Hydrogen Free Sulfidation Corrosion in High Temperature Environments

Corrosion is a pervasive challenge in the oil and gas industry, particularly in high-temperature environments where hydrogen-free sulfidation corrosion can wreak havoc on equipment integrity. To address this issue and ensure the safe and reliable operation of facilities, stringent guidelines and material selection strategies are required. In this article, we explore the intricacies of managing hydrogen-free sulfidation corrosion, with a particular focus on piping and vessel materials selection.

Menace of Hydrogen-Free Sulfidation Corrosion

Hydrogen-free sulfidation corrosion, though less discussed than its hydrogen sulfide counterpart, is a formidable adversary in the oil and gas industry. This form of corrosion occurs in environments where there is a lack of hydrogen sulfide (H2S) but an abundance of sulfur-containing compounds. When exposed to high temperatures exceeding 450°F, susceptible materials can degrade rapidly, posing significant safety and operational risks.

Piping Materials Selection

1. Materials Selection Criteria

Choosing the right materials is paramount when dealing with hydrogen-free sulfidation environments. This selection is guided by modified McConomy Curves, which offer a graphical representation of material compatibility in such conditions. The key considerations include:

a. Sulfur Content Reporting

In hydrogen-free hydrocarbon streams operating above 450°F, it’s imperative to report the weight percent of sulfur. Additionally, downstream of pressure letdown valves in hydroprocessing units, both weight percent of sulfur in the liquid phase and H2S mole percent in the vapor phase must be reported.

b. Corrosion Rate Estimation

The selection of materials downstream of pressure letdown valves involves estimating corrosion rates using both modified McConomy Curves and Couper Gorman Curves. The higher corrosion rate should dictate material selection. For piping located immediately downstream (within 10 times the pipe diameter) of pressure letdown valves, materials may require upgrading or extra corrosion allowance.

c. Addressing High Corrosion Rates

For systems like Product Stripper and Main Fractionator bottom reboilers in hydroprocessing units, where high corrosion rates are anticipated, Type 347 stainless steel must be used for fired heater tube metal temperatures exceeding 700°F. For reboiler/fired heater inlet and outlet piping, a minimum metallurgy of 9Cr-1Mo steel is mandated.

d. Special Consideration for Overhead Vapor Streams

Materials for hydrogen-free overhead vapor streams containing sulfur, such as coke drum overhead vapor lines and FCCU reactor overhead vapor lines, should be proposed by the licensor/designer and approved accordingly.

2. Materials to Avoid

To prevent corrosion in hydrogen-free sulfidation environments, several materials should be avoided:

a. 1 1/4Cr-1/2Mo and 2 1/4Cr-1Mo

These materials should not be used for corrosion control in such environments due to their susceptibility to degradation.

b. 5Cr-1/2Mo

The use of 5Cr-1/2Mo should be minimized across refinery applications. A judicious business decision should be made during the project’s feed stage to determine if 9Cr-1Mo can replace 5Cr-1/2Mo. While the construction cost difference is not significant, avoiding the mixing of 5Cr-1/2Mo materials with 9Cr-1Mo materials is crucial to prevent sulfidation-related issues.

Vessel Materials Selection

Carbon Steel Clad with Stainless Steel

In high-temperature environments prone to hydrogen-free sulfidation corrosion, pressure vessels are particularly vulnerable. To mitigate this risk, the use of 5Cr-1/2Mo and 9Cr-1Mo materials for building pressure vessels is prohibited. Instead, a more robust approach involves employing carbon steel vessels clad with Type 405 or 410S stainless steel.

This approach offers the corrosion resistance of stainless steel while retaining the structural strength and cost-effectiveness of carbon steel. By cladding the vessel with stainless steel, operators create a robust barrier that protects against sulfidation corrosion, ensuring the long-term integrity and safety of the vessel.

Conclusion

Managing hydrogen-free sulfidation corrosion is a critical aspect of ensuring the safety and reliability of equipment and facilities in the oil and gas industry. By adhering to stringent materials selection criteria, avoiding susceptible materials, and implementing innovative strategies like cladding pressure vessels, the industry can effectively combat this pervasive threat. As technology and materials science continue to evolve, so too will the industry’s ability to protect its assets from the hidden menace of hydrogen-free sulfidation corrosion.