GB/T 8650 Hydrogen Induced Cracking (HIC) Testing

GB/T 8650 Hydrogen Induced Cracking (HIC) testing is a critical standard used to evaluate the resistance of pipeline and pressure vessel steels to hydrogen embrittlement in sour service environments. This article provides a comprehensive technical overview of the GB/T 8650 standard, detailing the testing methodology, sample preparation, evaluation criteria, and its significance in the oil and gas industry.

Technical Overview

GB/T 8650 is the Chinese national standard titled "Evaluation of pipeline and pressure vessel steels for resistance to hydrogen induced cracking." This standard is technically equivalent to the international NACE TM0284 standard. It specifies the method for evaluating the capability of steel plates and pipes to resist hydrogen induced cracking (HIC) when exposed to wet hydrogen sulfide (H2S) environments. The test is essential for ensuring material integrity in sour oil and gas fields where hydrogen sulfide corrosion is a prevalent risk.

The mechanism of HIC involves atomic hydrogen entering the steel matrix during the corrosion process. This hydrogen diffuses and accumulates at inclusions or voids within the steel. As hydrogen atoms combine to form molecular hydrogen, the pressure increases, leading to internal cracking. GB/T 8650 provides a standardized procedure to quantify a material's susceptibility to this failure mode.

Testing Samples

Sample Dimensions

The standard specifies strict requirements for the dimensions of test specimens to ensure reproducibility. Typically, rectangular samples are used. The standard dimensions generally follow the guidelines:

• Length: 100 mm ± 1 mm
• Width: 20 mm ± 1 mm
• Thickness: The full thickness of the product being tested, up to a maximum of 30 mm

Sample Preparation

Sample preparation is critical for accurate results. The surfaces of the specimens must be ground to remove scale and imperfections. The final grinding stage should be completed using 320 grit abrasive paper or finer. After grinding, samples must be cleaned thoroughly, usually with acetone or alcohol, to remove any grease or contaminants that could affect the corrosion process. Typically, three samples are tested simultaneously to represent the material batch adequately.

Testing Items

The primary objective of GB/T 8650 testing is to measure the extent of internal cracking caused by hydrogen absorption. The testing items focus on quantifying crack parameters rather than simply passing or failing a sample. The key metrics evaluated include:

• Crack Length Ratio (CLR): The ratio of the total length of all cracks to the total inspection length.
• Crack Thickness Ratio (CTR): The ratio of the total thickness of all cracks to the total inspection thickness.
• Crack Sensitivity Ratio (CSR): The ratio of the total crack area to the total inspection area.

These ratios are calculated based on microscopic examination of the cross-sections of the test samples after exposure.

Testing Methods

Solution Preparation

The test environment simulates a sour service condition. The standard defines two primary test solutions:

• Solution A: A synthetic seawater solution saturated with hydrogen sulfide.
• Solution B: A brine solution (standard brine) saturated with hydrogen sulfide.

The pH of the solution is strictly controlled, typically required to be between 4.5 and 5.2 at the end of the test. The solution is deoxygenated by purging with inert gas (usually nitrogen) before introducing the toxic hydrogen sulfide gas.

Exposure Duration

The standard exposure duration for GB/T 8650 testing is 96 hours (4 days). During this period, the samples are fully immersed in the test solution under ambient temperature and pressure conditions. Continuous saturation of the solution with hydrogen sulfide must be maintained throughout the duration to ensure a steady driving force for hydrogen entry.

Post-Exposure Evaluation

After the 96-hour exposure, samples are removed and cleaned. To reveal internal cracks, the samples are sectioned, polished, and etched. A metallographic microscope is then used to examine three specific sections of each sample. Measurements of crack length and width are taken, and the CLR, CTR, and CSR values are calculated using the formulas provided in the standard.

Applications

The GB/T 8650 test is widely applied in industries where materials are exposed to environments containing hydrogen sulfide. Its primary applications include:

• Oil and Gas Transmission Pipelines: Ensuring line pipes can withstand sour gas transport.
• Pressure Vessels: Qualifying steel plates used in refinery equipment and separators.
• Offshore Platforms: Evaluating structural steel for resistance to seawater and H2S corrosion.
• Material Procurement: serving as a mandatory acceptance criterion for steel manufacturers supplying to sour service projects.

FAQ

What is the difference between HIC and SCC?

Hydrogen Induced Cracking (HIC) is a form of cracking that occurs due to hydrogen build-up inside the steel, often without external applied stress. Sulfide Stress Cracking (SSC) involves the combined action of tensile stress and a corrosive environment containing hydrogen sulfide. GB/T 8650 specifically addresses HIC, whereas standards like GB/T 4157 address SSC.

What are the acceptance criteria for GB/T 8650?

The GB/T 8650 standard itself does not define specific pass/fail acceptance criteria. Instead, it provides the method for generating data (CLR, CTR, CSR). The acceptance criteria are typically specified by the end-user, engineering design codes (such as NACE MR0175/ISO 15156), or material specifications. Common industry limits are CLR <= 15%, CTR <= 5%, and CSR <= 2%.

Is GB/T 8650 equivalent to NACE TM0284?

Yes, GB/T 8650 is technically aligned with NACE TM0284 "Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking." The test procedures, sample dimensions, and calculation methods are virtually identical, allowing for international recognition of test results generated under the Chinese standard.