High-Temperature Oxidation and Sulphidation Resistance: Comparing P5, P9, P11, and P22 for Refinery Service
In high-temperature refinery service — hydrotreater furnace outlets, hydrocracker hot separators, atmospheric and vacuum distillation transfer lines — two distinct degradation mechanisms determine material selection: oxidation (the reaction of steel with oxygen at high temperature) and sulphidation (the reaction of steel with hydrogen sulphide or sulphur vapour). These two mechanisms have different chemistry and different Cr dependency. Understanding both is necessary to select the right CrMo grade for a given refinery service temperature and sulphur content — and to avoid the common error of specifying P11 in a location where P5 or P9 is required for sulphidation resistance.
Oxidation Resistance — The Cr₂O₃ Mechanism
At elevated temperature in oxygen-bearing atmosphere, iron oxidises to form FeO, Fe₃O₄, and Fe₂O₃ scales. Chromium in the alloy preferentially diffuses to the surface and forms a thin, adherent Cr₂O₃ layer that limits further oxygen ingress — the higher the Cr content, the more protective the scale. As a rough guide: 1–1.25% Cr (P11) provides useful oxidation protection to ~590°C; 2.25% Cr (P22) to ~650°C; 5% Cr (P5) to ~700°C; 9% Cr (P9, P91) to ~750°C. Above these limits, excessive scaling occurs and wall thinning accelerates rapidly.
Sulphidation Resistance — The McConomy Curves
Sulphidation is attack by H₂S or sulphur vapour — the standard reference is the API RP 939-C McConomy curves, which plot corrosion rate in mils per year against temperature for each steel family. The key finding: carbon steel corrodes rapidly in high-H₂S atmospheres above 260°C; 5% Cr alloy (P5) provides approximately 10× better sulphidation resistance than carbon steel at the same temperature; 9% Cr (P9) provides approximately 100×. The sulphidation driving force increases with H₂S partial pressure — vacuum column bottoms at 0.5 ppm H₂S behaves very differently from coker fractionator outlets at 5,000 ppm H₂S.
Grade Selection Summary for Refinery High-Temperature Service
| Service | Temperature | H₂S Level | Fitting Grade |
|---|---|---|---|
| Boiler steam / utility steam | 400–530°C | None | P11 |
| HDS reactor outlet / hydrotreater | 350–450°C | High (H₂ + H₂S) | P11 (Nelson curve check required) |
| Vacuum distillation transfer line | 350–420°C | Moderate | P5 (sulphidation) or P11 (economics) |
| Coker fractionator overhead | 450–540°C | Very high | P5 minimum; P9 for high H₂S >400 ppm |
| Catalytic reformer heater outlet | 500–560°C | Low (clean H₂) | P22 — Nelson curve for H₂ partial pressure |