Q: When specifying valves for hydrogen service, what are some of the material considerations I should keep in mind?
A: Hydrogen can cause a number of different adverse effects in metallic materials. The specific problems that can occur, and the methods for avoiding them, depend upon the service conditions. Although the subject is much too vast to cover completely in this column, following are descriptions of the predominant hydrogen damage mechanisms, along with some suggestions for avoiding problems.
Hydrogen embrittlement, also called hydrogen stress cracking or hydrogen induced cracking, is a condition of low ductility in metals resulting from the absorption of hydrogen. Hydrogen embrittlement is mainly a problem in steels with ultimate tensile strength greater than 90 ksi, although a number of additional alloys are susceptible. Most hydrogen embrittlement failures occur as a result of absorption of hydrogen that is generated during plating, pickling, or cleaning operations. However, hydrogen charging may also occur in-service. This usually occurs in cases where hydrogen is generated due to corrosion, although it can also occur in high-temperature hydrogen applications. Hydrogen embrittlement failures are most often characterized as delayed, catastrophic failures occurring at temperatures near ambient, at stresses below the yield strength, and exhibiting single, non-branching cracks. However, failures deviating from these characteristics can and do occur.
The hydrogen embrittlement phenomenon requires a source of hydrogen ions (H+) or monatomic hydrogen (H). Diatomic (molecular) hydrogen (H2) will not cause hydrogen embrittlement, because the H2 molecules are too large to diffuse into the metallic crystal structure.
Hydrogen ions are created during any electrolytic or aqueous corrosion process, including general corrosion, galvanic corrosion, pitting corrosion, electrocleaning, electropolishing, pickling, and electroplating processes.
Monatomic hydrogen (H) is formed by dissociation of diatomic hydrogen (H2) at high temperatures. Reportedly, this dissociation begins to occur at around 350°F(175°C), with the proportion of H/H2 increasing as temperature increases.
It should be mentioned that although hydrogen embrittlement is most likely to occur at ambient temperatures, ambient-temperature failure may occur in a material that was "charged" with hydrogen during exposure at elevated temperature.
Since sulfide stress cracking is essentially hydrogen embrittlement catalyzed by the presence of sulfide ions, NACE MR0175/ISO 15156, Petroleum and Natural Gas Industries - Materials for Use in H2S-containing Environments in Oil and Gas Production, and/or NACE MR0103, Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments, can be used as guidelines for general materials selection to avoid hydrogen embrittlement. However, the requirements in these standards are somewhat conservative for avoidance of conventional hydrogen embrittlement. In general, steels below approximately 35 HRC are generally acceptable for applications where conventional hydrogen embrittlement is a concern, whereas the NACE standards would require steels to meet a 22 HRC maximum hardness requirement. Austenitic stainless steels, most nickel and copper alloys, and aluminum alloys are generally resistant to hydrogen embrittlement, although certain precipitation-hardened and/or strain-hardened grades in these material families can suffer hydrogen embrittlement.
When carbon and low-alloy steels are exposed to high-pressure, high-temperature hydrogen, the monatomic hydrogen can diffuse into the steel and combine with the carbon in the steel to form methane gas, which becomes trapped at grain boundaries and other discontinuities in the material. The resulting internal decarburization and grain boundary fissuring degrades the mechanical properties of the material. Resistance to hydrogen attack increases with increasing chromium and molybdenum levels, since these elements form more stable carbides than iron, and do not release the carbon to the hydrogen as readily. API-recommended Practice 941, Steels for Hydrogen Service at Elevated Temperatures and Pressure in Petroleum Refineries and Petrochemical Plants, includes a diagram (commonly called a Nelson curve), which shows zones where the carbon and alloy steel materials are acceptable as a function of hydrogen partial pressure and temperature.
Hydrogen blistering is the formation of blisters containing hydrogen gas in steels. This occurs when monatomic hydrogen (H) diffuses through the steel and recombines into molecular hydrogen (H2) at internal defects such as voids, laminations, and non-metallic inclusions. Molecular hydrogen cannot diffuse back out through steel, so the gradual buildup of molecular hydrogen results in increased pressure inside the defect cavities, eventually causing blistering of the material. Killed steels often are specified for elevated-temperature hydrogen applications or for applications where it is known that ionic hydrogen is generated. Killed steels are steels treated with a strong deoxidizing agent such as silicon or aluminum in order to reduce the oxygen content in the molten ingot, which in turn reduces the level of gas porosity in the finished steel. Killed steels are more resistant to hydrogen blistering than non-killed steels due to their relative lack of internal voids. The term "killed" actually only pertains to wrought products; however, cast steels are also deoxidized with elements such as silicon or aluminum to prevent the formation of gas porosity.