Subscribe to our newsletter to receive the latest news and events from TWI:

Subscribe >
Skip to content

Performance of Sweet-Rated Steel in Souring Environment

Project Code: 33553


  • Define the transition point from CO2 to CO2/H2S conditions based on scaling properties.
  • Define transition (CO2 to CO2/H2S) severity domains as tool of risk assessment for H2S-related degradation mechanisms on sweet rated material.
  • Investigate the effect of transition from CO2 to CO2/H2S on the surface of sweet-rated carbon steel and indicate the degree of risk for H2S-related degradation mechanisms.

Project Outline

When existing sweet wells turn into sour, the risk of sweet-rated material suffering degradation (i.e. cracking) may increase, depending on the established pre-sour, sour transition, and post-sour conditions. Previous work examined the behaviour of carbon steel upon transition from pure sweet (60 days) to sour (1% H2S in CO2 for 120 days) conditions at 40°C. It was shown that no steady-state conditions were achieved during the pure sweet exposure (no protective carbonate scales were formed), which did not counteract the upset in the test environment or inhibit the aggressive action of the H2S on the material. In the event of souring, especially where non–sour rated material is used, the risk of introducing hydrogen and inducing hydrogen-based cracking in the material could be significantly increased.

This project will constitute a first step towards eventually setting more realistic transition criteria from sweet to sour, adapted for each different situation. In particular, it involves the continuous long-term corrosion monitoring of small-scale samples of parent carbon steel material, using advanced electrochemical equipment. Long-term testing is highly desirable for industry. TWI has an extended history of undertaking very long term testing of materials in aggressive environments while maintaining excellent monitoring and control of environmental variables. This specific testing includes the variation in only two factors: the exposure period for each condition (i.e. ‘sweet period’ and ‘sour period’) and the test pressure.


Relevant Industry Sectors

Benefits to Industry

Transition severity domains will be set as a tool of risk assessment for H2S-related degradation mechanisms on sweet rated material. New capabilities and expertise in performing in situ corrosion monitoring at high partial pressure of H2S will allow efficient and reliable assistance to industry in ensuring the safe operation of pipelines.


Research Board Portal

For more information please email: