A complete guide to hydrogen sulfide stress corrosion testing: NACE TM0177 standard + equipment selection

In the process of oil and gas extraction and transportation, hydrogen sulfide (H₂S) is one of the most common and destructive corrosive media.Sulfide stress corrosion cracking (SSC) and stress corrosion cracking (SCC) in hydrogen sulfide environments are the main causes of failure of oil well pipes, pipelines, drilling tools and other equipment.NACE TM0177 is the globally recognized standard for hydrogen sulfide environmental stress corrosion testing.This article will explain in detail the core content of the NACE TM0177 standard, introduce the methods, processes and equipment selection requirements for hydrogen sulfide stress corrosion testing, and help laboratories and quality managers in the oil and gas industry conduct relevant tests in a standardized manner.

1. The hazards and test significance of hydrogen sulfide stress corrosion

1.1 What is sulfide stress corrosion cracking (SSC)

Sulfide stress corrosion cracking (SSC) refers to the brittle fracture phenomenon of metal materials in a hydrogen sulfide environment under the combined action of tensile stress and corrosion.The mechanism is: hydrogen sulfide ionizes in the aqueous solution to produce hydrogen ions. The hydrogen ions obtain electrons on the metal surface to generate hydrogen atoms. The hydrogen atoms penetrate into the metal and are enriched at stress concentrations, causing the material to embrittle and produce cracks.SSC has the following characteristics:

• low stress fracture: SSC can occur under tensile stress that is far lower than the yield strength of the material, and usually cracks at 60%-80% of the yield strength.

• brittle fracture: There is no obvious plastic deformation before SSC fracture. It is a brittle fracture and it is difficult to give early warning.

• High strength steel sensitive: The higher the strength of the material (the greater the hardness), the higher the SSC sensitivity. Generally, carbon steel with a hardness exceeding HRC22 is at risk of SSC.

• Low temperature sensitive: SSC is most sensitive near normal temperature (20℃-40℃), and the sensitivity decreases as the temperature increases (but other types of corrosion will occur)

1.2 Why do hydrogen sulfide stress corrosion tests need to be carried out?

The oil and gas industry is the hardest hit area by hydrogen sulfide stress corrosion.According to statistics, there are more than 1,000 oil well pipe failure accidents caused by hydrogen sulfide corrosion around the world every year, causing huge economic losses and safety risks.In the development of hydrogen sulfide-containing oil and gas fields, all equipment and materials exposed to hydrogen sulfide (oil well pipes, drill pipes, casings, valves, pressure vessels, pipelines, etc.) must pass hydrogen sulfide stress corrosion tests to verify their corrosion resistance.

The NACE TM0177 standard is the most authoritative hydrogen sulfide environmental stress corrosion test method standard in the world. It is widely cited by international organizations such as API, ISO, and ASTM. It is also a must-do item for material procurement and quality inspection in the domestic oil and gas industry.By passing the NACE TM0177 test, you can:

• Evaluate the SSC/SCC resistance of materials in hydrogen sulfide environments

• Screening of materials and heat treatment processes suitable for use in sour oil and gas fields

• Verify whether the material meets the sulfur resistance requirements of API 5CT, API 5L and other product standards

• Provide a basis for material selection and safety assessment for oil and gas field development

2. Interpretation of NACE TM0177 standard

2.1 Standard Overview

NACE TM0177 "Laboratory Testing of Metal Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in Hydrogen Sulfide Environments" was published by the American Society of Corrosion Engineers (NACE International, now renamed AMPP). The latest version is the 2023 version.This standard specifies laboratory test methods for evaluating the resistance of metallic materials to sulfide stress cracking (SSC) and stress corrosion cracking (SCC) in acidic environments containing hydrogen sulfide.The corresponding Chinese national standard is GB/T 4157 "Laboratory test methods for resistance to sulfide stress cracking and stress corrosion cracking of metals in hydrogen sulfide environments".

2.2 Four test methods in the standard

The NACE TM0177 standard specifies four test methods, which are suitable for different test purposes and material types:

In addition, the NACE TM0177 standard also cites slow strain rate testing (SSRT) as a complementary method for rapid screening of materials for SCC susceptibility.The SSRT method is specified in ISO 7539-7 and GB/T 15970.7 and is also widely used in hydrogen sulfide environmental assessment.

2.3 Standard test solution

The NACE TM0177 standard specifies two standard test solutions:

Solution A (NACE A solution):

• Ingredients: 5.0% NaCl + 0.5% glacial acetic acid (CH₃COOH) aqueous solution

• pH value: about 2.7 (initial), after saturation with H₂S, the pH is about 2.5-3.0

• Purpose: The most commonly used standard test solution, used to simulate acidic sulfur-containing oil and gas field environments

• H₂S partial pressure: usually 0.1MPa (1atm), saturated solution H₂S concentration is about 2000-3000mg/L

Solution B (NACE B solution/synthetic seawater):

• Ingredients: Artificial seawater prepared according to ASTM D1141. The main ingredients include NaCl, MgCl₂, Na₂SO₄, CaCl₂, etc.

• pH value: about 8.2 (initial), pH decreases after adding H₂S

• Purpose: Simulate marine sulfur-containing environment, used for material testing of ocean platforms and submarine pipelines

In addition to standard solutions, NACE TM0177 also allows the use of simulated solutions prepared according to actual working conditions (such as simulated oil field produced water), but the solution composition and preparation method must be detailed in the test report.

2.4 Test conditions and judgment criteria

• Test temperature: The standard temperature is 24℃±3℃ (room temperature), other temperatures can also be selected according to actual working conditions, but need to be stated in the report

• Test cycle: Method A constant load test is usually 720 hours (30 days), and can be extended to 1000 hours or longer if no breakage occurs within 720 hours

• Loading stress: Method A is usually loaded at 80%-100% of the actual yield strength of the material, or according to API standards.

• Judgment criteria: If the sample breaks during the test, the SSC fails; after the test, the unbroken sample will be subjected to a bending test or metallographic inspection to confirm whether there are microcracks.

• Parallel specimen: At least 3 parallel samples for each condition

3. Hydrogen sulfide stress corrosion test operation process (taking Method A constant load as an example)

3.1 Sample preparation

1. sampling: Take samples according to the direction and position specified in product standards (such as API 5CT). Pipes usually take longitudinal samples, and plates can take transverse or longitudinal samples.Heat affected zones and work hardening should be avoided when sampling.

2. processing: Processed into cylindrical tensile specimens according to NACE TM0177 standard, the gauge section diameter is usually 6.35mm±0.1mm (0.25 inches), and the gauge length is 25.4mm (1 inch) or 31.75mm (1.25 inches).The clamping end is designed according to the testing machine fixture.

3. surface treatment: The surface of the gauge length section is polished step by step with sandpaper to 600 mesh, the surface roughness Ra≤0.8μm, and the machining tool marks are removed.The grinding direction should be along the axis of the specimen.

4. Clean: Clean the sample surface with acetone or alcohol to remove oil stains, rinse with deionized water, and blow dry with cold air.

5. Measure: Use a micrometer to measure the diameter of the three positions of the gauge section, and take the minimum value, accurate to 0.01mm, for calculating the cross-sectional area and loading force.

6. Hardness testing: Test the Rockwell hardness (HRC) of each sample or the same batch of materials to ensure compliance with material standard requirements (sulfur-resistant steel usually requires HRC ≤ 22).

3.2 Solution preparation and ventilation

1. Use analytical grade NaCl, glacial acetic acid, and deionized water to prepare a 5% NaCl + 0.5% acetic acid solution.

2. Pour the solution into the test container (made of glass or acid-resistant plastic). The amount of solution should be enough to immerse the gauge length of the specimen and there should be enough head space for ventilation.

3. Pour in nitrogen (N₂) to remove oxygen for more than 30 minutes to remove dissolved oxygen in the solution (oxygen will accelerate corrosion and affect SSC results).

4. Pour in a saturated solution of hydrogen sulfide (H₂S) gas. The ventilation time is usually no less than 2 hours to ensure that the H₂S concentration reaches saturation.The ventilation process should be carried out in a fume hood equipped with an H₂S detection alarm.

5. Measure and record the pH value of the solution (should be between 2.5-3.0 after saturation) and H₂S concentration (can be detected by iodometric method or special test paper).

3.3 Sample installation and loading

1. Install the specimen onto the constant load loading device, ensuring that the specimen is centered.Constant load devices usually use weight-lever systems or hydraulic constant load systems.

2. According to the set loading stress (usually 80% or 100% of the material yield strength) and the specimen cross-sectional area, calculate the required loading force: F = σ × S₀.

3. Pour the prepared solution saturated with H₂S into the test container so that the gauge length section of the specimen is completely immersed.

4. Continuously introduce H₂S gas to maintain the H₂S saturated state of the solution, and control the gas flow rate within an appropriate range (usually 50-100mL/min).

5. Apply the set load and record the start time.Loading should be smooth and avoid shock.

3.4 Test process monitoring

• Check the operating status of the test device every day to confirm whether the load is stable, whether the solution is leaking, and whether the H₂S ventilation is normal.

• Record the ambient temperature, solution temperature, and pH value every day (can be measured once a week)

• Check whether the H₂S detection alarm is working properly and ensure that the laboratory is well ventilated

• If the sample breaks, record the break time and stop testing the sample immediately.

• Regularly replenish the solution lost due to volatilization (replenish with a solution saturated with H₂S in advance)

3.5 End of test and judgment of results

1. After reaching the specified test period (usually 720 hours), remove the load and take out the sample.

2. Rinse the sample with deionized water, dehydrate with acetone, and blow dry with cold air.

3. Visually inspect the specimen surface for cracks.

4. Conduct a bending test on the unbroken sample (bend the sample at 90° or 180°). If cracks or fractures appear after bending, it indicates the existence of SSC microcracks and is judged to be failure.

5. If necessary, conduct metallographic analysis to observe whether there are secondary cracks.

6. Record the test results of each specimen (breaking time/non-breaking, bending test results).

7. Based on the results of parallel specimens, it is comprehensively determined whether the material has passed the NACE TM0177 test (usually all parallel specimens are required to be unbroken and bent without cracks to pass).

4. Selection of hydrogen sulfide stress corrosion testing equipment

4.1 Constant load test device (Method A)

The constant load tensile test is the most commonly used method of NACE TM0177. The required equipment includes:

• Constant load loading frame: Weight - lever type or hydraulic servo type, load range is usually 1kN-50kN, load accuracy ±1%.The weight type has a simple structure and high reliability, but has limited loading force; the hydraulic type can achieve large loads and automatic loading, but the cost is higher.

• Corrosion test container: Made of glass or acid-resistant plastic, with sealing cover and gas inlet and outlet, can accommodate samples and solutions.

• Hydrogen sulfide gas supply system: H₂S cylinder, pressure reducing valve, flow meter, gas pipeline, tail gas absorption device (use sodium hydroxide solution to absorb tail gas H₂S).

• safety protection system: H₂S detection alarm (fixed and portable), powerful fume hood, emergency sprinkler device, personal protective equipment (gas mask, chemical-resistant gloves, goggles).

• temperature control system: If temperature control testing is required, a constant temperature water bath or heating jacket is required.

4.2 Slow strain rate stress corrosion testing machine (SSRT)

Although the slow strain rate test is not a standard method of NACE TM0177, it is widely used in rapid screening of SCC in hydrogen sulfide environments.Key points for selecting an SSRT testing machine include:

• strain rate range: Covering 10⁻⁷ s⁻¹ to 10⁻⁴ s⁻¹, stable operation at low speed

• Maximum test force:10kN-100kN, select according to sample size

• corrosive environment chamber: It must be made of materials resistant to hydrogen sulfide corrosion (Hastelloy C276, titanium alloy or lined with polytetrafluoroethylene), with good sealing performance and can pass into H₂S gas

• temperature control: Temperature control from room temperature to 200℃, temperature control accuracy ±1℃

• Security protection: H₂S leakage alarm, automatic shutdown protection, exhaust gas absorption interface

• Data collection software: Real-time collection of force, displacement, time, and temperature, and automatic calculation of SCC sensitivity index

4.3 Other method equipment

• Method B bent beam test: Three-point bending or four-point bending fixture is required, which can be performed on a universal testing machine or special bending device and used with corrosion containers

• Method C C-ring test: Requires C-ring specimens and loading bolts, simple equipment, mainly specimen processing and corrosion containers

• Method D DCB test: Requires double cantilever beam specimen, wedge loading device and crack length measurement system (such as reading microscope, DCPD DC potential drop system)

5. Safety operating specifications for hydrogen sulfide testing

Hydrogen sulfide is a highly toxic, flammable and explosive gas, and safe handling is crucial.The following safety regulations must be followed when carrying out hydrogen sulfide stress corrosion testing:

1. Laboratory requirements: The test must be conducted in a dedicated fume hood or a well-ventilated laboratory, equipped with a fixed H₂S detection alarm (the alarm threshold is usually set to 10ppm), and a warning sign is set at the entrance of the laboratory.

2. personal protection: Operators must wear portable H₂S detection alarms, gas masks (with special H₂S canisters or positive pressure air respirators), chemical protective gloves, goggles, and chemical protective clothing.

3. Gas cylinder management: H₂S cylinders must be firmly fixed and placed in a well-ventilated gas cylinder cabinet, equipped with pressure reducing valves and check valves to prevent solution backflow.Keep gas bottles away from fire and heat sources.

4. Exhaust gas treatment: The H₂S tail gas discharged from the test container must be absorbed and treated by sodium hydroxide solution (5%-10% NaOH) and must not be directly discharged into the atmosphere.Change the absorbent regularly.

5. Emergency treatment: The laboratory is equipped with emergency spray and eyewash devices, and an H₂S leakage emergency plan is formulated.If leakage occurs, immediately stop ventilation, evacuate personnel, start forced ventilation, and have professionals wearing positive-pressure air respirators deal with the problem.

6. Personnel training: All operators must undergo hydrogen sulfide safety operation training, be familiar with the hazards, protection methods and emergency response procedures of H₂S, and can only take up their posts after passing the assessment.

6. About Shandong Huayan Intelligent Equipment Group

Shandong Huayan Intelligent Equipment Group Co., Ltd. is a professional testing machine R&D and manufacturing enterprise. The core team has 20 years of industry experience and can provide complete equipment solutions for hydrogen sulfide stress corrosion testing.Huayan's Slow Strain Rate Stress Corrosion Tester (SSRT) uses a Hastelloy/titanium alloy environmental chamber that is resistant to hydrogen sulfide corrosion. It is equipped with an H₂S leakage alarm and safety protection system. The strain rate covers 10⁻⁷ s⁻¹ to 10⁻⁴ s⁻¹, which can meet the hydrogen sulfide environment SCC testing requirements of NACE TM0177 and GB/T 4157 standards.

Huayan Intelligent Equipment can also provide supporting equipment such as constant load hydrogen sulfide stress corrosion test equipment, corrosion test containers, H₂S gas supply and tail gas treatment systems, as well as test method consultation and operation training services.The company has passed ISO9001 quality management system certification, participated in the formulation of more than ten industry group standards, holds 17 national patents, and is the vice-chairman unit of the Shandong Electrotechnical Society.The group has Tianhua Measurement and Testing (Shandong) Co., Ltd., which can provide measurement and calibration services for test equipment.

Huayan has strong non-standard customization capabilities and can customize exclusive hydrogen sulfide corrosion testing systems according to customers' special testing needs (such as high temperature and high pressure hydrogen sulfide environment, special sample size, multi-channel parallel testing, etc.).If you need to carry out hydrogen sulfide stress corrosion testing or purchase related equipment, please contact Shandong Huayan Intelligent Equipment. We will provide you with safe, reliable and professional solutions.

7. FAQ

Q1: What is the relationship between NACE TM0177 and GB/T 4157?

A: GB/T 4157 "Laboratory Test Methods for Resistance of Metals to Sulfide Stress Cracking and Stress Corrosion Cracking in Hydrogen Sulfide Environments" is a Chinese national standard modified from NACE TM0177. The technical content is basically the same, and the test methods and solution formulas are the same.Hydrogen sulfide stress corrosion testing in China can be carried out in accordance with GB/T 4157 or NACE TM0177.For export products or international projects, it is recommended to cite NACE TM0177; for domestic product testing and scientific research, GB/T 4157 can be cited.The test results of the two are comparable.

Q2: Why should the hydrogen sulfide stress corrosion test be controlled at room temperature?What happens when the temperature gets too high?

A: Sulfide stress cracking (SSC) is very sensitive to temperature, with the highest sensitivity near room temperature (20℃-40℃).When the temperature rises, the diffusion rate of hydrogen atoms in the metal accelerates, but the solubility and penetration of hydrogen atoms decrease instead. At the same time, the toughness of the material increases as the temperature rises, so the SSC sensitivity decreases as the temperature rises.The NACE TM0177 standard selects 24°C ± 3°C as the standard test temperature because this is the most demanding SSC condition and is also the downhole temperature range of most sour oil and gas fields.If the actual operating temperature is higher, the test can be conducted at the corresponding temperature, but the results cannot be directly compared with the room temperature results.

Q3: In the NACE TM0177 test, if the sample does not break for 720 hours, does it pass?

A: Not necessarily.The standard cycle of NACE TM0177 Method A constant load test is 720 hours (30 days). No fracture after 720 hours just means that no SSC fracture occurred during this time, but microcracks may have occurred inside the sample.The standard requires that unbroken specimens be subjected to a bending test (usually bent at 90°). If cracks or fractures appear after bending, it indicates the existence of SSC microcracks and is still judged as failure.Only samples that have not broken and have no cracks in the bending test for 720 hours will be judged to have passed.For critical applications, test times are sometimes extended to 1,000 hours or more to obtain more reliable results.

Q4: Can the slow strain rate test (SSRT) replace the NACE TM0177 constant load test?

A: It cannot be completely replaced.SSRT is an accelerated screening test with a short test cycle (hours to days) that can quickly compare the SCC sensitivities of different materials. However, the test conditions (strain rate, loading method) are quite different from the actual service conditions. The results are mainly used for material screening and sorting and cannot be directly used as a basis for determining whether a product is qualified.The NACE TM0177 constant load test is an arbitration test method stipulated in product standards (such as API 5CT, API 5L). The test period is long (30 days), but it is closer to actual service conditions. The results are the basis for material sulfur resistance certification.Usually SSRT is used for rapid screening in the research and development stage, and NACE TM0177 is used for qualification determination of final products. The two are used in combination.

Q5: How to deal with the waste liquid and waste samples from the hydrogen sulfide test?

A: The waste liquid from the hydrogen sulfide test contains dissolved H₂S and heavy metal ions. It is a hazardous waste and cannot be discharged directly.Treatment method: (1) Collect waste liquid in a sealed container, add excess sodium hydroxide solution or sodium hypochlorite solution to convert H₂S into non-toxic sodium sulfide or sulfate, adjust the pH value to neutral and then treat it as hazardous waste; (2) Waste sampleIf sulfide is adsorbed on the surface, it should be fully rinsed with clean water and then cleaned with acetone. It should be treated as general industrial solid waste or handed over to a qualified hazardous waste treatment agency; (3) The treatment of all waste liquids and waste samples should be recorded and comply with local environmental regulations.Huayan Intelligent Equipment's equipment is equipped with an exhaust gas absorption system, which can effectively reduce H₂S emissions.