How to do stress corrosion test?ISO 7539-7 standard interpretation + operation process

Stress Corrosion Cracking (SCC) is a brittle fracture phenomenon that occurs in metal materials under the combined action of tensile stress and corrosive media. It is one of the main causes of material failure in oil and gas, marine engineering, nuclear power, aerospace and other industries.The slow strain rate test (SSRT) is currently the most commonly used accelerated test method to evaluate the stress corrosion susceptibility of materials. Its international standard ISO 7539-7 and national standard GB/T 15970.7 provide normative guidance for the test.However, many laboratory personnel are not familiar with the specific requirements and operating procedures of the standards.This article will explain in detail the core requirements of the ISO 7539-7 standard and give a complete stress corrosion test operation process.

1. Interpretation of the core content of the ISO 7539-7 standard

1.1 Standard Scope and Applicable Objects

ISO 7539-7 "Corrosion of metals and alloys - Stress corrosion tests - Part 7: Slow strain rate tests" specifies a method for evaluating the susceptibility to stress corrosion cracking of metals and alloys using slow strain rate tensile tests.This standard applies to:

• Various metals and alloys, including carbon steel, low alloy steel, stainless steel, aluminum alloy, titanium alloy, nickel-based alloy, copper alloy, etc.

• Various corrosive media environments, including aqueous solutions, organic solutions, molten salts, gas environments, etc.

• Various temperature conditions from room temperature to high temperature

• Smooth specimens and notched (pre-cracked) specimens

The purpose of this standard is to evaluate the stress corrosion cracking susceptibility of materials by comparing the differences in mechanical properties of materials in corrosive environments and inert environments. It is suitable for scenarios such as material screening, process evaluation, and environmental factor research.

1.2 Test principle

The principle of the slow strain rate test is to expose the specimen to a specific corrosive environment and axially stretch the specimen at a constant strain rate that is much lower than that of conventional tensile testing (usually 10⁻⁴ s⁻¹ to 10⁻⁷ s⁻¹) until fracture.During the slow stretching process, the material has enough time to interact with the corrosive medium, promoting the initiation and expansion of stress corrosion cracks.

SCC susceptibility is evaluated by measuring and calculating the following parameters and comparing the differences in corrosive environments and inert environments (such as air, nitrogen, silicone oil):

• Breaking time (t_f)

• Total elongation at break (δ_f)

• Area shrinkage (ψ_f)

• Maximum tensile strength (σ_max)

• Fracture energy (area under the stress-strain curve)

• Fracture morphology analysis (transgranular/intergranular fracture ratio)

1.3 Calculation of stress corrosion sensitivity index

The ISO 7539-7 standard recommends the following formula to calculate the stress corrosion susceptibility index:

ElongationSensitivity index I_δ = (δ_air - δ_sol) / δ_air × 100%

SectionShrinkageSensitivity index I_ψ = (ψ_air - ψ_sol) / ψ_air × 100%

Among them, δ_air and ψ_air are the elongation and area reduction in an inert environment (air) respectively, and δ_sol and ψ_sol are the corresponding values in corrosive media respectively.The larger the value of I_δ or I_ψ, the higher the stress corrosion sensitivity of the material in this corrosive environment.It is generally believed that:

• I_δ or I_ψ < 10%: No obvious stress corrosion sensitivity

• I_δ or I_ψ = 10%-30%: mild stress corrosion sensitivity

• I_δ or I_ψ = 30%-50%: moderate stress corrosion susceptibility

• I_δ or I_ψ > 50%: High stress corrosion susceptibility

It should be noted that the above thresholds are only for reference. Different industries and different material systems may have different judgment standards. Comprehensive judgments should be made based on specific application scenarios and fracture morphology analysis.

2. Preparation before test

2.1 Sample preparation

Sample preparation is a key link in stress corrosion testing. The size, shape, and surface condition of the sample directly affect the test results.The ISO 7539-7 standard has the following requirements for specimens:

2.2 Preparation of corrosive medium

The composition, concentration, pH value, temperature and other parameters of the corrosive medium should be formulated in strict accordance with the test plan or relevant standards.Common standard corrosive media include:

• NACE A solution: 5% NaCl + 0.5% glacial acetic acid aqueous solution, pH value is about 2.7, used for hydrogen sulfide stress corrosion test

• NACE B solution: Synthetic seawater, used for stress corrosion testing in marine environments

• artificial seawater: Prepared according to ASTM D1141, used for seawater environment testing

• 3.5%NaCl solution: The most commonly used neutral salt spray environment simulation fluid

• other media: Prepared according to the actual service environment, such as oil field produced water, chemical media, etc.

Analytical grade reagents and deionized or distilled water should be used when preparing corrosive media. After preparation, measure and record the pH value.For media that require the introduction of gas (such as H₂S, CO₂, N₂), they should be fully ventilated and saturated before the test, and the gas should be continuously introduced or the environment should be sealed during the test.

2.3 Equipment inspection and calibration

The slow strain rate stress corrosion testing machine should be inspected and calibrated before testing:

• Check whether the calibration certificate of the force sensor and displacement sensor is within the validity period

• Check whether the temperature control system of the environmental chamber is normal and whether the temperature display is accurate

• Check whether the corrosive medium circulation system (if any) is normal

• Check whether safety protection devices (leak alarm, emergency shutdown, etc.) are effective

• Run the equipment without load and check whether the low-speed operation is stable and whether there is any abnormal noise, crawling or shaking.

• Confirm that the data acquisition software functions normally and the sampling frequency is set appropriately (usually 1Hz-10Hz)

3. Test operation process

3.1 Specimen installation

1. Use a vernier caliper to measure the diameter (or width and thickness) of the gauge section of the sample at three locations at both ends and in the middle of the gauge section, and take the minimum value for cross-sectional area calculation, accurate to 0.01mm.

2. Mark the gauge length by marking the gauge length at both ends of the gauge section of the specimen, and mark it with a marker or hardness tester to facilitate the measurement of the elongation after breaking.

3. Install the specimen into the fixture of the testing machine, ensuring that the specimen is centered to avoid eccentric loading.Clamp the upper clamp first, then install the environmental chamber, and then clamp the lower clamp.

4. Inject the corrosive medium into the environmental chamber to ensure that the gauge length section of the specimen is completely immersed in the medium.

5. If temperature control is required, turn on the temperature control system and wait until the medium temperature reaches the set temperature and stabilizes (usually 30-60 minutes) before starting stretching.

6. If gas needs to be introduced, ventilate to saturation according to standard requirements, and then maintain a stable gas flow.

3.2 Parameter setting and test start

1. Enter the sample information in the control software: sample number, material, gauge length, initial cross-sectional area, etc.

2. Set the strain rate: Set the strain rate according to the test plan (usually 10⁻⁶ s⁻¹ to 10⁻⁵ s⁻¹), and the software will automatically convert it to the beam displacement speed based on the gauge length.

3. Set data collection parameters: sampling frequency, recording interval (usually record data every 1 second or every 5 seconds).

4. Set shutdown conditions: usually set to automatically stop when the force value drops to 20%-30% of the peak value after the sample breaks, or set maximum displacement protection.

5. After confirming that all parameters are correct, start the test.After the test starts, record the start time, medium temperature, pH value and other environmental parameters.

3.3 Test process monitoring

Slow strain rate tests usually last from hours to days and require regular monitoring during the test:

• Check the operating status of the equipment every 2-4 hours to confirm that the force value and displacement curve are normal and there are no abnormal fluctuations.

• Check the level of the corrosive medium and replenish it in time if it evaporates.

• Check whether the medium temperature is stable within the set value ±1℃

• For tests in which corrosive gases are introduced, check gas flow and leak alarms

• Record any abnormal conditions during the test (such as equipment alarms, media leakage, abnormal deformation of the sample, etc.)

3.4 End of test and sample storage

1. After the sample breaks, the equipment automatically stops and the break time is recorded.

2. Drain the corrosive medium, rinse the sample surface with deionized water, then dehydrate with acetone or alcohol, and blow dry with cold air.

3. Align the two broken specimens together, measure the gauge length after breaking, and calculate the elongation after breaking.

4. Measure the minimum diameter (or width and thickness) at the fracture and calculate the reduction of area.

5. Store the fracture specimen in a desiccator to avoid oxidation and contamination for subsequent fracture morphology analysis (SEM scanning electron microscope observation).

6. Export test data (force-displacement curve, time-force curve, etc.) and save the original data file.

4. Data processing and result analysis

4.1 Calculation of mechanical property parameters

Based on the test data, the following mechanical property parameters are calculated:

• Tensile strengthσ_b= Maximum force F_max / Initial cross-sectional area S₀

• Elongation after fracture δ= (gauge length after breaking L₁ - original gauge length L₀) / L₀ × 100%

• Area shrinkage ψ= (Initial cross-sectional area S₀ - Minimum cross-sectional area of fracture S₁) / S₀ × 100%

• Breaking time t_f: The time from the start of the test to the specimen fracture

• Fracture energy W: Integrated area under the stress-strain curve

4.2 Stress corrosion susceptibility assessment

Calculate the stress corrosion susceptibility index according to the method recommended by the ISO 7539-7 standard:

I_δ = (δ_air - δ_sol) / δ_air × 100%

I_ψ = (ψ_air - ψ_sol) / ψ_air × 100%

At the same time, combined with fracture morphology analysis: observe the fracture morphology under a scanning electron microscope (SEM). If the fracture surface in a corrosive environment has obvious intergranular fractures, secondary cracks, corrosion products and other characteristics, it indicates the existence of stress corrosion cracking.

4.3 Contents of test report

A complete stress corrosion test report should include the following:

• Standards based on the test (ISO 7539-7 or GB/T 15970.7)

• Sample material information (material grade, heat treatment status, chemical composition, mechanical properties)

• Sample size and shape, surface treatment

• Corrosion medium composition, concentration, pH value, temperature

• Strain rate, test temperature, test environment

• The number of parallel samples for each set of tests and the test data of a single sample

• Mean and standard deviation of mechanical property parameters

• Stress corrosion susceptibility index calculation results

• Fracture morphology analysis results (with SEM photos)

• Conclusion and evaluation opinions

• Tester, auditor, test date

5. Test Precautions and Frequently Asked Questions

5.1 Key considerations

• Specimen centering: The specimen installation must ensure centering. Eccentric loading will cause additional bending stress and affect the test results.It is recommended to use self-aligning clamps.

• Strain rate selection: Strain rate is the most critical parameter that affects the SSRT test results. It must be set strictly in accordance with the standard or test plan. The results at different strain rates cannot be directly compared.

• Media consistency: The composition, concentration, pH value, and temperature of the corrosive medium should remain stable during the entire test process, and should be checked and recorded regularly.

• Parallel specimen: The stress corrosion test data is highly discrete. Each group should have at least 3 parallel samples. It is recommended to conduct more than 5 important tests. Take the average and calculate the standard deviation.

• Security protection: The corrosive medium (especially hydrogen sulfide) is toxic and harmful. Protective equipment must be worn during operation and carried out in a fume hood with a leak alarm device.

5.2 Frequently asked questions and solutions

6. About Shandong Huayan Intelligent Equipment Group

Shandong Huayan Intelligent Equipment Group Co., Ltd. is a professional testing machine R&D and manufacturing enterprise that can provide slow strain rate stress corrosion testing machines and complete testing solutions that comply with ISO 7539-7 and GB/T 15970.7 standards.Huayan's SSRT testing machine is equipped with a precision low-speed drive system, a corrosion-resistant environmental chamber, a high-precision data acquisition system and professional testing software. The software has built-in ISO 7539-7 standard test procedures and data processing templates, which can automatically calculate the stress corrosion sensitivity index and generate standard test reports.

Huayan Intelligent Equipment can also provide technical consultation, operational training and equipment calibration services for stress corrosion testing.Tianhua Metrology and Testing (Shandong) Co., Ltd. is a subsidiary of the group, which has metrology and calibration qualifications and can provide customers with calibration services for force value, displacement, temperature and other parameters of SSRT testing machines.The company's core team has 20 years of experience in the testing machine industry, has 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.

If you need to carry out stress corrosion testing or purchase an SSRT testing machine, please contact Shandong Huayan Intelligent Equipment. We will provide you with one-stop service from equipment selection, installation and commissioning, operation training to measurement calibration.

7. FAQ

Q1: What is the relationship between ISO 7539-7 and GB/T 15970.7?

A: GB/T 15970.7 "Corrosion of Metals and Alloys - Stress Corrosion Test Part 7: Slow Strain Rate Test" is a national standard equivalent to ISO 7539-7, and the technical content is completely consistent.SSRT testing in China can be carried out in accordance with GB/T 15970.7 or ISO 7539-7, both of which are equivalent.If the test report is used for international exchanges or export product testing, it is recommended to cite ISO 7539-7; if it is used for domestic testing and scientific research, GB/T 15970.7 can be cited.

Q2: Is air control test necessary for SSRT test?

A: Yes, it must be done.The core of the SSRT test is to compare the difference in mechanical properties between corrosive environments and inert environments. Without a control test, the stress corrosion sensitivity index cannot be calculated.The control test should use exactly the same specimen, strain rate and temperature as the corrosive environment test, except that the environment is changed to an inert environment such as air, nitrogen or silicone oil.It is recommended that half of the same batch of samples be subjected to corrosive environment tests and half to inert environment control tests.

Q3: Can the SSRT test specimen be reused?

A: No.The SSRT test is a destructive test, and the sample cannot be reused after it breaks.Even if the sample does not completely break in some tests (such as early shutdown), plastic deformation and micro-cracks have occurred inside the sample and cannot be used for new tests.Each test condition requires the use of new specimens.

Q4: Does the fracture surface of stress corrosion test need to be analyzed by SEM?

A: The ISO 7539-7 standard does not mandate SEM fracture analysis, but it is strongly recommended.Changes in mechanical property parameters can only indicate degradation of material properties, but cannot determine whether the degradation is caused by stress corrosion cracking.SEM fracture morphology analysis can directly observe whether there are intergranular fractures, secondary cracks, corrosion products and other SCC characteristics, which is direct evidence to judge stress corrosion.For important experiments and controversial results, SEM analysis is essential.

Q5: Can SSRT test results be directly used in engineering design?

A: It cannot be used directly in engineering design.The SSRT test is an accelerated screening test. The test conditions (strain rate, medium concentration, etc.) are different from the actual service conditions. The results are mainly used for comparison and screening of the relative sensitivity of materials and cannot be directly used as the basis for allowable stress or life prediction in engineering design.Engineering design should be comprehensively judged based on the results of constant load tests, actual working condition exposure tests, fracture mechanics analysis and other methods.The SSRT test is suitable as a rapid evaluation tool during the materials development and screening stages.