presence of non-metal materials on the surface of a metal
When metals that aren’t properly treated are freely exposed to the elements, chemical reactions between metals and the environment form compounds like ferrous oxide (FeO)—more commonly known as rust.
Preventing pitting corrosion
Preventing pitting corrosion starts early in a metal asset’s useful life. First and foremost, choosing the right metal makes a big difference. The risk of pitting corrosion is greatly reduced when you know ahead of time how materials react in different environments. Higher alloys resist corrosion more strongly. Next, control the environment to the extent it’s possible. For indoor or sheltered assets, keeping environmental factors like temperature, pH and chloride concentration in check minimizes the risk of pitting corrosion, ensuring a long useful life for your assets. Finally, apply the proper industrial coating to your assets and have them inspected regularly.
Invest in corrosion control
A world-class provider of industrial coating inspections, surveys and coating project management, FeO can help you ensure your assets stand up to the harshest environments. Contact us to learn more about keeping your assets protected.