Sulfidation
Sulfidation is a common high-temperature corrosion-failure mechanism. As the name implies, it is related to the presence of contamination by sulfur compounds. When examining this form of damage microscopically, a "front" of sulfidation is often seen to penetrate into the affected alloy. Localized pitting-type attack is also possible. A distinction can be made between sulfidation in gaseous environments and corrosion in the presence of salt deposits on corroding surfaces. Only the former is considered in this section; the latter is included in the section on salt and ash deposit corrosion. It is possible to divide gaseous environments associated with sulfidation into the following three categories:
- Hydrogen-hydrogen sulfide mixtures or sulfur vapor of a highly reducing nature. Hydrogen sulfide in the presence of hydrogen becomes extremely corrosive above 260 to 288 °C. Sulfidation problems may also be encountered at lower temperatures. Increased temperatures and higher hydrogen sulfide contents generally lead to higher degradation rates. Hydrogen-hydrogen sulfide mixtures are found in catalytic reformers in oil refining operations. For catalytic reforming, the 18Cr-8Ni austenitic stainless steels grades are considered to be adequately resistant to sulfidation.
- Mixed gas that contains mixtures of hydrogen, water, carbon dioxide, carbon monoxide, and hydrogen sulphide: The presence of oxidizing gases such as H2O (steam) or CO2 slow the sulfidation rate below that expected if only the H2S-H2 concentrations were considered. This can be important because gases, which are thought to contain only H2S-H2, often also contain some H2O.
- Sulfur dioxide-containing atmospheres: Atmospheres high in sulfur dioxide are encountered in sulfur furnaces, where sulfur is combusted in air for manufacturing sulfuric acid. Lower levels of sulfur dioxide are encountered in flue gases when fossil fuels contaminated with sulfur species are combusted. It has been pointed out that relatively little corrosion data exist for engineering alloys in these atmospheres. Beneficial effects (retardation of sulfidation) of chromium alloying additions and higher oxygen levels in the atmosphere have been noted.