Hydrogen damage refers to the mechanical damage of metal caused by the interaction with or presence of hydrogen. Atomic hydrogen, with a radius of 1.1, can diffuse through many metals and steels and is highly reactive. Molecular hydrogen, however, is stable and cannot diffuse.

(a) Hydrogen Blistering

Hydrogen blistering occurs when atomic hydrogen diffuses into a metal that contains voids or empty spaces. Within these voids, the atomic hydrogen recombines to form molecular hydrogen (H2​). Since molecular hydrogen cannot diffuse out of the metal, it builds up immense pressure inside the voids, which can cause the material to deform locally, swell, or even rupture. This form of damage is common in the petroleum industry, such as during refining or in storage tanks.

Prevention: To prevent hydrogen blistering, you can:

• Use Coatings: Apply metallic, organic, or inorganic coatings and liners that are impervious to hydrogen penetration. Examples include rubber, plastic, brick linings, and nickel or austenitic steel cladding.
• Use Inhibitors: Add inhibitors to closed systems to reduce the rate of corrosion and hydrogen ion reduction.
• Use Clean Steels: Utilize materials with minimal internal voids, such as killed steel instead of rimmed steel.
• Remove Poisons: Eliminate substances like phosphorus compounds, sulfide ions, and arsenic compounds that can hamper the formation of molecular hydrogen, leading to a buildup of atomic hydrogen.
• Substitute Alloys: Use nickel-containing steels or nickel alloys, which have very low hydrogen diffusion rates.

(b) Hydrogen Embrittlement

Hydrogen embrittlement is the penetration of hydrogen into a metal, which causes it to become brittle and lose its tensile strength. This is often seen in high-strength steels and can be caused by dissolved hydrogen reacting with hydride-forming metals (like titanium) to create brittle hydride compounds. The buildup of hydrogen near micro-voids and dislocation sites can interfere with the material's slip mechanisms. Cracking can occur with just a few parts per million of absorbed hydrogen.

Prevention: You can prevent hydrogen embrittlement by:

• Reducing Corrosion: Decrease the overall corrosion rate to lower the rate of hydrogen evolution.
• Baking: Heat the steel at relatively low temperatures to bake out and remove the absorbed hydrogen. This process is often reversible.
• Altering Plating Conditions: Carefully select plating baths and control the current during electroplating to avoid hydrogen evolution.
• Proper Welding: Maintain dry conditions and use welding rods with low hydrogen content, as water and water vapor are sources of hydrogen.
• Substituting Alloys: Use alloys that are less susceptible, such as steels alloyed with molybdenum and nickel.

(c) Decarburization

Decarburization is the high-temperature removal of carbon from steel. This process typically occurs in moist, high-temperature environments. When carbon is removed from the steel, it loses its tensile strength. It is a form of hydrogen damage caused by a high-temperature hydrogen attack.

Prevention: To prevent decarburization, you must control the sources of nascent hydrogen. The general prevention methods for hydrogen attack apply, which include using appropriate alloys and controlling the high-temperature, moist atmosphere.

(d) Hydrogen Attack

A hydrogen attack is the interaction between hydrogen and a constituent of an alloy at high temperatures. In steel, this high-temperature interaction can lead to decarburization. Atomic hydrogen reacts with the carbon in the steel to form methane gas (CH4​). The methane gas cannot diffuse out, leading to internal pressure buildup and cracking, similar to hydrogen blistering. This process degrades the mechanical properties of the steel.

Prevention: The primary prevention method is to use alloys that are resistant to hydrogen attack. The Nelson Curves are a widely used industry standard for selecting materials based on operating temperature and hydrogen partial pressure to avoid this type of damage.