Metallic structures in high-demand industrial settings are constantly threatened by various forms of material degradation. One severe form of damage is Hydrogen Induced Cracking (HIC), a mechanism that can lead to catastrophic failure without a significant external load. HIC is caused by the absorption of hydrogen into the metal lattice, which then accumulates internally to create fissures. This degradation is a serious concern across the energy sector, affecting the reliability of pipelines, pressure vessels, and storage tanks used in the oil, gas, and petrochemical industries.
Defining Hydrogen Induced Cracking
Hydrogen Induced Cracking is a form of internal material damage characterized by the formation of non-surface-breaking cracks within the bulk steel. These cracks are typically laminar, meaning they are flat and primarily oriented parallel to the rolled surface of the metal plate or pipe wall. The damage often begins as small, isolated internal voids or blisters that develop at specific points of weakness.
A hallmark of this damage mechanism is its progression into what is known as stepwise cracking (SWC). SWC occurs when multiple internal cracks, lying on parallel planes, link together by fracturing the thin ligaments of metal between them. This linking process happens through the thickness of the material, creating a characteristic, layered appearance. HIC is a pressure-driven phenomenon that can occur in the absence of any significant applied external stress.
The Internal Mechanism of Failure
The process of HIC begins with the generation and absorption of atomic hydrogen (\(\text{H}\)) onto the surface of the steel structure, typically as a byproduct of a corrosion reaction. Because the hydrogen atom is the smallest element, it readily diffuses and penetrates the crystal lattice structure of the metal.
Once inside the metal, the hydrogen atoms migrate through the lattice, drawn toward internal discontinuities known as trap sites. These sites include non-metallic inclusions, grain boundaries, or microscopic voids. When a sufficient concentration of atomic hydrogen gathers at one of these trap sites, the atoms recombine to form molecular hydrogen (\(\text{H}_2\)).
The molecular hydrogen gas is too large to diffuse back out of the metal lattice. As corrosion continues and more atomic hydrogen is absorbed, the concentration of \(\text{H}_2\) gas within the void increases dramatically. This continuous influx causes a massive buildup of pressure within the confined space.
The internal pressure eventually exceeds the local yield strength of the steel surrounding the void. This initiates a crack that propagates outward from the internal defect. As adjacent internal cracks form and grow, the high shear stresses generated between them cause the cracks to link up, resulting in the through-thickness stepwise cracking that defines HIC.
Environmental and Material Susceptibility
The formation of HIC is highly dependent on both the external environment and the internal quality of the steel. The most aggressive environmental condition is known as “sour service,” which refers to environments containing water and hydrogen sulfide (\(\text{H}_2\text{S}\)). The \(\text{H}_2\text{S}\) molecule acts as a “recombination poison” on the steel surface.
Normally, atomic hydrogen produced by corrosion would immediately recombine to form harmless \(\text{H}_2\) gas and bubble away. However, the presence of \(\text{H}_2\text{S}\) inhibits this surface recombination reaction, forcing a significantly higher concentration of atomic hydrogen to be absorbed into the steel. This process is further accelerated in acidic conditions, meaning a lower \(\text{pH}\) level increases the rate of hydrogen absorption.
The material’s susceptibility is directly related to its cleanliness and content of impurities. Non-metallic inclusions, such as manganese sulfides, are a major factor in HIC susceptibility.
These inclusions are elongated during the rolling process of manufacturing steel plate, creating flat, internal surfaces that serve as preferred accumulation and recombination sites for the diffusing hydrogen atoms. Steel with a high volume of these impurities offers numerous trap sites, making it vulnerable to the internal pressure buildup. Conversely, cleaner steels with reduced sulfur content and fewer inclusions exhibit a much higher resistance to HIC.
Strategies for Prevention and Control
Preventing HIC requires a two-pronged approach focusing on both material selection and environmental management. A primary strategy involves specifying the use of HIC-resistant steel, often referred to as “clean steel,” for new construction. These materials are manufactured with extremely low sulfur content, drastically reducing the number of non-metallic inclusions that act as hydrogen trap sites.
Environmental control measures are employed to minimize the generation and absorption of hydrogen at the material surface. This includes the continuous injection of corrosion inhibitors into process streams to slow down the corrosion rate. Additionally, strict \(\text{pH}\) control is implemented to ensure the environment remains less acidic, thereby reducing the efficiency of the hydrogen sulfide as a surface poison.
For structures already in service, regular inspection is a necessary control measure. Non-destructive testing (NDT) techniques, such as ultrasonic testing (UT), are used to locate and characterize the internal, laminar cracks. A high-temperature heat treatment, known as a hydrogen bake-out, can be applied to remove absorbed hydrogen from the steel.