General Sulfidation Resistance
Inconel 600 (UNS N06600) is a Ni‑Cr (≈72Ni–15Cr) austenitic alloy that forms a protective surface scale primarily composed of chromium oxide (Cr₂O₃) under oxidizing or mildly reducing conditions. This oxide layer acts as a barrier to sulfur ingress, providing good resistance to sulfidation in many industrial scenarios.
Key reasons for its performance:
Chromium reacts preferentially with sulfur to form relatively stable chromium sulfides (e.g., Cr₂S₃) and helps maintain protective scales.
Nickel has a lower affinity for sulfur than iron, reducing the formation of brittle, low‑melting‑point sulfides (a common problem in Fe‑based alloys).
The alloy's high nickel content also provides good resistance to sulfide stress corrosion cracking (SSCC) in many cases, particularly compared to carbon and low‑alloy steels.
Performance in Different Sulfur‑Containing Environments
1) High‑Temperature Sulfidation (e.g., H₂S, SO₂, elemental sulfur, sulfiding gases)
Moderate temperatures (≈500–700°C): Inconel 600 typically shows good sulfidation resistance in environments where a protective Cr₂O₃ scale can form and be maintained.
Higher temperatures (>700–800°C): Sulfidation rates increase significantly. At these temperatures, sulfur can react with chromium to form sulfides, and the protective oxide scale may become less stable or be converted to sulfide, leading to:
Accelerated metal loss
Formation of porous or non‑adherent scales
Potential for metal dusting in some sulfur‑containing, carbon‑rich atmospheres
Very aggressive sulfidizing conditions (high sulfur activity, low oxygen partial pressure, reducing atmospheres with H₂S/SO₂): The protective oxide/sulfide layer may not form effectively, resulting in poor resistance and rapid attack.
2) Corrosive Sulfur in Liquid Phases (e.g., molten sulfur, sulfide salts)
Molten sulfur: Inconel 600 can be used in molten sulfur service at moderate temperatures, but performance depends on impurities (e.g., H₂S, polysulfides) and temperature. At higher temperatures, reaction with sulfur can cause corrosion and embrittlement.
Molten sulfide salts (e.g., Na₂S, K₂S, sulfide mixtures): These environments can be highly aggressive, especially at elevated temperatures, and may lead to significant sulfidation or corrosion of the alloy.
3) Low‑Temperature Sulfur‑Containing Corrosion (e.g., H₂S in oil and gas)
Inconel 600 is generally resistant to sulfide stress corrosion cracking (SSCC) and hydrogen embrittlement (HE) in H₂S environments at moderate temperatures and pressures, particularly compared to carbon steels.
However, in severe sour service (very high H₂S partial pressure, low pH, high chloride, and tensile stress), localized corrosion or cracking risks may still exist, and careful material selection and stress management are recommended.
Factors Affecting Performance in Sulfur Environments
Temperature: Sulfidation attack increases rapidly with temperature; resistance decreases significantly above approximately 700–800°C in many cases.
Sulfur activity / partial pressure: Higher sulfur activity leads to more aggressive sulfidation.
Oxygen availability: Sufficient oxygen helps form and maintain the protective Cr₂O₃ scale; low oxygen partial pressure favors sulfide formation and poor resistance.
Gas composition: The presence of H₂, CO, CO₂, H₂O, and other species can alter the stability of oxide vs. sulfide scales and influence the rate of attack.
Alloy composition: Higher chromium content generally improves sulfidation resistance; alloys with more chromium (e.g., Inconel 601) may be better suited for severe sulfidizing conditions.
Summary
Inconel 600 provides good resistance to sulfidation and sulfur‑containing corrosion in many environments, particularly at moderate temperatures where a protective chromium oxide scale can be maintained. However, its performance degrades at high temperatures (>700–800°C) and in strongly reducing, high‑sulfur‑activity environments, where rapid sulfidation or other forms of attack may occur.
