Broadly speaking, corrosion can be separated into two distinct types: generalized and localized. It’s possible to further subdivide corrosion from here, but dividing a corrosion problem is a good place to start. If corrosion on a particular asset is generalized, diagnosis and treatment will be relatively straightforward. Localized corrosion, however, can be a bit trickier both to spot and to remedy.
Generalized corrosion, as its name suggests, affects the entire surface area of an asset. It can take the form of a more or less uniform loss of surface material or a generalized thinning which spans the entirety of a metal surface. It is also sometimes referred to as “uniform corrosion.”
Because generalized corrosion is predictable, treatable and easy to detect, it is widely regarded as the less treacherous of the two types of general corrosion. Metal surfaces affected by generalized corrosion, for the most part, show visible signs of being affected prior to becoming structurally compromised. Hence, maintenance normally occurs long before this type of corrosion completely ruins an asset. If left unchecked, however, uniform corrosion will eventually result in the complete degradation of an asset.
Localized corrosion, once again as its name implies, is corrosion affecting a specific location on a metal surface. Because localized corrosion usually occurs in areas not plainly visible, it is often the more difficult of the two types of corrosion to detect. It is commonly the result of a failed or improperly applied coating. Additionally, since localized corrosion often occurs in areas that are ostensibly already protected against such corrosion, asset owners often don’t even suspect they should be on guard against it, making it even more likely to evade detection.
According to NACE, the two most predominant forms of localized corrosion are pitting corrosion and crevice corrosion. Filiform corrosion is another type of localized corrosion that can become an issue given the appropriate conditions.
- Pitting corrosion– Pitting corrosion occurs when localized holes or cavities form at points of failure in a passivation corrosion control system. Compared to generalized corrosion, the relatively small areas that characterize pitting corrosion incidents make this type of corrosion more difficult to spot, and hence more likely to progress to the point of becoming a serious failure. Pitting corrosion also has a tendency to appear small and concentrated on the surface of a metal, when it is, in fact, large and widespread beneath the surface.
- Crevice corrosion– Also known as contact corrosion, crevice corrosion occurs at a point of contact between either a metal and a metal, or a metal and a non-metal. This type of corrosion typically occurs under gaskets, washers, clamps, or even between a metal and barnacles in the case of permanently and frequently submerged assets. Because the surfaces afflicted by crevice corrosion are partially shielded from exposure to the outside environment by the materials adjacent to them, this type of corrosion is also often difficult both to detect and to defend against. Often crevice corrosion is addressed during the building phase, where instances in which it might occur are deliberately engineered out.
- Filiform corrosion– Filiform corrosion occurs when moisture penetrates a protective barrier and settles between the barrier and the metal surface. The barrier could be a protective coating, metal plating or some other measure meant to provide added protection for a surface. It often results from improper surface preparation. Corrosion under insulation (CUI), a common problem in many processing facilities, is a form of filiform corrosion.
All of the above forms of corrosion are avoidable. Prevention requires the right product, the right experience, and most of all the right applicator. To find out what makes an applicator “the right one,” download the guide below.