Today, with the continuous growth of energy demand, the development of high-sulfur Gas Fields (Sour Gas Fields) has become an important part of the global energy strategy. However, such gas fields contain large amounts of hydrogen sulfide (H₂S), carbon dioxide (CO₂), and other acidic components, posing a severe challenge to the collection, transportation, and processing equipment. Among them, hydrogen sulfide corrosion is one of the most significant risks leading to pipeline failure. It not only may cause huge economic losses but also may trigger catastrophic safety accidents.

H₂S corrosion mainly manifests in the following forms: hydrogen-induced cracking (HIC), sulfide stress corrosion cracking (SSCC), and electrochemical weight loss corrosion. The hydrogen atoms generated by the reaction between the wet H₂S environment and the surface of the steel pipe penetrate into the steel, accumulate and combine at inclusions or defects to form hydrogen molecules, generating huge pressure and causing internal cracks (HIC) to appear. Under tensile stress, these microcracks are highly prone to propagate into macroscopic cracks (SSCC), causing sudden brittle fracture of the pipeline. Such failures often occur without warning and are extremely harmful.

Therefore, in a high-sulfur natural gas environment, choosing anti-corrosion pipeline steel pipes is by no means a simple material selection, but a systematic project involving materials science, corrosion engineering, coating technology and quality control. This article will systematically expound on how to build a safe and reliable acid gas transportation pipeline through a comprehensive solution.

The first line of defense: Material selection that meets strict standards

Material selection is the cornerstone for resisting H₂S corrosion. The core principle is to select steel specifically designed for acidic environments, and the key lies in strict chemical composition control and advanced production processes.

The core position of the NACE MR0175/ISO 15156 standard

The international authoritative standard NACE MR0175 (now merged with ISO 15156) is the ultimate guideline for selecting materials for acidic environments. This standard not only specifies the chemical composition, mechanical properties and heat treatment process of steel, but also elaborates on the applicability of the material under specific H₂S partial pressure, pH value and chloride content environments.

Control of key chemical components

· Sulfur (S) content: It must be extremely low. Sulfur forms non-metallic inclusions such as MnS in steel, and these inclusions are the main source of HIC initiation. It is usually required that the S content be ≤ 0.002%.

· Phosphorus (P) content: It also needs to be strictly controlled as it will intensify the central segregation of steel, reduce toughness, and promote corrosion. The P content is required to be ≤ 0.015%.

· Calcium (Ca) treatment: Through the calcium treatment process, long strip-shaped MnS inclusions can be transformed into spherical CaS inclusions, thereby significantly reducing HIC sensitivity.

· Copper (Cu) content: An appropriate amount of Cu (0.2% – 0.3%) can form a protective oxide film on the surface of steel, enhancing its resistance to atmospheric corrosion. However, its impact on HIC needs to be comprehensively evaluated.

Table: Typical Chemical Composition Requirements for Pipeline Steel Used in Acidic Service Environments (Examples)：

 Element  Standard Requirements (Typical Values, wt%)  Functions and Control Reasons
 C  ≤ 0.06% Reduce carbon equivalent, improve weldability and resistance to SSC
  Mn   ≤ 1.30% Excessive content will aggravate central segregation and needs to be controlled together with carbon content
 S  ≤ 0.002% Extremely low requirement to reduce MnS inclusions and prevent HIC initiation
 P  ≤ 0.015%  Reduce cold brittleness and segregation
 Ca  in moderation  Control inclusion morphology and transform type II sulfides into spherical ones
O  ≤ 0.002%  Reduce oxide inclusions and improve steel purity

Microstructure and mechanical properties

Apart from the composition, a uniform and fine microstructure is the key to resisting HIC and SSCC. The acicular ferritic pipeline steel produced by the thermal mechanical control process (TMCP) has excellent comprehensive performance. Its high strength, high toughness and uniform microstructure can effectively prevent the initiation and propagation of cracks.

· Special attention should be paid to mechanical properties:

Hardness control: NACE MR0175 has a definite upper limit for the hardness of weld seams and heat-affected zones (HAZ) (typically HRC 22). The higher the hardness, the more likely SSCC is to occur.

· Yield strength: Although high-strength steel can thin the pipe wall, in an ultra-high H₂S environment, the conventional custom yield strength should not exceed 80ksi (550MPa) to ensure sufficient resistance to SSC.

The second line of defense: Advanced coating system

Even if the best pipe material is selected, a high-performance external anti-corrosion coating is indispensable. As a physical barrier, it isolates the steel pipe from moist, chloride-rich soil or Marine environments, which is the key to extending the pipeline’s lifespan and ensuring its integrity.

Coating selection: 3LPE vs. 3LPP

In harsh environments, three-layer polyethylene (3LPE) and three-layer polypropylene (3LPP) are currently the mainstream and most technologically mature coating systems.

· Three-layer polyethylene (3LPE)

Structure: Epoxy powder (FBE) bottom layer + polymer adhesive middle layer + polyethylene (PE) top layer.

Advantages: Excellent mechanical properties, cathodic disbondment resistance, and water resistance. The technology is mature and widely applied.

Limitation: The upper limit of temperature resistance of PE is usually 70°C. For high-temperature conveying conditions, its performance will decline.

· Three-layer polypropylene (3LPP)

Structure: Epoxy powder (FBE) bottom layer + polymer adhesive middle layer + polypropylene (PP) top layer.

Advantages: Outstanding high-temperature resistance (up to 125°C), higher mechanical strength, and better impact and wear resistance. It is highly suitable for high-temperature gathering and transportation pipelines, submarine pipelines or pipe sections that require directional drilling to pass through.

Cost: Usually higher than 3LPE.

Performance Indicators 3LPE 3LPP
Maximum Operating Temperature 60 – 70 °C 100 – 125 °C
Impact Resistance Good Excellent
Adhesion Excellent Good Good
Flexibility Excellent Excellent
Chemical Resistance Good Good
Applicable Environments Excellent Excellent
Selection Suggestion Most land and submarine environments high temperature, rocky areas, directional drilling

Selection suggestion: For high-sulfur natural gas fields, especially high-temperature flowlines near wellheads, 3LPP is often a better choice. Its higher temperature resistance and mechanical damage resistance provide additional guarantees for the long-term safe operation of pipelines.

3lpp anti corrosion coated pipes for sour gas.


Our comprehensive solution for acidic service environments

The selection of anti-corrosion pipeline steel pipes is not an isolated decision, but a system that runs through the entire process from design, procurement, manufacturing, inspection to installation. The core of our solution is the three-in-one guarantee:

1. Strict control over raw materials and manufacturing

We start from the source and cooperate with top steel mills that comply with API 5L and NACE MR0175/ISO 15156 standards to ensure that the chemical composition and microstructure of the base material fully meet the standards. The steel pipe is formed by advanced JCOE/UOE forming process and double-wire/multi-wire submerged arc welding (SAW) technology, and is combined with online heat treatment to ensure that the performance of the weld seam and heat-affected zone (especially hardness) meets the requirements of acidic environment.

2. High-performance composite protection system

We recommend a composite protection solution of “anti-HIC /SSCC substrate + 3LPP (or 3LPE) outer coating + matching inner coating”.

Inner coating: Epoxy resin inner coating can be selected based on the gas quality conditions (such as containing CO₂) to reduce the contact of corrosive media on the inner wall and lower the frictional resistance of fluid transportation.

Cathodic protection: Designed in conjunction with the coating system as the final protective barrier.

3. Rigorous non-destructive testing and quality verification

Quality is designed and manufactured, and even more so, inspected. We carry out strict inspections far beyond the conventional projects:

HIC test: In accordance with the NACE TM0284 standard, A 96-hour immersion test was conducted in the solution (Solution A) simulating the service environment. Subsequently, the crack sensitivity rate (CLR), crack length rate (CTR), and crack thickness rate (CSR) were evaluated through ultrasonic scanning detection.

SSC test: In accordance with the NACE TM0177 standard, the four-point bending method or tensile test method was adopted to continuously load stress (usually 80% AYS) in saturated H₂S solution for 720 hours to verify its anti-SSCC performance.

Comprehensive UT/RT inspection: 100% ultrasonic and X-ray inspection is carried out on the pipe body and butt welds to ensure there are no defects at all.

Coating inspection: including holiday detection (leak detection), cathodic disbondment test, impact resistance test, etc.

A partner ensuring safety in harsh environments

The development of high-sulfur natural gas fields is a high-tech, high-risk and high-investment systematic project. Among them, the safety of the conveying pipeline is the lifeline of the entire project’s success. Simply choosing “high-grade” steel is not sufficient to address the unique challenges brought by H₂S.

The secret to success lies in the details: meticulous calculation of the percentage of chemical components, precise control of production process parameters, strict quality control of every millimeter of coating, and almost exacting non-destructive testing of every meter of steel pipe.

We are well aware that what we offer is not merely sections of steel pipes, but a weighty commitment to safety. As a long-term partner dedicated to providing harsh working condition solutions for the oil and gas industry, we, with our profound understanding of the standard system, strategic cooperation with top suppliers, and our own strong technical quality and project management capabilities, are able to offer customers comprehensive services ranging from technical consultation, material selection, pipe supply to on-site technical support.

Choosing us means you have chosen a reliable partner with the goal of zero failure. Let’s join hands to minimize the development risks of high-sulfur natural gas fields and jointly ensure the safety, efficiency and long-term effectiveness of energy transmission.