Pitting corrosion is a localized breakdown of metal manifesting in small cavities or “pits” visible on a metal surface. The damage that these tiny, sometimes nearly invisible, pits cause can be deadly.
Case in point: Pitting corrosion is believed to be the cause of the 1967 collapse of the U.S. Highway 35 bridge between Point Pleasant, WV, and Kanauga, OH. Forty-six people died when that structure suddenly fell into the Ohio River. Investigators determined the cause of this disaster had begun decades earlier with a small crack that formed during the casting of the bridge’s I-beams. The I-bar subsequently broke under the compounding stresses of a corrosive environment and newer, heavier vehicles crossing the bridge.
According to Michael Harkin, an NACE and SSPC coating inspector and president of FEO Inc. (feoinc.com, Virginia Beach, VA), understanding how to prevent pitting corrosion goes a long way to ensuring long, safe, and useful service for metal assets exposed to the elements. He offers the following insight into the problem and approaches for combating it.
— Jane Alexander, Managing Editor
There’s more to the pits indicative of a pitting corrosion attack than meets the eye. Far more damage is done beneath the metal surface because the corrosion bores inward. Pitting corrosion causes the loss of metal thickness, translating to a loss of structural integrity that 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. The attack could go unnoticed but, over time, the metal fatigue it causes could lead to formation of cracks. Cracks can quickly lead to beam failure and set off a catastrophic chain reaction as unplanned stresses multiply.
There are several causes of pitting corrosion, including:
• localized mechanical or chemical damage to a metal’s protective oxide film
• improper application of corrosion-control products
• presence of non-metal materials on the surface of a metal.
When metals aren’t properly treated and freely exposed to the elements, chemical reactions between them and the environment form compounds such as ferrous oxide, more commonly known as rust.
Preventing pitting corrosion starts early, beginning with the choice of the right metal during the design of an asset. The risk of pitting corrosion is greatly reduced when users know ahead of time how materials react in different environments. Higher-alloy metals resist corrosion more strongly than low-alloy materials.
Next, to the extent that it’s possible, control the operating environment. For indoor or sheltered assets, keeping environmental factors such as temperature, pH, and chloride concentration in check minimizes the risk of pitting corrosion.
Finally, apply the proper industrial coating to your assets and have them inspected with non-destructive testing (NDT) methods. MT
Notes on Non-Destructive Testing (NDT)
According FEO’s Michael Harkin, non-destructive testing is the only legitimate option for inspecting coatings systems that are already in service (and intended to be kept in service). NDT is a subset of non-invasive procedures that don’t compromise the integrity of a tested system or material. As applied to coatings, these procedures can include using electromagnetic waves to gauge the thickness of a coating, infrared thermography to measure heat distribution and determine how well a coating is binding to its substrate, or lasers to measure surface profile without physically contacting the substrate.
FEO Inc., Virginia Beach, VA, is a QP5-certified coating inspection and consulting company. For more information, visit feoinc.com.