Sulfur resistance working condition

Sulfur resistance working condition

Due to the acidic working environment due to oil production, very strict requirements are placed on the metal materials of the valves in order to prevent the occurrence of hydrogen sulfide cracks. It is therefore necessary to analyze the standards relating to metallic materials for anti-vulcanization stress cracking issued by the American Association of Anti-Corrosion Engineering (NACE), commonly used in the valve industry.

Commonly used criteria

Sulfidic stress cracking (SSC) is the cracking of metals under the combined action of tensile stress and corrosion in the presence of water and H2S. In simple terms SSC is: a form of hydrogen stress cracking caused by the absorption of atomic hydrogen

produced by the corrosion reactions of sulfur on the metal surface.

The appearance of SSC is mainly related to the strength of the material (in terms of hardness): it is believed that materials with high hardness have an increased susceptibility to SSC cracking.

Valves with anti-sulfur requirements are the standards:

• NACEMR0175
• NACEMR0103

The NACEMR0103 standard proposes the maximum or minimum chemical composition requirements for austenitic stainless steels and nickel alloys and the ferrite volume content for duplex stainless steels. However, these requirements are not as detailed in NACEMR0175. 

The Organization for Standardization (ISO) has synchronized the NACEMR0175 standard as an ISO standard and divided it into three standards, NACEMR0175 / ISO15156.1-3, according to the selection principle and different materials, with minor additions to its content. In the sulfur-resistant environment, the partial pressure of H2S in the wet gas phase (with water as the liquid medium) as gas, condensate and crude acid is equal to or greater than 0.0003 MPa (0.05 psi). 

The scope of the NACE standard is generally exposed to oil and gas field drilling equipment in an acidic environment, i.e. in the working conditions of hydrocarbon media containing hydrogen sulfide (H2S) such as oil extraction and transportation of natural gas. Long-term exposure to an acidic environment causes the breakdown and damage of unselected and treated metal materials. Therefore, the NACEMR0175 standard proposes a number of metallic material grades, heat treatment methods and mechanical property requirements including hardness to prevent SSC. In use, SSC is influenced by many factors. Chemical composition, strength, hardness, heat treatment and microstructure, local differences of materials.