Sulfidic Corrosion | Materials and Corrosion Control
| Damage Mechanism | Sulfidic Corrosion |
| Damage Description | · High-temperature sulfidic corrosion of carbon and low alloy steel components is a well-recognized phenomenon in petroleum refineries. Crude oils often contain from 0.5 to 5 wt.% sulfur in a variety of different sulfur compounds, but it is normally reported as total sulfur content.
· Sulfidic corrosion is primarily caused by the reactive sulfur compounds present in hydrocarbon streams. · Damage is a concern within the temperature ranges 450° and 800°F (230° – 425°C). Corrosion rates begin to decrease above 800°F. · The presence of hydrogen accelerates corrosion to the extent that it is considered separately as High-Temperature H2/H2S corrosion |
| Affected Materials | Carbon steel, low alloy Cr-Mo steel and austenitic stainless steel. |
| Control Methodology | · Principal factors are temperature, reactive sulfur content, and alloy composition. Material selection is normally based on the Modified McConomy curves, which predict uniform corrosion rates as a function of temperature for carbon steel and various Cr steels and % sulfur content. However, the actual experienced corrosion rate may depart from the predicted values by a factor of 10.
· Different sulfur-containing compounds react at varying rates, the silicon content may also affect the sulfidation, so predictions based only on total sulfur levels can be misleading. · Resistance to high-temperature sulfidic corrosion increases with increasing chromium content in the steel but 1 ¼ and 2 ¼ Cr steels offer only marginal improvement with respect to carbon steel. So, when required, the metallurgical upgrading goes directly from carbon steel to 5Cr – ½ Mo steel. · 300 Series SS such as Types 304, 316, 321 and 347 are highly resistant to this corrosion mechanism but not often necessary. |
| Monitoring Techniques
| · Verify the material selection can handle the Process conditions regarding temperatures & sulfur content.
· Verify that operating temperatures are less than or equal to the design. · OSI: thinning can be detected using external UT measurements and profile RT. · Carry out PMI programs to verify that the intended alloy has been used; rogue material has been the cause of major fires in refineries (e.g. carbon steel mistakenly installed instead of · Verify the sulfur content of the stream to make sure it has stayed within the anticipated amount. |
| Inspection Frequency | · As determined by OSI |
| KPIs | · Ensure alloy installed suffered corrosion rates below 5 MPY, and 10 MPY if fouling is a concern. |
| References | · API RP 571 (DM #1), API RP 939C, API RP 581, NACE PUBL 34103 |