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Different Types of Corrosion
- Recognition, Mechanisms & Prevention

High-temperature Hydrogen Attack (Decarburization)

Recognition

What is high-temperature hydrogen attack?

High-temperature Hydrogen Attack (HTHA) refers to the loss of strength and ductility of steel by high-temperature reaction of absorbed hydrogen with carbides in the steel, resulting in decarburization and internal fissuring.

High-temperature hydrogen attack is also referred to as hot hydrogen attack or decarburization. It occurs in carbon and low-alloyed steels exposed for an extended period to hydrogen under high pressure and at high temperature.

This micrograph shows intergranular cracking caused by high temperature hydrogen attack in a C-0.5Mo steel.

Mechanisms

What causes High-temperature hydrogen attack?

Atomic hydrogen (formed in corrosion processes or by dissociation of molecular hydrogen in a gas stream at the steel surface) diffuses into steel. At grain boundaries, crystal imperfections, inclusions, discontinuities and other defects, the atomic hydrogen reacts with the dissolved carbon or with the metal carbides, forming methane:

8H+C+Fe₃C =2CH₄+3Fe       (eq.1)

Because of the pressure build-up of the methane in the steel, this results in the formation of intergranular cracks (refer to the micrograph above), fissures and blisters, often extending to the surface of the steel. This form of hydrogen damage sometimes resembles the low-temperature hydrogen blistering.  Moreover, the decarburization process leads to the loss of carbon in the steel and hence a reduction in tensile strength and an increase in ductility and creep rate. Interestingly, the reverse process (Eq.1 above), carburization, can also occur in hydrogen-hydrocarbon mixtures such as that encountered in petroleum-refining operations.

Nelson curves are commonly used to select the various grades of steels and the safe operating limits of temperature and hydrogen partial pressure.

Prevention

How to prevent High-temperature hydrogen attack? High-temperature hydrogen attack, decarburization and fissuring can be prevented through:

• avoid high carbon steels

• use higher alloyed steels

• following the safe operating limits defined in the Nelson curves.

• use a safety margin of 30°C when using Nelson curves

For more details

More details on high-temperature hydrogen attack are included in the following corrosion courses which you can take as in-house training courses, course-on-demand, online courses or distance learning courses:

Corrosion and Its Prevention (5-day module)
Corrosion and Its Prevention (2-day module)
Corrosion, Metallurgy, Failure Analysis and Prevention (3 days)
Marine Corrosion, Causes and Prevention (2 days)
Materials Selection and Corrosion (2 days)
Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days)

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