|Content Type||Conference Paper|
|Title||Cracking Susceptibility Of Duplex Stainless Steel At An Intermediate Temperature In The Presence Of H2s Containing Environments|
|Authors||Julio G. Maldonado, InterCorr International; James W. Skogsberg, ChevronTexaco|
|Source||CORROSION 2004, March 28 - April 1, 2004 , New Orleans, La|
|Copyright||2004. NACE International|
|Keywords||Duplex stainless steels, solution-annealed, SCC, SSC, sulfide stress cracking, sour service|
The use of duplex stainless steels in the oil and gas industry has received special consideration due to their good corrosion resistance, mechanical properties and weldability. Specifically, duplex stainless steels possess excellent chloride pitting resistance and stress corrosion cracking resistance. In this study, the cracking susceptibility of duplex stainless steel at 80°C in the presence of H2S containing environments was investigated. Specifically, environments containing 1,000 ppm and 100,000 ppm Cl- brines with a pH of 3 and 4.5, respectively, and with H2S partial pressures of 1.5, 5 and 10 psia (10, 34 and 69 kPa abs) were employed for the evaluation. The cracking susceptibility of the duplex stainless steel material was only evaluated with respect to the solution-annealed condition. For the experimental work, subsize tensile specimens of annealed 2205 duplex (UNS S31803/S32205) stainless steel under a sustained load, with the use of proving rings, were tested in triplicate with exposure durations of up to 90 days. Cyclic polarization scans were also conducted for each of the environments selected for the cracking susceptibility study. The electrochemical work was conducted to ascertain the differences in the breakdown potential, hence, pitting potential and resulting cracking susceptibility of the material under the environmental conditions evaluated. The results of this work indicated that duplex stainless steel in the solution-annealed condition was resistant to sulfide stress cracking up to a partial pressure of H2S of 5 psia (34 kPa abs). Only one specimen in each of the two Cl-/pH combined conditions with an applied H2S partial pressure of 10 psia (69 kPa abs) failed during the 90-day exposure. These results suggest a possible extension on the applicability of the material in terms of the maximum allowed H2S partial pressure in the NACE MR0175 document.
INTRODUCTION AND BACKGROUND
The use of duplex stainless steels in the oil and gas industry has received special consideration due to their good corrosion resistance, mechanical properties and weldability. Specifically, duplex stainless steels possess excellent chloride pitting resistance and stress corrosion cracking resistance. This combination of properties is important when considering the fact that recent oil and gas developments are taking place in increasingly challenging environmental conditions. The use of these materials is economically advantageous in comparison with the use of the more expensive CRA nickel alloys. In general, duplex stainless steels in the solution-annealed condition would be considered suitable for lower pressure applications, but not suitable for production tubing.
Duplex stainless steels have shown to suffer a loss in ductility due to hydrogen pickup during cathodic polarization, i.e. cathodic protection, which has resulted in the failure of cold worked components. Additionally, failures in the field are usually due to localized corrosion and sulfide stress cracking (SSC). NACE MR01751 establishes a maximum H2S partial pressure of 1.5 psia (10 kPa abs) for solution-annealed duplex stainless steel with a pitting resistance equivalent number (PREN) between 30 and 40, as it is the case for the duplex stainless steel material employed in this investigation (UNS S31803/S32205). However, recent work conducted by Woollin and Maligas2 has shown that material limits set by the MR01751 document should not be considered in isolation but considered along with other environmental and material variables as they relate to a specific application.
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