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Effect of Microstructural Particularities on the Corrosion Resistance of Nickel Alloy UNS N07718 – What Really Makes the Difference

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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.

Product Number: 51317--9068-SG
ISBN: 9068 2017 CP
Author: Jutta Kloewer
Publication Date: 2017
Industries: Materials Selection and Design , Corrosion Monitoring and Control
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The precipitation hardenable alloy 718 (UNS N07718) is the most commonly used Nickel alloy in upstream oilfield technology. However it has been determined that alloy 718 can be susceptible to localised corrosion and environmentally assisted cracking under certain service conditions. This susceptibility is affected by its complex microstructure containing several intermetallic phases nitrides carbides and carbo-nitrides. In addition the size and amount of these precipitates depend sensitively upon ageing time and temperature selected for the heat treatment. However a complete understanding of the influence of microstructural particulatries on the corrosion resistance of alloy 718 has not been reached to date. To address this a large research program dealing with the effect of precipitates on the susceptibility of alloy 718 to localised corrosion and hydrogen embrittlement was conducted.Electrochemical results have shown that that d-phase reduces pit nucleation time and deteriorates the passivity of alloy 718. Nevertheless the presence of d-phase had no effect on the electrochemical potential necessary for pit initiation. The location of the nucleated pits was also independent of the presence d-phase in the microstructure. The galvanic differences between the microstructural phases in alloy 718 appears to be crucial for pit initiation at carbo-nitrides. Diffusion of hydrogen in alloy 718 was measured by permeation tests. Both g’/g’’- and d-phase delay hydrogen diffusion with g’/g’’-phase having a more pronounced effect than d-phase. On the other hand the effect of both d-phase and g’/g’’-phase on hydrogen trapping was neglectable. Using in-situ tensile testing under electrochemical hydrogen charging it was found that d-phase increases the susceptibility of alloy 718 to hydrogen embrittlement by initiating cracks in the matrix/precipitate interface. Several mechanisms including hydrogen-enhanced localised plasticity (HELP) have been identified contributing to the hydrogen embrittlement susceptibility of alloy 718. These results can be used for redesigning alloy 718 with improved corrosion resistance.

Key words: UNS N07718, hydrogen embrittlement, pitting, hydrogen charging, delta-phase

The precipitation hardenable alloy 718 (UNS N07718) is the most commonly used Nickel alloy in upstream oilfield technology. However it has been determined that alloy 718 can be susceptible to localised corrosion and environmentally assisted cracking under certain service conditions. This susceptibility is affected by its complex microstructure containing several intermetallic phases nitrides carbides and carbo-nitrides. In addition the size and amount of these precipitates depend sensitively upon ageing time and temperature selected for the heat treatment. However a complete understanding of the influence of microstructural particulatries on the corrosion resistance of alloy 718 has not been reached to date. To address this a large research program dealing with the effect of precipitates on the susceptibility of alloy 718 to localised corrosion and hydrogen embrittlement was conducted.Electrochemical results have shown that that d-phase reduces pit nucleation time and deteriorates the passivity of alloy 718. Nevertheless the presence of d-phase had no effect on the electrochemical potential necessary for pit initiation. The location of the nucleated pits was also independent of the presence d-phase in the microstructure. The galvanic differences between the microstructural phases in alloy 718 appears to be crucial for pit initiation at carbo-nitrides. Diffusion of hydrogen in alloy 718 was measured by permeation tests. Both g’/g’’- and d-phase delay hydrogen diffusion with g’/g’’-phase having a more pronounced effect than d-phase. On the other hand the effect of both d-phase and g’/g’’-phase on hydrogen trapping was neglectable. Using in-situ tensile testing under electrochemical hydrogen charging it was found that d-phase increases the susceptibility of alloy 718 to hydrogen embrittlement by initiating cracks in the matrix/precipitate interface. Several mechanisms including hydrogen-enhanced localised plasticity (HELP) have been identified contributing to the hydrogen embrittlement susceptibility of alloy 718. These results can be used for redesigning alloy 718 with improved corrosion resistance.

Key words: UNS N07718, hydrogen embrittlement, pitting, hydrogen charging, delta-phase

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