Corrosion Protection and Monitoring of Oilfield Pipelines Against CO₂ and H₂S
Corrosion Protection and Monitoring of Oilfield Pipelines Against CO₂ and H₂S
CO₂ /H₂S coexistence : the partial pressure ratio determines which is the dominant corrosive agent .
In actual oil and gas fields, CO₂ and H₂S almost always coexist. However, corrosion behavior is not a simple superposition of the two, but rather the partial pressure ratio determines the dominant mechanism .
| Partial pressure ratio p(H₂S ) /p( CO₂ ) | Dominant corrosion type | Key risks |
| < 0.01 | CO₂ Dominant | FeCO₃ films are unstable and have a high risk of localized corrosion . |
| 0.01~0.5 | Competition Formation | FeS and FeCO₃ compete for deposition, resulting in a complex film structure . |
| > 0.5 | H₂S dominance | The risk of hydrogen damage is rising sharply; SSC/HIC should be prioritized for prevention and control. |
Engineering Significance | Within the same block, the partial pressure ratio may vary at different wellheads and at different production stages. Corrosion prevention solutions cannot be applied uniformly and require dynamic evaluation.
02
Material selection: Temperature is a "watershed"
▍2.1 Low-temperature zone (<60℃): Carbon steel + corrosion inhibitor is an economical choice
NACE standards and domestic oil and gas field practices in western China show that in CO₂/H₂S environments below 150℃ , carbon steel and low-alloy steel combined with corrosion inhibitors can effectively control corrosion. However, there are two prerequisites:
• Uniform corrosion is predominant, with localized corrosion ("spot corrosion" and "mesa corrosion") not being significant;
• Corrosion inhibitors are continuously added, and their concentration and compatibility have been verified on-site.
Once localized corrosion becomes significant and corrosion inhibitors are insufficient to cover the damage, the materials must be upgraded.
▍2.2 Medium and High Temperature Zone (60~150℃): Corrosion-resistant alloys come into view
| Material | Applicable Environment | Limitations |
| 316L stainless steel | Contains CO₂ , low H₂S | H₂S partial pressure reduces pitting corrosion resistance. |
| 2205 duplex stainless steel | CO₂ / H₂S coexistence | H₂S has limited application limits; use with caution at high temperatures . |
| 825 nickel-based alloy | High H₂S , High Temperature | High cost and strict welding process requirements |
CO₂- containing environments with temperatures above 60℃ in oil and gas fields in western China , 316L, 2205, or composite pipes are recommended; however, in high H₂S environments , 825 alloy is a more reliable choice.
▍2.3 Ultra-high temperature/high sulfur content: Nickel-based bimetallic composite pipe
For highly corrosive operating conditions (high H₂S + high CO₂ + high Cl⁻ ) , China has developed nickel-based bimetallic composite pipe technology, along with a corrosion inhibitor treatment process, which has reached the international advanced level.
03
Corrosion inhibitor systems: CO₂ and H₂S require different "prescriptions" .
▍3.1 CO₂ Environment : Film Formation Protection is Key
Imidazoline - based corrosion inhibitors are the mainstream for CO₂ corrosion , and their mechanism of action is clear:
• The N atom on the imidazoline ring forms a coordinate bond with the empty d orbital of Fe, resulting in chemisorption ;
• Long-chain alkyl groups are perpendicular to the metal surface, forming a hydrophobic protective film;
• It blocks the migration of H₂CO₃ , H⁺ , and Cl⁻ to the metal surface .
Typical data: Gemini imidazoline quaternary ammonium salts can achieve a corrosion inhibition rate of 90.74% and reduce the corrosion rate to 0.0569 mm/a in a 5% NaCl saturated CO₂ aqueous solution at 40℃ and a concentration of 100 mg/L .
▍3.2 H₂S Environment : It needs to both inhibit corrosion and prevent hydrogen contamination.
H₂S corrosion inhibitors is not "reducing the corrosion rate," but rather preventing hydrogen from entering the steel . Therefore , formulation design has two levels:
| level | Target | Common components |
| First layer: Film-forming corrosion inhibition | Reduce anodic dissolution and cathode hydrogen evolution | imidazoline derivatives, quaternary ammonium salts |
| Second layer: Hydrogen permeability barrier | Inhibit hydrogen atom recombination and reduce hydrogen penetration | Sulfur-containing organic components (such as mercaptoethanol), iodides |
Typical formulation: five-fingered domidazoline quaternary ammonium salt (10~30%) + OP-10 (1~10%) + mercaptoethanol (1~10%) + thiourea (1~10%). The multi-quaternary ammonium salt increases adsorption centers, the multi-hydrophobic chains improve film strength, and mercaptoethanol and thiourea synergistically inhibit hydrogen permeation.
▍3.3 Coexistence of CO₂ / H₂S : Synergistic effects are key
In a coexisting environment, a single corrosion inhibitor often proves ineffective. The industry consensus is to use a compound system .
• Imidazolin quaternary ammonium salt (main agent, film-forming corrosion inhibitor);
• Acynyloxymethyl quaternary ammonium salt (excipient, to enhance adsorption stability).
• Thiourea/iodide ( hydrogen permeation barrier );
• Surfactants (dispersing, wetting, improving film uniformity).
The combination of imidazoline benzoate and imidazoline laurate achieves a corrosion inhibition efficiency of 97.31% at a concentration of 600 mg/L.
Important Reminder | Corrosion inhibitors are not a "cure-all". Their effectiveness in the field is greatly affected by the composition of the medium, temperature, H₂S / CO₂ partial pressure, flow rate, and material. The same formulation may have vastly different effects in different gas fields. Simulation of actual operating conditions is essential for evaluation and selection before use.
04
Corrosion monitoring: From "post-disaster remediation" to "pre-disaster early warning"
▍4.1 Internal Corrosion Monitoring
| method | principle | Applicable Scenarios |
| Resistance probe method | The resistance of the metal sample increases as it thins due to corrosion. | Continuous monitoring, suitable for critical pipe sections |
| Electrochemical impedance spectroscopy (EIS) | Changes in interfacial impedance reflect the state of the film. | Laboratory evaluation and field verification |
| Ultrasonic thickness measurement | Direct measurement of wall thickness reduction | Regular inspections and fixed-point monitoring |
China has been applying spiral-shaped resistance corrosion probes and pressurized recovery devices, combined with automatic signal acquisition and remote wireless data transmission, to achieve unattended automatic monitoring.
▍4.2 External Corrosion and Leakage Monitoring
Distributed fiber optic sensing technology is a recent breakthrough; however, we are using higher-resolution flexible ER corrosion rate probes and EIS coating degradation online monitoring technology.
