12.2.1 Concept, Mechanism, Detection, and Prevention

Since the early ages of industrial usage of hydrogen, the selection of appropriate materials has been a concern for industrial gas operators. These operators have paid special attention to the issues raised by the specific interaction of this molecule with metallic materials, specifically with steel, in order to ensure safety and reliability of hydrogen from manufacture to cylinder filling, to distribution, and to usage at the customer site.

Hydrogen energy development will face the same constraints of material compatibility, with even stricter requirements, as the equipment will be owned and partially operated by a wide range of nonspecifically trained users and will be operated at very high pressures, 700 bar or more.

The phenomenon of internal hydrogen embrittlement due to excessive amounts of hydrogen introduced during the manufacture of steels has been known since the end of the nineteenth century. The phenomenon of internal hydrogen embrittlement when welding steels was discovered later. These two problems are now well mastered and are not covered here. This chapter only concerns the phenomenon of external hydrogen embrittlement caused by pressurized gaseous hydrogen in contact with steels, essentially at high temperatures. The hydrogen penetrates into the steel (during operation in the case of pressurized tanks) and diminishes local or overall mechanical properties of the steel. This can lead to bursting of these pressurized tanks under certain conditions.

The hydrogen that penetrates into steels can be found either in metallic solution or in combined state (H₂, CH₄ molecules).

When hydrogen is found in metallic solution, the phenomenon of steel deterioration is called gaseous hydrogen embrittlement; this phenomenon generally takes place at temperatures close to ambient, and the penetration or transport of the hydrogen takes place essentially by transport by dislocations when the material is undergoing deformation. This phenomenon therefore occurs essentially in areas with local plastic deformations.

When hydrogen is present in a combined state, it is a matter of hydrogen attack (HA) or hydrogen damage. The hydrogen reacts with the carbon in the steel to form molecules of methane; this leads to the formation of microcavities in the steel and to a lack of carbon in the steel, which lead to a reduction in the overall strength of the material. Hydrogen is therefore transported by diffusion. This is why this phenomenon mainly takes place at high temperature. In addition, contrary to the phenomenon of hydrogen embrittlement, this phenomenon is reversible as long as microcavities have not formed.

One of the most important parameters to be considered here is temperature; and, indeed, hydrogen embrittlement of steels mainly occurs around ambient temperature and tends to disappear at high temperature. On the contrary, HA only takes place at high temperature. Depending on whether the operating temperature is higher or lower than 300 °C, one or the other of the two phenomena mentioned above should be taken into account. In other words, HA is not to be confused with hydrogen embrittlement or other forms of low temperature hydrogen corrosion. Surface decarburization and internal decarburization are important effects of HA.

Surface decarburization results in a decrease in hardness and increase in ductility of the material near the surface. This is usually only a minor concern for these types of application. However, internal decarburization, and in particular the formation of methane and consequent development of voids, can lead to substantial deterioration of mechanical properties due to loss of carbides and formation of voids and catastrophic failure.

The main factors influencing HA are the hydrogen partial pressure, the temperature of the steel, and the duration of the exposure. Damage usually occurs after an incubation period, which can vary from a few hours to many years depending on the severity of the environment. High temperatures and low hydrogen partial pressures favor surface decarburization, while the opposite conditions (lower temperature, high hydrogen partial pressure) favor fissuring. In addition, the composition of the steel influences the resistance to HA; in particular elements that tie up carbon in stable precipitates, such as Cr, Mo, and V, are very important. Increasing content of such elements increases the resistance to HA, and Cr–Mo steels with more than 5% Cr, and austenitic stainless steels, are not susceptible to HA.

Aside from Cr and Mo, other elements that tend to bind carbon in the form of stable metallic carbide, such as Ti, W, etc., have the same favorable effect.

On the contrary, a high carbon content is detrimental. Similarly, the addition of Al, Ni, or excess of Mn has a detrimental effect on the behavior of welds.

The residual elements that lead to the formation of nonmetallic inclusions have a detrimental effect. To prevent risks of HA, stress relief treatment at temperatures equal to or greater than 650 °C is needed. These types of treatment are covered by standards and/or should be carried out in accordance with the stipulations of the local code or the code of the manufacturer.

It also appears that the risk of HA increases when the level of mechanical stresses increases. The hydrogen partial pressure should not be considered alone. Total pressure and additional stresses during operation or residual stresses, if not completely eliminated by heat treatment, should also be taken into account. In addition to the treatment following welding recommended above, it is better to limit the hardness of the weld bead and the heat affected zone.

The operating limits for steels can be empirically described using the operating temperature and hydrogen partial pressure, as originally discussed by Nelson (1949) and in the American Petroleum Institute (API) recommended practice 941, “Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants” (Das 1996; Parthasarathy 1985).

Since the 1970s, empirical data have been collected from operating plants and tests to establish operating limits of carbon steel and low‐alloy steel equipment in hydrogen service at elevated temperatures. API 941 provides guidance on those limits.

Using API 941, if a piece of equipment or piping is operated above the API 941 Nelson curve, then the material is not suitable for service under those conditions. For example, if the normal operating conditions are a temperature of 488 °C (900 °F) and 2.23 MPa (323 psig) hydrogen partial pressure, as illustrated in Figure 12.1, then the carbon steel in this case and even the 1.0Cr–0.5Mo steel are not suitable for service under those conditions. There would be a risk of premature failure in a relatively short time of exposure. Either the temperature or the pressure would have to drop below the carbon steel curve, or chromium alloyed steel should be considered for use instead. The selection of a 1 1/4Cr–1/2Mo material would be the preferred choice.

Using API 941, the following practices should be considered:

1. Selecting the proper material for the operating conditions, and for increased temperatures, considering the use of alloys with higher weight percents of chromium and molybdenum.
   
2. Using actual operating temperatures for assessing HA susceptibility and validating that the actual operating temperatures and pressures are below API 941 curve by a defined amount.
   
3. Employing experienced individuals who understand the HA phenomenon as well as the API 941 recommended practices.

For corrosion purposes, sometimes vessels are clad, lined, or weld overlaid to protect the vessel surface. This can provide initial protection, provided hydrogen does not diffuse through the liner or migrate behind the lining or cladding. If that occurs, then the vessel may be susceptible to HA.

Refractory lining is often used to insulate a pipe or vessel to lower the metal wall temperature and is an effective way to reduce the effects of HA. However, the refractory can degrade, crack, or deteriorate due to operating conditions or even flexure of the refractory, allowing hot spots to form, which would elevate the metal wall temperature and possibly result in exceeding the HA operating limits of the equipment. Figure 12.2 illustrates how a degraded refractory hot spot could result in exceeding the operating temperature limit for a carbon steel line.

One way to monitor the condition of the refractory is to perform regular infrared imaging of the equipment.

For clad, lined, or overlaid equipment, the following practices should be considered:

1. Ensuring that proper foundation support for refractory‐lined equipment is in place to reduce flexure of the refractory.
   
2. Performing regular infrared inspections, especially on the refractory‐lined equipment.
   
3. Ensuring that the operating limit is understood and appropriate actions are taken if the limit is exceeded.

There is increasing concern that the Nelson curves may not be relevant for the newer steels being used in high temperature hydrogen service, or may be overly conservative, and there are increasing trends toward risk‐based inspection of items in hot hydrogen service.

In this context, unsatisfactory service experience with the carbon–1/2Mo steel has led to consideration of the Nelson curves. Currently, API 941 warns against new construction with the alloy and urges inspection and monitoring of existing equipment.

There are a number of inspection methods available. Most of them are based on ultrasonics:

1. Ultrasonic echo attenuation method.
   
2. Amplitude‐based backscatter.
   
3. Velocity ratio.
   
4. Creeping waves/time‐of‐flight measurement.
   
5. Pitch‐catch mode shear wave velocity.
   
6. Ultrasonic method based on backscatter and velocity ratio measurement.
   
7. AUBTs – advanced ultrasonic backscatter techniques.
   
8. Method based on time‐of‐flight diffraction (TOFD), thickness mapping, backscatter, and velocity ratio.
   
9. In situ metallography – replicas.

The overview of these techniques is given in Table 12.1.

In summary, in this section we have introduced the concept of HA and described the influence of various parameters on HA of materials, namely, steels. In order to safely use such materials in the presence of hydrogen, it is recommended to have an internal specification. This specification must cover:

• The “scope,” i.e. the hydrogen pressure, the temperature, and the hydrogen purity.
  
• The “material,” i.e. the mechanical properties, chemical composition, and heat treatment.
  
• The stress level of the equipment.
  
• The surface defects and quality of finish.
  
• The welding procedure, if any.

The two main sources of hydrogen that contribute to HA of steels in industries are from the corrosion of steel by boiler water in the waterwall tube for coal‐fired boilers and from high pressure, high temperature hydrogen‐containing atmospheres used in petroleum refining. HA problems related to these two separate areas are briefly discussed in Section 12.2.2.

12.2.2 Attack in Coal‐Fired Boilers and Petroleum Refining

Subcritical drum boilers that use a recirculating steam‐generating system are prone to high temperature corrosion, particularly if the water chemistry is not properly controlled. Control requires (i) removal of impurities to purify the water and (ii) chemical treatments to control pH, electrochemical potential, and dissolved oxygen concentration. With adequate control, the waterwall steel tube forms a protective magnetite (Fe₃O₄) scale when steel is corroded by water under normal operating conditions (Cohen 1989):

12.1

whose by‐product is hydrogen. Growth of magnetite follows a parabolic rate law with the scale growth rate diminishing with time. Accordingly, the generation of hydrogen likewise diminishes with time once a steady state is reached. The atomic hydrogen produced combines with other hydrogen atoms to form molecular hydrogen that is then dispersed in the boiler water as a gas or in solution. However, when water chemistry is poorly controlled, the boiler deposits formed on the surface of the waterwall tube may lead to acidic corrosion attack on the tube under the deposits, leading to large atomic hydrogen absorption by the steel. Hydrogen atoms in the steel then react with iron carbide (Fe₃C) to form methane (CH₄):

12.2

Methane gas then accumulates at grain boundaries and other interfaces due to its low diffusivity. Microcracks and microfissures are eventually developed by increasing local gas pressure created by the increasing amount of methane gas produced by the iron carbide/hydrogen reaction. Furthermore, because of iron carbides being reduced to iron by reaction 12.2, the affected area is decarburized. The locations that are often susceptible to HA in the boiler are the burner zone and the bull nose area (Stultz and Kitto 1992). Monitoring and controlling boiler water chemistry is critical in preventing internal tube deposits and HA (Dooley 1987).

HA is also a serious materials issue in the design and operation of refinery equipment, such as reactors in hydrotreating, reforming, and hydrocracking units. The mechanism for HA of steels in petroleum refining is essentially the same as that described in the waterwall tube that suffers HA in the boiler. As discussed earlier, adding Cr and/or Mo to the steel to increase the stability of iron carbides can increase the resistance of the steel in HA. But concerns have been raised as discussed by Parthasarathy (1985). Thus, Cr–Mo steels are much more resistant to HA than carbon and C–0.5Mo steels. The conditions under which carbon and Cr–Mo steels can be used in high temperature hydrogen service are described in detail in API 941 (Parthasarathy 1985). The behavior of carbon and Cr–Mo steels with respect to their resistance to HA is summarized in Nelson curves, as shown in Figures 12.1 and 12.2 (Chiba et al. 1985; Merrick and Ciuffreda 1982; Sorell and Humphries 1978; Turnbull 1995).

12.3.1 Kinetics and Mechanisms of Oxidation

It has been suggested that the presence of water vapor affects the oxidation behavior of metals; therefore, the morphology of the scales is also affected (Rahmel and Tobolski 1965). Depending on the type of metal, the oxidation is, in some cases, enhanced or does not change the oxidation rates of alloys compared with the oxidation in dry air. For example, iron oxidizes 1.6 times faster at 950 °C in H₂/H₂O and O₂/H₂O mixtures than in dry air. The oxidation rate of a MCrAlY coating is 25 times greater in H₂/H₂O than in dry air at 1080–1100 °C (Leyens et al. 1996). The growth rates of chromia formers are proved to be higher in moisture‐containing gases (Schütze et al. 2005; see also Section 7.7.2). However, water vapor has generally little effect on alumina‐forming alloys (Saunders et al. 2008). The oxidation rate of Fe–21.5Cr–5.6Al is even slightly decreased in wet air compared with dry air at 1000 °C, whereas the growth rate of PM 2000, a mechanical alloyed material, is slightly increased during isothermal oxidation. Moisture has little effect on the oxidation of commercial Ni‐based superalloys and coatings, as shown by Onal et al. (2003), but is detrimental for the scale adhesion.

The presence of water vapor affects the selective oxidation of species responsible for the formation of a protective scale. Kvernes et al. (1977) reported a decrease in the duration of the initial oxidation stage with water content in the ambient gas and an increase in the subsequent reaction rate under the same conditions for the high temperature oxidation of Fe–13Cr–xAl alloys. Though thicker scales formed, they were also less protective, including spinel in the inner part of the scale. Buscail et al. (1997) found that the water vapor decreases the isothermal oxidation rate of Fe–Cr–Al at 1000 °C. Although the effect was not significant, the study proposed that the initial transient oxidation processes were affected by the wet conditions and that water vapor might have significant effects on the cyclic oxidation of certain alloys.

It is well known that most technical steels oxidize faster in water vapor or in air or combustion gases containing water vapor than in dry air, but, according to Kofstad (1988), the mechanisms are not well understood. Therefore, it is important to know the composition and defect structure of the surface as they play an important role in the diffusion process.

Anghel et al. (2004) found that upon addition of water to CO gas, the dissociation rate of CO on Cr decreased, and after subsequent removal of water, the dissociation rate increased again. In this case, the surface activity for CO dissociation should be a key factor for carbon uptake in certain applications and can be reduced by adsorbed water. These results suggest the following adsorption ranking: N₂ < H₂ < CO < H₂O. Increased dissociation of oxygen (O₂ → 2O) and complete dissociation of molecular water (H₂O → 2H + O) on surfaces of metal–oxide promote an increased transport of both O and O²⁻ in the oxide due to increased concentration gradient of O and O²⁻ over the oxides (Hultquist 1997).

Cation sites are Lewis acids and may interact with electron donor molecules, such as water that has a lone pair of electrons, while oxide ions may act as basic sites interacting with acceptor ions, such as proton to produce a hydroxyl group. Where selective oxidation occurs, oxygen may be added to the adsorbate, not as O²⁻, but as a neutral oxygen atom. There will be a corresponding reduction of the substrate in the form of electrons that might be free carriers, but more often this will lead to a localized decrease in the oxidation state of the metal at the surface. Hydrogen is a nonpolar molecule with low polarizability and has a weak donor or acceptor properties. Water has a large dipole moment and lone pair of electrons and is therefore a good donor. Adsorption occurs by acid–base process through interaction with metal ions; non‐dissociative molecular adsorption also occurs. Oxygen is a very powerful electron acceptor and can be reduced in several steps. Nitrogen and argon are generally inert, as expected.

The presence of water in the oxidizing atmospheres causes the formation of various defects, such as pores and whiskers. Whiskers are generally formed on the surface at the end of a dislocation by diffusion through a hollow void (Raynaud and Rapp 1984). This process is possible due to faster dissociation of water compared to oxygen, which leads to surface reaction as rate determinant for their growth and therefore their linear kinetics; however, as the oxide thickens, diffusion becomes the rate‐controlling mechanism. Rahmel and Tobolski (1965) stated that pores occur at the iron/wustite interface during the oxidation of iron at 850 °C. In the presence of water vapor or carbon, a H₂/H₂O or CO/CO₂ mixture is formed, respectively, in these pores, which transports oxygen to the iron surface by an oxidation/reduction mechanism. Therefore, oxide bridges are built upward, from the metal to the scale, which enables the further oxidation of the metal without substantial inhibition. For this mechanism to take place, water vapor should penetrate the scale, and this occurs through microcracks or by proton transport. The pores are generally formed at the scale/oxide interface due to vacancies coalescence, especially for scale growing by outward transport of cations. The enhanced plasticity (ability to creep), possibly due to the incorporation of hydrogen in the lattice observed by Tuck et al. (1969), allows the scale to maintain the contact with the substrate. Quadakkers et al. (2005) showed that the scale spallation of oxides formed on Fe/12–19% Cr in steam is accompanied by the formation of a rapidly growing magnetite layer and an inner scale consisting of Cr₂–O₃ precipitates in a FeO matrix, whereby the two layers are separated by a gap due to pores. Additionally, molecular gas transport occurs through the outer scale. As the overall scale thickness increases, the Fe activity at the scale/gas interface gradually decreases due to an increasingly difficult transport of Fe cations to the oxide surface due to the presence of the large gap that develops. The purity of the alloy and exposure temperature plays an important role as they determine how the vacancies coalesce within the scale and/or at the interface scale/metal to form pores.

Porosity is observed at metal/scale interface during oxidation in dry air of chromia‐formed alloys, while the pores are distributed through the entire scale. The effect of water vapor on transport mechanisms within chromia scales is to increase cation vacancies, and thus chromium diffusion is enhanced, leading to vacancy condensation and pore formation. At the same time, however, there is an increase in inward diffusion of oxidant due to the effective diffusion of OH⁻, and Henry et al. (2000) suggest that the pores become incorporated in the growing scale. If wet gas is exchanged with dry gas during the oxidation, the scale remains permeable, indicating that water vapor is preferentially adsorbed in the internal surfaces, preventing further reaction with oxygen, as well as closure, as observed by Ehlers et al. (2006) during the oxidation of 9% Cr steels.

12.3.2 Protons Incorporation

Oxidation of metals in water vapor‐containing atmospheres at high temperature is accompanied by the formation of hydrogen defects, which are mainly dissolved protons (Norby 1993; Tveten et al. 1999). The reaction can be written as

12.3