Common Types of Corrosion
The nation’s crumbling transportation infrastructure, and the lack of funding it receives, is making headlines on a daily basis these days. While there are many causes for collapsed bridges, potholed highways and inefficient railway networks, one natural phenomenon stands out as a threat to nearly every sector that falls under the umbrella of “America’s transportation infrastructure problem.” The National Association of Corrosion Engineers (NACE) estimates that corrosion costs the U.S. economy $276 billion annually.
More than just something unpleasant to look at, corrosion represents a serious drag on the economy and a danger to human health and safety. It is a natural, albeit preventable, process that’s spawned an entire industry dedicated to battling it. Corrosion services have become an important part of maintaining steel and iron assets.
Broadly, corrosion is defined as the naturally occurring degradation of a surface (usually a metal), when exposed to the atmosphere. A more relevant definition for our purposes concerns what happens to iron, a major component of steel and the most commonly used alloy in infrastructure, when it is exposed to the elements.
When iron is exposed to oxygen and moisture, an electrochemical process known as oxidation occurs. Oxidation results in iron oxide, or rust, forming on the metal’s surface and corrosion begins to set in. As long as steel remains an integral component in the makeup of the country’s transportation infrastructure, corrosion is not a problem that will disappear anytime soon.
Generalized Corrosion and Localized Corrosion
The many types of corrosion that can affect an asset add to the difficulty of protecting against it. Generally speaking, there are two, broad categories of corrosion: generalized and localized. As their names suggest, generalized corrosion attacks the entire surface area of an asset, while localized corrosion is limited to certain, often irregularly shaped, areas of a particular asset.
Because generalized corrosion is predictable, treatable and fairly easy to detect, it’s often seen as the less dangerous of the two types, assuming prevention methods are in place initially. Localized corrosion, on the other hand, can be more difficult to detect and is more likely to occur even after an asset has apparently been protected. Here are the most common localized corrosion types, according to NACE:
- Pitting corrosion– Pitting corrosion is the result of localized failures in a coating system. At these points of failure, small holes begin to form and increase in size if the problem is not addressed. Because pitting corrosion is more difficult to spot, and often occurs on assets that owners consider adequately protected, pitting corrosion is much more likely to progress to the point of seriously degrading the integrity of an asset, making it a far more dangerous type of corrosion.
- Crevice corrosion– Also sometimes called “contact” corrosion, crevice corrosion occurs in those micro-spaces where two different materials overlap or otherwise touch one another. This could be a metal-on-metal or metal on a non-metal point of contact, but it usually occurs around bolts, gaskets, washers, clamps or other fastening devices that form small spaces where corrosion process can begin.
- Filiform corrosion– Filiform corrosion occurs when moisture is allowed to penetrate the small gap between a coating and the substrate, usually at a natural edge on the substrate or at a defect in the coating system. This type of localized corrosion is often distinguishable by bubbles forming beneath the coating.
The most successful method for controlling these types of corrosion involves the one-two-punch of an effective prevention strategy and diligent upkeep of that means of prevention.