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Sour Service Test Qualification of a New High-Strength Titanium Alloy

Picture for Sour Service Test Qualification of a New High-Strength Titanium Alloy
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Product Number: 51315-5794-SG
ISBN: 5794 2015 CP
Author: Ronald Schutz
Publication Date: 2015
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A newly developed high-strength highly corrosion-resistant alpha-beta titanium alloy developed specifically for HPHT oil and gas production service was laboratory tested in severe sour environments for NACE MR01-75/ISO 15156 Standard sour service acceptance. Extruded pipe and forged plate products from three heats were tested in both alpha-beta and transformed-beta conditions at yield strengths exceeding 930 MPa and up to 41 HRC via four different SSC/SCC test methods: NACE TM01-77 Method A tensiles C-rings per NACE TM01-77 Method C slow strain rate tensiles per NACE TM01-98 and bolt-open-loaded C(T) Kscc testing per NACE TM01-77 Method D. The Method A iron-coupled and non-coupled tensiles stressed to 100% actual YS resisted cracking in the 30 day NACE Solution A exposure. All C-rings stressed to 100% actual YS survived 90 day exposure to NACE VII plus conditions involving 25 wt.% NaCl brine with 3.45 MPa H2S and CO2 and elemental sulfur at 288°C. Slow strain rate tensile testing in the same 288°C sour brine and even in 33 wt.% NaCl brine with 3.45 MPa H2S 6.9 MPa CO2 and sulfur failed to reveal any indications of SSC or SCC susceptibility. Pipe and plate Method D Kscc values proved to be greater than 90% of Kair values after the 30 day sour brine exposure at 260°C.Since resistance to chloride crevice corrosion is an essential implicit requirement for successful performance of any titanium alloy component exposed to either sweet or sour high temperature aqueous chloride media accelerated brine crevice testing of this new alloy was also included in this qualification program. Severe crevices formed by mating alloy plate coupons with PTFE sheet gaskets were exposed to naturally-aerated seawater acidified with HCl to pH 3.0 at 550°F for 90 days. These very conservative aggressive tests revealed no measurable crevice or other localized corrosion attack on all coupons.Based on this sour test database combined with these favorable chloride crevice corrosion test results it is recommended that this new titanium alloy (Note: Alloy UNS# application underway and to be assigned shortly) be considered for acceptance in the NACE/ISO sour standard to a maximum hardness of 41 HRC.KEY WORDS: Titanium Titanium Alloy Alpha-beta titanium alloy High strength Sour service Sour brine High pressure-high temperature Sulfide stress cracking Stress corrosion cracking Crevice corrosion Sour qualification testing
A newly developed high-strength highly corrosion-resistant alpha-beta titanium alloy developed specifically for HPHT oil and gas production service was laboratory tested in severe sour environments for NACE MR01-75/ISO 15156 Standard sour service acceptance. Extruded pipe and forged plate products from three heats were tested in both alpha-beta and transformed-beta conditions at yield strengths exceeding 930 MPa and up to 41 HRC via four different SSC/SCC test methods: NACE TM01-77 Method A tensiles C-rings per NACE TM01-77 Method C slow strain rate tensiles per NACE TM01-98 and bolt-open-loaded C(T) Kscc testing per NACE TM01-77 Method D. The Method A iron-coupled and non-coupled tensiles stressed to 100% actual YS resisted cracking in the 30 day NACE Solution A exposure. All C-rings stressed to 100% actual YS survived 90 day exposure to NACE VII plus conditions involving 25 wt.% NaCl brine with 3.45 MPa H2S and CO2 and elemental sulfur at 288°C. Slow strain rate tensile testing in the same 288°C sour brine and even in 33 wt.% NaCl brine with 3.45 MPa H2S 6.9 MPa CO2 and sulfur failed to reveal any indications of SSC or SCC susceptibility. Pipe and plate Method D Kscc values proved to be greater than 90% of Kair values after the 30 day sour brine exposure at 260°C.Since resistance to chloride crevice corrosion is an essential implicit requirement for successful performance of any titanium alloy component exposed to either sweet or sour high temperature aqueous chloride media accelerated brine crevice testing of this new alloy was also included in this qualification program. Severe crevices formed by mating alloy plate coupons with PTFE sheet gaskets were exposed to naturally-aerated seawater acidified with HCl to pH 3.0 at 550°F for 90 days. These very conservative aggressive tests revealed no measurable crevice or other localized corrosion attack on all coupons.Based on this sour test database combined with these favorable chloride crevice corrosion test results it is recommended that this new titanium alloy (Note: Alloy UNS# application underway and to be assigned shortly) be considered for acceptance in the NACE/ISO sour standard to a maximum hardness of 41 HRC.KEY WORDS: Titanium Titanium Alloy Alpha-beta titanium alloy High strength Sour service Sour brine High pressure-high temperature Sulfide stress cracking Stress corrosion cracking Crevice corrosion Sour qualification testing
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