Hydrogen sulfide (H₂S) is highly corrosive to metals in humid environments, primarily causing sulfide stress cracking (SSC) and stress corrosion cracking (SCC). In wet H₂S environments, atomic hydrogen generated during corrosion reactions may diffuse into the metal and cause material embrittlement. Under tensile stress, brittle fracture may occur even when the applied stress level is far below the material’s yield strength. This presents a major risk for high-strength stainless steels used in sour oil and gas service conditions.

NACE standards, developed by the National Association of Corrosion Engineers (NACE), provide unified technical guidelines and specifications for preventing and controlling material corrosion in industrial applications. Among these standards, the following two are particularly important in the oil and gas industry and serve as the primary basis for material selection in sour service environments containing hydrogen sulfide (H₂S).

• NACE MR0175 / ISO 15156: The most stringent and widely adopted standard for oil and gas production, primarily applicable to upstream wellhead equipment and production facilities.

   

• NACE MR0103 / ISO 17945: Developed specifically for downstream processing environments such as refineries and petrochemical plants. Unlike upstream extraction environments, H₂S concentrations in refinery processes are relatively lower, but the risk of material cracking still exists.

FITOK offers tube fittings made from materials compliant with NACE standards, suitable for sour service. Available materials include 316 stainless steel, 6Mo alloys, super duplex 2507 stainless steel, Alloy 625, Alloy 825, and Alloy C-276. Among these, super duplex 2507 tube fittings feature several unique characteristics in both material selection and manufacturing processes, which are elaborated in detail below.

Figure 1 Structure of Super Duplex 2507 Tube Fitting

Based on the structure of the double ferrule tube fitting (Figure 1), a complete assembly generally consists of four parts: the fitting body, front ferrule, rear ferrule, and nut. After the the double ferrule tube fitting is properly assembled with the tubing, the front ferrule forms two sealing surfaces with the fitting body and the tubing respectively, and its front end is in direct contact with the process medium. Relying on the sealing and isolation of the front ferrule, the rear ferrule and nut are not exposed to the system medium; the rear ferrule mainly clamps the tubing and provides structural support for the front ferrule.

Table 1 – Materials of Super Duplex 2507 Tube Fitting Components

The material of each component of the 2507 super duplex stainless steel double ferrule tube fitting is shown in Table 1. The rear ferrule is made of 6Mo alloy, and the nut is machined from 2507 super duplex stainless steel bar stock.

Neither component comes into contact with the process medium. And NACE MR0175 only imposes material restrictions on components exposed to hydrogen sulfide sour environments. Therefore, when only the inside of the pipeline is in contact with sour media, the material selection for the rear ferrule and nut is not subject to the relevant requirements of NACE MR0175.

Table 2 – Reference to NACE MR0175:2015 Table A.24: Environmental and materials limits for duplex stainless steel used for any equipment or component

For FITOK 2507 super duplex stainless steel double ferrule tube fittings, both the body and front ferrule are wetted parts.

The fitting body is manufactured from solution-annealed 2507 super duplex stainless steel, fully complying with Table A.24 (Table 2) of the NACE MR0175 standard, and suitable for a wide range of equipment and service conditions.

In contrast, the front ferrule is fabricated from cold-drawn bar stock. This often raises customer questions: Is it non-compliant with NACE MR0175 for the medium-wetted front ferrule to be made from cold-drawn material?

The fundamental reason why NACE MR0175 requires wetted components in sulfide-containing sour service to use annealed 2507 super duplex stainless steel is as follows:

Cold-worked materials feature high residual stress and high dislocation density, making them more susceptible to stress corrosion cracking compared with annealed materials.

However, according to the applicable equipment list specified in NACE MR0175 (Table 3), components subjected only to compressive stress are not restricted by this standard.

Failure modes such as stress corrosion cracking (SCC) and sulfide stress cracking (SSC) require tensile stress within the component as a prerequisite.

Under actual assembly and service loading conditions, only the front end of the front ferrule is exposed to the process medium in the two-ferrule structure, and this contact area remains under compressive stress with no tensile stress present.

Accordingly, the front ferrule poses no risk of stress corrosion cracking or sour gas cracking, and is exempt from the annealed material requirements specified in NACE MR0175.

Table 3 – Reference to NACE MR0175:2015 Table 1: List of equipment for which exceptions are permitted

On the other hand, to ensure that the 2507 super duplex stainless steel double ferrule tube fitting delivers sufficient structural strength and surface hardness when clamping tubing of the same material, the front ferrule must be manufactured from cold-drawn bar stock. Using annealed material would fail to achieve reliable tubing clamping and long-term sealing performance.

The FITOK 2507 super duplex stainless steel double ferrule tube fitting adopts a differentiated material selection structure, with the back ferrule made of 6Mo alloy.

6Mo alloy delivers stable mechanical properties and offers corrosion resistance comparable to 2507 super duplex stainless steel. The two materials have a low potential difference, resulting in minimal risk of galvanic corrosion. When paired with other 2507 super duplex stainless steel components, it achieves more stable mechanical clamping and locking performance.

In summary, provided the exterior of the fitting is not exposed to sour environments and only the internal flow path is in contact with process media, the FITOK 2507 super duplex stainless steel double ferrule tube fitting with a cold-drawn front ferrule can still be safely applied in sulfide-containing sour service conditions.

In addition to tube fittings compliant with NACE MR0175 / ISO 15156, FITOK also offers a wide range of sour service products compliant with NACE MR0175 / ISO 15156, including S type medium and high pressure fittings 20M and 60H series, pipe fittings PMH series, adapter fittings AMH series, and matching tubing T20M and T60H series. Moreover, type S valves compliant with NACE MR0175 / ISO 15156 are also available, including ball valves, needle valves, relief valves, gauge valves, and check valves.

Should you have any questions or require additional information, please contact FITOK or your local authorized distributor. We will reach you as soon as possible!