Susceptibility of Material to Cracking in Sour Service Environments
Hydrogen-induced cracking / Hydrogen-assisted cracking (HIC) is caused by the diffusion of hydrogen atoms into the metal. The presence of hydrogen in the lattice weakens the mechanical integrity of the metal and leads to crack growth and brittle fracture at stress levels below the yield strength. Like stress corrosion cracking, it can lead to sudden failure of metal parts without any detectable warning signs. In common applications, hydrogen damage is usually only relevant for high-strength steel with a tensile strength of approximately 1000 N/mm2 or higher.
CONDITIONS TO START HYDROGEN INDUCED CRACKING
The source of hydrogen can be the production process such as steel making, pickling and electro galvanizing (primary hydrogen). A secondary source can be the hydrogen formed during a corrosion process. During the corrosion process, hydrogen is formed and diffuses into the material. This hydrogen intake leads to a decrease in the toughness or ductility of a metal.
As for HIC, three different conditions must be present at the same time.
- Mechanical (load, stress)
- Material (hardness structure)
- Environment (external, internal hydrogen)
TEST SPECIFICATION FOR HYDROGEN INDUCED CRACKING
Hydrogen induced cracking testing apparatus (HIC test) to test and accurately determine the hydrogen induced cracking over exposed test specimens as per NACE standards requirements. HIC testing is recommended to evaluate the resistance of pipeline and pressure vessel plate steels to hydrogen induced cracking corrosion caused by hydrogen absorption from aqueous sulfide corrosion. Unstressed test specimens are exposed to a solution at temperature of 25° with continuous H2S gas flow. The test specimens are finally removed and evaluated.
Test specimens determine susceptibility of metals in H2S service. HIC testing apparatus meet requirements of ASTM / NACE Standard TM0284 and suitable to test material as per requirements of NACE MR0175.
The test methods are listed below.
- NACE TM0284 – Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking.
- NACE MR0175 – Petroleum and natural gas industries – Materials for use in H2S containing environments in oil and gas productions.
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