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Sulfidation Corrosion

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Sulfidation Corrosion (also known as sulfidic corrosion) is a result of naturally occurring sulfur compounds found in crude oil. In the absence of hydrogen, corrosion due to sulfur compounds in the crude is thought to occur at temperatures above 500°F (260 °C). Up to that temperature, corrosion rates due to sulfidation are relatively low, even for carbon steels, unless there is naphthenic acid present in the crude. High temperature sulfidation is one of the most well-known corrosion mechanisms in the oil refining industry because it can occur in multiple sections of the refinery.

Sulfidation corrosion results in the thinning of the pressure containment envelope, affecting components such as piping and pipe fittings, heater tubes, and pressure vessels. Most industry incidents have occurred in piping, due to lower nominal wall thicknesses compared to other equipment types. Sulfidation can be localized or general in nature for a given component; although the majority of cases exhibit general thinning. When the damage is general and thinning occurs over a large area, ruptures are possible and can lead to the potential release of large quantities of hydrocarbon streams.

Corrosive thinning of piping walls and equipment due to sulfidation depends on many factors, such as materials of construction, sulfur content of the oil, temperature, flow rate, and H2 concentration, making it hard to predict accurately. However, one predictor of higher sulfidation rates is low silicon content in carbon steel components.

Inspection methods for finding low silicon components vary, and programs differ from stripping all insulation, to the use of radiographic methods or long range guided wave UT methods to find all welds and selectively stripping insulation. This allows access to the piping on either side of the weld, as well as the weld itself.

Of course, rather than stripping insulation, there are more practical ways to perform baseline thickness checks. For example, one could use an X-ray imager to find welds and then cut a cookie in the insulation to check the thickness of each piping segment. Once the cookie has been cut, thickness checks can be done and positive material identification (PMI) could be performed. Pulsed eddy current (PEC) is also useful NDE technique for finding thin components without stripping insulation.

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