High-Temperature Hydrogen Attack (HTHA)

High-Temperature Hydrogen Attack (HTHA) is a form of degradation occurring in steels and other alloys exposed to high temperatures and hydrogen. It occurs when hydrogen atoms penetrate the steel and react with carbon to form methane gas, which builds up pressure within the metal and causes cracking and other damage. HTHA is typically observed in refineries, petrochemical plants, and other high-temperature and high-pressure environments and can be a serious safety concern for workers and equipment. It is, therefore, important to carefully monitor and manage the conditions in which high-pressure equipment is operated to prevent HTHA from occurring.

HTHA is typically observed in equipment that operates at high temperatures and pressures and is exposed to hydrogens, such as in refineries, petrochemical plants, and other similar industrial processes. Some examples of process equipment that are susceptible to HTHA include:

• Reactors: These vessels are used for chemical reactions at high temperatures and pressures and are often exposed to hydrogen.
• Furnaces: These are used for heating and melting metals and may be exposed to hydrogen during the process.
• Boilers: These vessels generate steam and may be exposed to hydrogen due to using hydrogen-rich fuels.
• Pipelines: These are used for transporting gases and liquids and may be exposed to hydrogen due to the presence of hydrogen in the transported fluid.
• Pressure vessels: These are used for storing and transporting pressurized gases and liquids and may be exposed to hydrogen during operation.

It is important to note that any equipment exposed to high temperatures, high pressures, and hydrogen is potentially susceptible to HTHA and therefore requires careful monitoring and management to prevent this degradation.

There are several methods for inspecting equipment for HTHA, depending on the specific equipment and the severity of the expected damage. Here are some standard techniques:

• Visual Inspection: This is the simplest method, where inspectors look for signs of cracking, blistering, or other physical changes on the surface of the equipment. However, this method is unreliable, as HTHA damage can occur beneath the surface, which cannot be detected through visual inspection alone.
• Ultrasonic Testing: This method uses high-frequency sound waves to detect flaws, such as cracks or voids, beneath the surface of the equipment. This method is commonly used for inspecting welds and detecting HTHA damage in pressure vessels and pipelines.
• Radiographic Testing: This method uses X-rays or gamma rays to produce an image of the internal structure of the equipment. This method is particularly useful for inspecting thick-walled vessels and detecting HTHA damage that is not visible from the surface.
• Magnetic Particle Testing: This method uses magnetic fields and iron particles to detect cracks and other defects on the surface of the equipment. This method is particularly useful for detecting surface cracks in magnetic materials like carbon steels.
• Eddy Current Testing: This method uses electromagnetic fields to detect defects in conductive materials, such as stainless steel. It is commonly used for detecting cracks and other damage in heat exchangers, condensers, and other equipment.

It is important to note that each of these methods has advantages and limitations, and the choice of inspection method should be based on the specific equipment and the expected type of damage. In addition, regular monitoring and maintenance of the equipment are critical to prevent HTHA damage from occurring in the first place.

Several laboratory testing methods can be used to detect HTHA in metals and alloys. Here are some common methods:

• Hardness Testing: HTHA can cause a decrease in the hardness of the affected material, so hardness testing can be used to detect HTHA damage. The most commonly used method is the Rockwell hardness test, which measures the indentation hardness of the material.
• Metallography: This method involves examining the material's microstructure using a microscope. HTHA damage can cause changes in the material's microstructure, such as the formation of carbides and other phases. Metallography can be used to detect these changes and determine the severity of the HTHA damage.
• Chemical Analysis: HTHA damage can cause changes in the material's chemical composition, such as carbon depletion and methane formation. Chemical analysis, such as energy-dispersive X-ray spectroscopy (EDS), can detect these changes and determine the extent of HTHA damage.
• Tensile Testing: HTHA damage can cause a decrease in the tensile strength of the affected material. Tensile testing can be used to detect this decrease and determine the severity of the HTHA damage.
• Scanning Electron Microscopy (SEM): SEM can examine the material's microstructure at a higher magnification than metallography. SEM can also be used with EDS to identify the chemical composition of the material.

It is important to note that laboratory testing alone may not be sufficient to detect HTHA damage. This type of degradation can occur beneath the material's surface and may not be visible to the naked eye or detectable using laboratory testing. Therefore, laboratory testing should be combined with other inspection methods, such as ultrasonic or radiographic testing, to ensure the most accurate and reliable detection of HTHA damage.

The American Petroleum Institute (API) has developed several codes and standards for inspecting equipment for HTHA in the petrochemical industry. Here are some of the API codes commonly used for HTHA inspection:

• API 510: Pressure Vessel Inspection Code: This code provides guidelines for the inspection, repair, alteration, and rerating of pressure vessels. It includes procedures for HTHA inspection and monitoring.
• API 571: Damage Mechanisms Affecting Fixed Equipment in the Refining Industry: This code provides a comprehensive list of damage mechanisms that can affect fixed equipment in the refining industry, including HTHA. It includes information on the characteristics and detection of HTHA damage.
• API 579-1/ASME FFS-1: Fitness-For-Service: This code provides guidelines for evaluating the fitness-for-service of equipment damaged or degraded, including by HTHA. It contains procedures for determining the extent and severity of HTHA damage and assessing the remaining life of the equipment.
• API 580: Risk-Based Inspection: This code provides guidelines for implementing a risk-based inspection program for process equipment. It includes information on assessing the risk of HTHA damage and prioritizing inspection activities accordingly.
• API 941: Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants: This code provides guidelines for selecting materials for equipment exposed to hydrogen at high temperatures and pressures, such as in the petrochemical industry. It includes information on preventing HTHA damage by selecting appropriate materials and operating conditions.

It is important to note that the choice of API code will depend on the specific equipment and the requirements of the inspection program. Therefore, consulting with qualified inspection professionals and engineers is essential to determine the appropriate API codes for HTHA inspection.

The National Association of Corrosion Engineers (NACE) also provides guidelines and standards for inspecting equipment for HTHA in the petrochemical industry. Here are some of the NACE codes commonly used for HTHA inspection:

• NACE SP0296-2016: Evaluation of Pipeline and Pressure Vessel Steel Microstructure and Properties after Exposure to High Temperature and High-Pressure Service: This standard provides guidelines for evaluating the microstructure and properties of pipeline and pressure vessel steels after exposure to high temperature and high-pressure service, including HTHA. It includes procedures for sample preparation, metallography, and hardness testing.
• NACE SP0294-2016: Design, Fabrication, and Inspection of Tanks for the Storage of Concentrated Sulfuric Acid and Oleum at Ambient Temperatures: This standard provides guidelines for the design, fabrication, and inspection of tanks for the storage of concentrated sulfuric acid and oleum. It includes guidelines for HTHA inspection and monitoring.
• NACE MR0175/ISO 15156: Petroleum, petrochemical, and natural gas industries – Materials for use in H2S-containing environments in oil and gas production: This standard provides guidelines for selecting materials for use in atmospheres containing hydrogen sulfide (H2S), including those that may be susceptible to HTHA. It includes information on preventing HTHA damage by selecting appropriate materials and operating conditions.
• NACE SP0178-2007: Design Considerations for Corrosion Control of Reinforcing Steel in Concrete: This standard provides guidelines for designing and implementing corrosion control measures for reinforcing steel in concrete structures. It includes information on how to prevent HTHA damage in reinforced concrete structures.
• NACE RP0490-2000: Holiday Detection of Internal Tubular Coatings of 250 to 760 μm (10 to 30 mils) Dry Film Thickness: This standard provides guidelines for detecting holidays (pinholes, voids, or other discontinuities) in internal tubular coatings. It includes information on how to prevent HTHA damage by ensuring the integrity of coatings that may be used to avoid contact between the metal and hydrogen.

It is important to note that the choice of NACE code will depend on the specific equipment and the requirements of the inspection program. Therefore, consulting with qualified inspection professionals and engineers is essential to determine the appropriate NACE codes for HTHA inspection.