Module Six of CCE 281 Corrosion: Impact, Principles, and Practical Solutions
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Hydrogen Induced Cracking (HIC) |
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There are several categories of hydrogen phenomena that are localized in nature. Atomic hydrogen, and not the molecule, is the smallest atom and it is small enough to diffuse readily through a metallic structure. When the crystal lattice is in contact or is saturated with atomic hydrogen, the mechanical properties of many metals and alloys are diminished. Nascent atomic hydrogen can be produced as a cathodic reaction when certain chemical species are present which act as negative catalysts (i.e. poisons) for the recombination of atomic to molecular hydrogen as shown in the following equation.
If the formation of molecular hydrogen is suppressed, the nascent atomic hydrogen can diffuse into the interstices of the metal instead of being harmlessly evolved as a gaseous reaction product. There are many chemical species which poison this recombination (e.g. cyanides, arsenic, antimony, or selenium compounds). However, the most commonly encountered species is hydrogen sulfide (H2S), which is formed in many natural decompositions, and in many petrochemical processes (reference).
Processes or conditions involving wet hydrogen sulfide, i.e. sour services, and the high incidence of sulfide-induced HIC has resulted in the term sulfide stress cracking (SSC). The SSC of medium strength steels has been a continuing source of trouble in the oil fields, and from these troubles has evolved in international standards [6]. However, similar problems are encountered wherever wet hydrogen sulfide is encountered (e.g. acid gas scrubbing systems, heavy water plants, and waste water treatment).
Failures have occurred in the field when storage tank roofs have become saturated with hydrogen by corrosion and then been subjected to a surge in pressure, resulting in the brittle failure of circumferential welds. In rare instances, even copper and Monel 400 (N04400) have been subjected to HIC. More resistant materials, such as Inconels and Hastelloys often employed to combat HIC, can become susceptible under the combined influence of severe cold work, the presence of hydrogen recombination poisons, and a direct current from the galvanic couple due to electrical contact with a more anodic member.
The mechanism of HIC has not been definitely established. Various factors are believed to contribute to unlocking the lattice of the metal, such as hydrogen pressure at the crack tip, the competition of hydrogen atoms for the lattice bonding electrons, the easier plastic flow and dislocation formation in the metal at the crack tip in the presence of hydrogen, and the formation of certain metal hydrides in the alloy. The following phenomena have also been commonly reported in relation to hydrogen weakening of metallic components.
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