High temperature sulfidation is probably the most well-known corrosion mechanism in the oil refining industry because it occurs in large sections of the refinery. Sulfidation corrosion (also known as sulfidic corrosion) is a result of naturally occurring sulfur (S) 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 500F (260C). 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.
Sulfidation corrosion results in the thinning of the pressure containment envelope, affecting such components as piping and pipe fittings, heater tubes, and pressure vessels. Most industry incidents have occurred in piping, due to lower nominal wall thicknesses as compared to other equipment types. Sulfidation corrosion can be localized or general in nature for a given component, though 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 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.
According to the American Petroleum Institute (API) Recommended Practice 939-C (Guidelines for Avoiding Sulfidation Corrosion Failures in Oil Refineries), one-third of high-temperature sulfidic corrosion failures are the result of low silicon content in piping. Moreover, older pipe with low Si may corrode at rates 2 to 10 times faster than surrounding higher Si piping. Recent industry experience indicates that carbon steel components (primarily pipe) and their respective welds with low-Si (< 0.10 wt. %) content can corrode at an accelerated rate when exposed to H2-free sulfidation corrosion conditions. Piping welds, where the root passes were made with low-Si containing filler metals, have been shown to corrode at a greater rate than the base metal components when exposed to the same process conditions.