What is the NACE MR0175 Standard and the requirements for steel tubes resistant to SSC?

What is the Standard NACE MR0175?

During oil and gas extraction and processing, hydrogen sulfide (H₂S) is a common and highly destructive gas. It can not only cause personnel poisoning but also lead to catastrophic corrosion cracking of metal equipment, triggering safety incidents and significant economic losses.

NACE MR0175 specifies which materials can be used in hydrogen sulfide-containing environments in the oil and gas industry to prevent materials from cracking due to specific types of corrosion, thereby ensuring the safety of equipment and personnel.

What is the Relationship Between NACE MR0175 and ISO 15156?

In 2003, NACE collaborated with the International Organization for Standardization (ISO) to harmonize the content of the NACE MR0175 and ISO 15156 standards. Now, these two standard names refer to the same set of technical requirements. They are commonly referred to together as NACE MR0175/ISO 15156.

Why is H₂S So Dangerous?

The root cause of metal corrosion cracking in H₂S environments is actually hydrogen atoms.

①Electrochemical Reaction:

Metal undergoes an electrochemical corrosion reaction with H₂S in an aqueous environment. This reaction itself causes slow dissolution of the metal (general corrosion).

②Hydrogen Atom Penetration:

This reaction inevitably produces hydrogen atoms (H). These atoms are very small. Under normal circumstances, they would combine on the metal surface to form hydrogen molecules (H₂), which then bubble away from the surface.

However, H₂S strongly adsorbs onto the metal surface, hindering the combination of hydrogen atoms into hydrogen molecules. This leads to a large number of hydrogen atoms penetrating into the metals interior.

③Hydrogen Accumulation:

These tiny hydrogen atoms accumulate at microscopic defects within the metal (such as grain boundaries, inclusions) and combine to form hydrogen molecules (H₂).

④Internal Pressure:

Hydrogen molecules generate enormous pressure within these confined spaces. When this pressure exceeds the yield strength of the metal, it can cause the metal to form blisters (HIC) or undergo brittle cracking (SSC) at areas of stress concentration (such as welds, notches).

Which Materials Are Suitable for Use in H₂S-Containing Environments?

Carbon & Low-Alloy Steels: Primarily ensure resistance to SSC by controlling their hardness (typically required to be ≤ 22 HRC) and chemical composition.

Corrosion-Resistant Alloys (CRAs): Including austenitic stainless steels, duplex stainless steels, nickel-based alloys, titanium alloys, etc. These materials rely mainly on their specific alloy composition and microstructure to ensure corrosion resistance.

Evaluation of Steel Pipe Resistance to H₂S Corrosion per NACE MR0175/ISO 15156

The evaluation of a steel pipe materials ability to resist H₂S corrosion (primarily various forms of cracking) according to the NACE MR0175/ISO 15156 standard can be analyzed from the following aspects:

①Mandatory Requirements for Material Condition

Steel pipe materials must meet key parameter limits for use in H₂S environments to comply with NACE MR0175:

Chemical Element Control

Hardness Limit

Brinell Hardness HBW ≤ 237 (approx. Rockwell C Hardness HRC ≤ 22, Vickers Hardness HV ≤ 248).

Note: Can be relaxed to HRC 26 for low H₂S partial pressure (<0.3 kPa) or high pH environments, but requires verification by SSC testing.

Heat Treatment Condition

Normalized (N), Normalized & Tempered (N&T), or Quenched & Tempered (Q&T) to relieve residual stresses and homogenize the microstructure.

Service Environment Adaptability

Sour Service Threshold: H₂S partial pressure ≥ 0.0003 MPa (0.05 psi).

pH Value: ≥ 3.5 (Corrosion-Resistant Alloys may be needed for lower pH).

Temperature Effect: Carbon steel best SSC resistance range: ambient to 65°C;

Austenitic stainless steels require attention to Chloride Stress Corrosion Cracking above ~80°C.

Welding

Filler Metal: Matches base metal composition (low-hydrogen electrodes).

Weld & HAZ Hardness: ≤ HRC 22.

Post Weld Heat Treatment (PWHT): Temperature ≥ 620°C.

Cold Working

Deformation rate ≤ 5%; otherwise, annealing is required.

Non-Destructive Testing (NDT)

• Ultrasonic Testing (UT): According to SA-578 Level B/C.
• Magnetic Particle Testing (MT): To detect surface defects.

②Testing

If testing is required, it primarily evaluates the materials resistance to Sulfide Stress Cracking (SSC) and Hydrogen-Induced Cracking (HIC).

Sulfide Stress Cracking (SSC) Test:

Reference Standard: NACE TM0177 "Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments".

Methods: Four-Point Bend (FPB) or C-Ring tests, simulating stress of 90% of Actual Yield Strength (AYS) in the standard test environment.

Hydrogen-Induced Cracking (HIC) Test:

Reference Standard: NACE TM0284  "Standard Test Method - Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking".

Procedure: Immersion in standard solution for 96 hours.

Evaluation: Assess Crack Length Ratio (CLR), Crack Thickness Ratio (CTR), Crack Sensitivity Ratio (CSR).

Typical Acceptance Criteria: e.g., CLR ≤ 15%, CTR ≤ 5%, CSR ≤ 2%, or more stringent requirements may apply.