Carbon and low-alloy steels are common structural materials in high-pressure hydrogen fuel

tanks and pipelines. These steels are inexpensive, and a wide range of properties can be

achieved through alloying, machining and heat treatment. Making complex structures such as

fuel tanks and pipes is easy using steel because these materials can be formed, welded and

heat treated in large quantities. amounts parts. Isolating and transporting high-pressure

hydrogen gas in steel structures is a special challenge. Gaseous hydrogen can be

adsorbed and dispersed on the steel surface, producing atomic hydrogen. The subsequent

dissolution and diffusion of atomic hydrogen into steels can weaken mechanical properties

commonly called hydrogen embrittlement. The appearance of atomic hydrogen. hydrogen

embrittlement is an increased tendency to fracture. Hydrogen reduces typical parameters of

fracture toughness such as tensile strength, toughness and fracture toughness, accelerates

fatigue crack propagation and increases material failure modes. In particular, steel structures

that do not degrade under static electrical load in benign environments numerical

temperatures can become sensitive to time dependent crack propagation in hydrogen gas.

Industrial gas companies have decades of experience with hydrogen

transmission pipelines and currently operate more than 1000 miles of pipelines in the United

States and currently Europe. These pipelines have proven safe and reliable for certain

materials, environmental and mechanical pipelines, to the conditions. Even though hydrogen

can be an alternative for conventional fuel and environmentally friendly it doesn’t mean it

doesn’t pose any issues or catastrophise. The transportation of hydrogen comes with many

issues to the pipelines that is transporting it. At the moment in most countries are repurposing

the pipelines that are used for the transportation of natural gas for the transfer of hydrogen

across regions. In this section we’ll be discussing about Hydrogen Embrittlement in pipelines.

A known mechanism in this category is "hydrogen attack", which involves a chemical

reaction between hydrogen and carbon in the steel to produce methane. The generation of
high-pressure methane gas in internal cracks and the depletion of carbon in the steel allow

damage to the material. Other mechanisms not mentioned in this chapter include

an internal deposit of high-pressure hydrogen gas. Damages caused by the internal

generation of methane or hydrogen gas are not considered for steel structures used to

store and transport high-pressure hydrogen gas.

Gas pressure affects the severity of hydrogen embrittlement in steel because this

variable determines the amount of atomic hydrogen dissolved in the steel. The operating

pressure of steel vessels in hydrogen distribution applications is typically in the range of 20–

30 MPa. The inner surface of hydrogen fuel tanks is prone to local corrosion due

to possible impurities of steel and hydrogen gas. In addition to gas pressure,

hydrostatic tension increases the hydrogen content of metals. This leads to a high local

concentration of atomic hydrogen in voltage spikes, such as faults, which

promotes hydrogen embrittlement.

Although steel pipes were safely operated with hydrogen gas, special limitations were placed

on the properties of the steels. Relatively low strength carbon steel is specifically

designed for hydrogen gas piping.5 Examples of steels proven for hydrogen gas are ASTM

A106 Grade B and API 5L Grade X42 and API 5L Grade X52. API 5L steels containing

small amounts of niobium, vanadium and titanium are called "microalloyed"

steels. Microalloyed X52 steel has been widely used in hydrogen gas tubes. Steels for

hydrogen gas pipelines are machined to obtain uniform fine-grained

microstructures. Normalizing heat treatment can be used to obtain the desired microstructure

in ordinary steels. A typical normalizing heat treatment consists of heating the steel in the

austenite phase field, followed by air cooling. Fine-grained micro-alloy steel is produced

using a fine-grained hot rolling process in the austenitic ferrite phase field. Material strength

is an important variable affecting hydrogen embrittlement of pipeline steels. One of

limitation. The recommended maximum tensile strength of steel for Hydrogen gas

pipes is 800 Mpa.

Hydrogen Induced Blistering

Hydrogen induced blistering or hydrogen induced corrosion is one of

several interrelated mechanisms by which adsorbed hydrogen atoms can damage the

integrity of tubular steels. The formation of internal cracks line pipe steels due to the

absorption of hydrogen atoms (H atoms) remaining as gas molecules (H2 molecules)

at inhomogeneity is called HIC. The cracks are mostly (but not always)

parallel to the surfaces of the rolling surface and part of the steel. No residual or

applied tensile stress is required to develop HIC, Figure . HIC is usually associated with non-

metallic inclusions such as elongated manganese sulphides (MnS), Figure .

Figure : Hydrogen induced cracking of carbon steel pipeline showing (a) HIC; (b) crack at

MnS inclusion location and (c) near the pipe surface rough convex bumps called “blisters”.

When the hydrogen concentration exceeds a critical value due to hydrogen penetration,

hydrogen-induced cracking occurs, as shown in Figure 8a. It is known that cracking in

H2S media is related to the presence of non-metallic impurities, especially MnS and

bands. Interfaces between large MnS inclusions and/or cluster structure

and the matrix generally act as hydrogen sinks (Figure ). Hydrogen tends to fuse with these

interfaces and reach a critical value. Since cracks are initiated by elongated
MnS particles, HIC sensitivity decreases with decreasing sulphur content. Low temperature

rolling increases HIC sensitivity increases sulphur attachment. The increased sensitivity

maybe due to an interfacial reaction between the steel and the wet H2S medium.

(a) (b)

Figure: (a) Hydrogen blister in NPS 6 sour gas pipeline (Canada 1997 Failure); (b) Blistering

and through-wall crack in seamless line pipe.

When atomic hydrogen flows into the steel, bubbles form in microscopic cavities around the

non-metallic inclusions. Burst occurs when hydrogen – induced cracks fail to align in a

direction parallel to the surface and connect to HICs in adjacent planes of the steel figure b.

Its probably due to the non-uniformity held by the hydrogen atoms that the bubbles are short

lived figure a.

A special form of HIC occurs when there is a high local stress concentration in sour service

pipes, called Atmospheric Water Induced Hydrogen Cracking (SOHIC).

Cracking from Precipitation of Internal Hydrogen

en damage occurs in manufacturing, welding and casting, and is caused by the introduction of

hydrogen into the molten metal. Hydrogen sources are the main form of hydrogen through

reactions between metals and moisture in the environment. This source of hydrogen and the p

otential for hydrogen-induced post weld damage is one of the reasons why electrodes are

dried by storage and heating in low temperature ovens.

Figure : Cracks on the tensile specimen strained in the presence of high pressure hudrogen.

Type 304L stainless steel tested in 10000 psi Hydrogen at room temperature. The ¼”

specimen was not exposed to hydrogen prior to initiation of the tensile strength test.

The solubility of hydrogen decreases significantly during drying, and the hydrogen in the por

es, pores and other internal surfaces decreases. As the pressure in the hydrogen flow zone
increases, so does the stress at the pore end. The presence of hydrogen at high pressure on the

surfaces of many metals and alloys causes surface defects to develop macroscopically. The

tips cannot be blunt while being there. And the lack of bending makes these sharp tips to

extend to the metal. This is because ferrite and martensitic steel, the spread generally occurs

along prior austenite grain boundaries forming an intergranular appearance to the fracture

site. The pressure drops as the void increases in size.

The fisheye reduces the cross section of the metal, provides maximum stress and can act asa s

tarting point for fatigue. Protecting the metal from moisture is important to prevent

fisheyes from forming. This can be done by chemical waste handling, vacuum melting and

using a dry environment. Fisheye can be prevented even if hydrogen is present, by heating

the metal slightly above room temperature and by letting the hydrogen slowly exit the metal.

Hydrogen Attack

This happens when the absorbed hydrogen reacts with alloying, or impurities present in the

microstructure to form an insoluble, mostly gaseous phase. Two examples are that of copper-

oxide inclusions in copper and hydrogen interactions with carbides in steel. The exposure of

steel to high temperature and high-pressure hydrogen can change the microstructure, reduce

the strength and eventually cause failure. The degradation in mechanical terms only occurs

after a gestation period during which little to no change in microstructure has happened.

However, the decarburization occurs as soon as the steel is exposed to hydrogen at elevated

temperatures. This reaction takes place between hydrogen present and the carbon in the steel.

The reaction product is methane. This cause decarburization, and if continued will reduces

the carbon content in the steel to the point carbides like Fe 3C in pearlite, begins to dissociate.

The reaction also leads to the steel absorbing the hydrogen, which is very mobile and starts to

accumulate in the pores and micropores. There the hydrogen molecules recombine and react

with carbon to form methane. The reaction:

Coupled with cementite decomposition,

Fe3C = 3Fe + C

Thermodynamically favours the formation of methane when the temperature goes beyond

200 degrees Celsius. Moreover, methane may also be formed by direct interaction between

hydrogen and carbide particles.

Figure : Intergranular cracking and rupture due to hydrogen attack.