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 Understanding Hydrogen Failures
The effects of hydrogen can be lessened or even eliminated through careful selection of base metals, weld metals, and fabrication practices, as well as adherence to procedures
BY J. R. STILL
Fig. 1 —If hydrogen is introduced, the piping systems on this floating production storage and offloading vessel can be susceptible to corrosion and corrosion-assisted cracking.
The ingress of hydrogen into metals can occur in several ways depending on material type, the presence of hydrogen in the form of atomic hydrogen, and factors such as the solubility of atomic hydrogen, the pressure of hydrogen, and stress (Refs. 1, 2).
Carbon and carbon-manganese steels and weld metals are susceptible to hydrogen ingress from several sources. Two primary sources are the ingress of atomic hydrogen into the piping material and weld metal resulting from the processing of reservoir fluids and the ingress of hydrogen resulting from the presence of moisture during welding of process piping.
The first route involves separating reservoir fluids into oil, gas, and formation water. Figure 1 shows an offshore processing facility on a floating production storage and offloading (FPSO) vessel. The FPSO facility consists of piping systems linked to equipment associated with the separation process. Failures linked to the ingress of hydrogen in process piping systems are associated with corrosion or corrosion-assisted cracking. One of the products of corrosion is hydrogen.
The second route is associated with the introduction of hydrogen into the weld pool during welding (Refs. 3, 4). In this instance the failure mechanism results from the breakdown of moisture, where hydrogen is absorbed by the weld pool from the arc atmosphere. The failure mechanism is then similar to that experienced in the processing of reservoir fluids.
Corrosion and Corrosion-Assisted Cracking
Depending on the reservoir composition, compounds such as carbon dioxide (CO2), hydrogen sulfide (H2S), or a combination of both can be present in hydrocarbons. The terms "sweet" and "sour" are used in the oil and gas industry to identify hydrocarbons that contain CO2 and H2S, respectively. Carbon dioxide and hydrogen sulfide in the presence of water when subjected to pressure and temperatures during separation are two main contributors to corrosion of process facilities and piping systems. Following are examples of corrosion failure types experienced during processing of hydrocarbons.
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Weight Loss Corrosion
Fig. 2 —An example of weld metal failure due to weight loss corrosion.
An example of weight loss corrosion (Refs. 5, 6) is illustrated in Fig. 2. This form of failure is mainly attributed to carbon dioxide and water forming carbonic acid. The presence of carbonic acid (H2CO3) reduces the pH of the water, resulting in localized corrosion such as pitting. A similar effect occurs with hydrogen sulfide except that the reaction produces iron sulfide, which is unprotected and easily removed. One of the products of this reaction is atomic hydrogen. The reactions for H2CO3 and H2S are outlined below: |
H2CO3 Reaction:
Fe+H2CO3->FeCO3+H2
H2S Reaction:
Fe+H2S+H2O->FeS+2H
Environmental Cracking
Fig. 3 —Environmental cracking in the weld HAZ.
The term "environmental cracking" (EC) is used to describe sulfide stress cracking (SSC) and stress corrosion cracking (SCC) (Ref. 7). An example of environmental cracking (Ref. 8) in the heat-affected zone (HAZ) of a weld is illustrated in Fig. 3.
Materials and weld metals subjected to mechanical stresses such as thermal processing, residual stresses, and manufacturing practices within a low pH production fluid that contains hydrogen sulfide can experience cracking. Hydrogen sulfide dissociates into atomic hydrogen and ferrous sulfide.
Atomic hydrogen penetrates the weld metal, pipe material, or HAZ lattice structure and, when subjected to mechanical stress, local embrittlement occurs. Examples of environmental cracking include the following:
Cracking in an H2S Environment
Fig. 4 —Hydrogen-induced cracking in A333 Grade 6 pipe material used in an H2S environment.
Cracking associated with a sour environment is classified as hydrogen-induced cracking (HIC) (Ref. 9).
Failure in this instance is similar to environmental cracking (Ref. 10), where hydrogen sulfide dissociation into atomic hydrogen penetrates the weld metal, pipe material, or HAZ lattice structure. In this instance, the hydrogen atoms encounter nonmetallic inclusions and then the hydrogen recombines to form molecular hydrogen. This process results in the buildup of molecular hydrogen, with an increase in pressure within the nonmetallic inclusion. This continues until the pressure buildup is sufficient to initiate fracture. A number of factors such as pH, volume of hydrogen diffused, volume-fraction, and the shape of inclusions present, and the surrounding microstructure influence the process. Stress in this instance is not as critical compared with SCC and SSC (Ref. 7). Failure types include the following:
- Hydrogen-induced cracking (HIC)
- Stepwise cracking (SWC)
- Stress-oriented hydrogen-induced cracking (SOHIC)
- Hydrogen blistering
Figure 4 provides an example of hydrogen-induced cracking in which the material contained a high-volume-fraction of inclusions. It is recommended (Ref. 11) that sulfur levels of materials for wet H2S service be controlled at 0.002% maximum to avoid HIC. Additions of inclusion shape control elements, such as calcium or rare earth elements (Ref. 12), are used to prevent HIC.
Preferential Weld Corrosion
Fig. 5 —Schematic of the cathodic reactions that can occur in seawater injection systems, which result in preferential weld corrosion.
While not directly associated with separating hydrocarbons, seawater injection systems are used for injecting seawater into the reservoir to maintain pressure. They are susceptible to preferential weld corrosion (Ref. 13). This form of corrosion is the result of galvanic coupling, whereby the weld metal acts as an anode and the pipe material as a cathode (Ref. 14). The anodic and cathodic reactions involved are outlined in Fig. 5.
The corrosion reaction involved produces hydrogen, which reduces the pH of the seawater resulting in an increase in the acidity.
Fig. 6 —Preferential weld corrosion in A106 Grade B pipe.
Oxygen content of the seawater also has to be present to promote formation of ferrous oxide, which is readily removed when the velocity of the seawater is in the region of 2.5 m/s. The process continues until the outer surface of the weld is penetrated. An example of this form of corrosion is illustrated in Fig. 6. To avoid preferential weld corrosion, the oxygen content of the seawater has to be low and the weld deposit and HAZ must behave cathodically with respect to the pipe. This is achieved by selecting an electrode of suitable composition that behaves in a cathodic manner.
Hydrogen Cracking Associated with Welding Process Piping
Cracking associated with corrosion-assisted cracking such as HIC and SWC is similar to other forms of hydrogen cracking. This involves weld metal and HAZ cracking encountered during welding if procedures are not adhered to (Ref. 15). Types of weld metal and HAZ cracking attributed to hydrogen include the following.
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Heat-Affected Zone Cracking
Fig. 7 —An example of hydrogen-induced cracking in a weld heat-affected zone.
An example of HAZ cracking of a pipe HAZ is illustrated in Fig. 7. The HAZ cracking mechanism (Ref. 16) is similar to environmental cracking. Factors influencing HAZ cracking in carbon and carbon-manganese steels include the following: |
Hydrogen.The source of hydrogen is the moisture retained within the electrode coating. Baking of electrodes to ensure the moisture is removed prior to welding is essential. Other potential sources of hydrogen involve paint and grease and failure to remove moisture from the weld area prior to welding. Material hydrogen levels can also contribute to HAZ cracking.
Chemistry.The chemical composition provides a guide, through the carbon equivalent, to the level of preheat required to ensure that the HAZ microstructure hardness levels are within the NACE MR0175 requirements.
Preheat temperature. The preheat temperature applied must be capable of ensuring that the time to cool between øt 800 to 500°C produces acceptable microstructure and hardness levels.
Heat input. Heat input is determined by the welding parameters. Low heat inputs in the absence of preheat will produce high HAZ hardness levels. It is essential that the interpass temperature is not allowed to reach temperatures that affect the øt 800 to 500°C cooling rate and thus the properties of the material.
To avoid hydrogen cracking in welds and HAZs, the controls listed above must be strictly adhered to.
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Fisheyes
Fig. 8 —An example of a fisheye.
Fisheyes (Ref. 16) are a form of hydrogen embrittlement that is confined to as-welded structures. Hydrogen can be retained within a weld defect such as a pore. When a weld joint tensile test is subjected to a slow strain rate, necking of the tensile specimen starts. The hydrogen retained within the pore and surrounding area is subjected to a further increase in the applied strain, resulting in local embrittlement. When the tensile specimen fractures, the pore appears as a brittle area with a ductile structure, as illustrated in Fig. 8. This form of hydrogen failure is found mainly in tensile specimens from procedure pipe welds that have not been subject to the controls listed above. |
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Other Forms of Hydrogen Cracking
Fig. 9 —An example of chevron cracking.
The following is a brief description of types of hydrogen cracking associated with fabrication of process piping, pressure-retaining facilities, and structural fabrications: |
Chevron cracking. This type of cracking (Ref. 17) has never been known to occur in process piping and is mainly confined to thick-wall multipass welds deposited in carbon and carbon-manganese steels. The cracking mechanism is attributed to poor housekeeping of the submerged arc flux, which has been allowed to absorb moisture. During welding, the moisture dissociates into hydrogen and oxygen. The oxygen may form oxides within the weld, or recombine and evaporate. Hydrogen becomes trapped as the weld metal cools and, if sufficient hydrogen is present, cracking can occur. The cracks take the shape of a chevron, hence the name chevron cracking as illustrated in Fig. 9.
Fig. 10 —An example of lamellar tearing. The type of failure associated with hydrogen has been nearly eliminated since the introduction of "clean" steels with low-sulfur contents.
Lamellar tearing. The existence of lamellar tearing is widely accepted as being the result of weld shrinkage stress acting through the thickness direction of steels with high inclusion contents (Ref. 16). However, prior to the introduction of clean steels in the 1970s, the diffusion of hydrogen from deposited weld metals was recognized as one of the factors influencing lamellar tearing (Ref. 17). Research carried out in 1972 (Ref. 18) reported that lamellar tearing was influenced by a combination of contracting stress, presence of nonmetallic inclusion, and hydrogen diffused from the weld metal. Hydrogen in this instance acts in a similar way to that reported for cracking associated with an H2S environment. It was concluded that inclusion content was the main factor and must be limited if lamellar tearing was to be controlled. Figure 10 shows an example of lamellar tearing.
Lamellar tearing has virtually been eliminated due to the production of clean steels with sulfur levels below 0.005%.
Summary
The selection of suitable materials and weld metal on new and mature installations is now common practice, provided the fluid or gas composition, in conjunction with operating conditions, warrants their selection. Provided the material and weld metal selection is correct, the effect of corrosion can be stemmed and in some instances eliminated.
J. R. STILL (still@wcmltd.freeserve.co.uk) is a Consultant Welding/Materials Engineer in Aberdeen, Scotland.
Acknowledgments
The author would like to thank Dr. D. Shaw for commenting on the preparation of this paper and Dr. K. Prosser of Macaw Engineering for supplying Figs. 3, 4, and 9, and Mr. Ian Bradely of Oceaneering OIS Aberdeen for Fig. 2.
References
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Materials Park, Ohio: ASM International.
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- Bailey, N. Weldability of Ferritic Steels. Cambridge, England: Abington Publishing.
- Wright, V. S., and Davison, I. T. 1979. Chevron cracking in submerged arc welds. Metal Construction (3): 129-133.
- Still, J. R. 1995. Lamellar tearing —A ghost from the past. Welding and Metal Fabrication (11/12): 445-448.
- Nishio, Y., Yamamoto, Y., Kajimoto, K., and Hirozane, T. 1972. On the lamellar tearing in multirun fillet welds. Mitsubishi Heavy Industries Ltd., Technical Review (10): 19-27.
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