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Susceptibility to Hydrogen Induced Stress Cracking of Alloy 718 and Alloy 725 Under Cathodic Polarization

Picture for Susceptibility to Hydrogen Induced Stress Cracking of Alloy 718 and Alloy 725 Under Cathodic Polarization
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 A test project to examine the susceptibility of Hydrogen Induced Stress Cracking (HISC) has been executed. In this project hydrogen charged samples of Alloy 718 and Alloy 725 have been exposed under tensile stress to establish critical stress levels for initiation of HISC.

Product Number: 51315-5597-SG
ISBN: 5597 2015 CP
Author: Gaute Stenerud
Publication Date: 2015
Industry: Science of Corrosion
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Nickel alloys are used for subsea applications for components both exposed to seawater and well fluid. These alloys are normally selected due to acceptable corrosion resistance and high strength (precipitation hardened). However the high strength Ni-alloys can suffer from hydrogen embrittlement when exposed to a hydrogen source. A lot of work has been done to establish “safe operation” windows for different Ni-alloys exposed to well fluid containing H2S (reference to ISO 15156-3).When exposed subsea a component made from a Ni-alloy will be connected to a cathodic protection system. As part of the cathodic reaction hydrogen will develop on the surface to be protected. Parts of this hydrogen will absorb on the surface and diffuse as atomic hydrogen into the metal. Since nickel has an austenitic structure the overall hydrogen diffusion will be slow while the solubility will be high.Alloy 718 (UNS N07718) and Alloy 725 (UNS N07725) are used for subsea applications. Some un-expected failures have occurred and hydrogen has been explained as one reason for the fractures. A test project to examine the susceptibility of Hydrogen Induced Stress Cracking (HISC) has been executed. In this project hydrogen charged samples of Alloy 718 and Alloy 725 have been exposed under tensile stress to establish critical stress levels for initiation of HISC. The critical stress levels for hydrogen charged samples have been compared to fracture stress for un-charged samples. Examination in a scanning electro microscope (SEM) has been executed to document fracture morphology of fractured samples. In addition the hydrogen content of the test samples have been documented with melt extraction technique.

Keywords: Conference papers 2015, conference papers, Hydrogen Induced Stress Cracking, cathodic protection, seawater, Alloy 718, Alloy 725, constant load testing

Nickel alloys are used for subsea applications for components both exposed to seawater and well fluid. These alloys are normally selected due to acceptable corrosion resistance and high strength (precipitation hardened). However the high strength Ni-alloys can suffer from hydrogen embrittlement when exposed to a hydrogen source. A lot of work has been done to establish “safe operation” windows for different Ni-alloys exposed to well fluid containing H2S (reference to ISO 15156-3).When exposed subsea a component made from a Ni-alloy will be connected to a cathodic protection system. As part of the cathodic reaction hydrogen will develop on the surface to be protected. Parts of this hydrogen will absorb on the surface and diffuse as atomic hydrogen into the metal. Since nickel has an austenitic structure the overall hydrogen diffusion will be slow while the solubility will be high.Alloy 718 (UNS N07718) and Alloy 725 (UNS N07725) are used for subsea applications. Some un-expected failures have occurred and hydrogen has been explained as one reason for the fractures. A test project to examine the susceptibility of Hydrogen Induced Stress Cracking (HISC) has been executed. In this project hydrogen charged samples of Alloy 718 and Alloy 725 have been exposed under tensile stress to establish critical stress levels for initiation of HISC. The critical stress levels for hydrogen charged samples have been compared to fracture stress for un-charged samples. Examination in a scanning electro microscope (SEM) has been executed to document fracture morphology of fractured samples. In addition the hydrogen content of the test samples have been documented with melt extraction technique.

Keywords: Conference papers 2015, conference papers, Hydrogen Induced Stress Cracking, cathodic protection, seawater, Alloy 718, Alloy 725, constant load testing

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Several mechanisms including Hydrogen-Enhanced Localised Plasticity (HELP) and Hydrogen-Enhanced De-cohesion (HEDE) have been identified as contributing to the hydrogen embrittlement susceptibility of UNS N07718, depending upon microstructural particularities in the material.
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