ABSTRACT

In this paper, the pitting failure boundary of duplex stainless steel under sour condition was investigated by electrochemical and immersion tests. Polarization measurements are employed to investigate the effect of temperature and H2S on the corrosion behavior of Duplex Stainless Steel. And the pitting susceptibility is estimated by the further analysis of polarization curves and surface characterization. The results show that the pitting resistance of S31803 duplex stainless steel decreases with the increase of temperature and the H2S partial pressure. S31803 duplex stainless steel is pitting sensitive at around 85°C in 50kPa H2S, 130,000ppm Cl- condition.

INTRODUCTION

The fast increasing energy demand has boosted the exploitation of much deeper and sourer oilgas field with high pressures, high temperatures and extreme corrosive environments, which brings potential threat of sudden material failure. The typical failure cases for stainless steels are caused by pitting and stress corrosion creaking (SCC) 1-5. Due to the higher mechanical properties and excellent general and localized corrosion resistance comparing with either austenitic stainless steels or ferritic stainless steels, S31803 duplex stainless steel is widely used in the extreme oil-gas conditions to meet the requirement for exploitation 6. The high pitting corrosion resistance of duplex stainless steel is related to the composition and microstructure, formed by roughly equal percentages of austenite and ferrite, which is the reason why the material is widely used in oil-gas industries 7-8.

Duplex stainless steels (DSSs) are characterized by the precipitation of many secondary phases, carbides and nitrides for tempering temperatures. This phenomenon implies a high susceptibility to localized corrosion, however better than austenitic and ferritic grades. Many recorded researches and case studies have been conducted on the corrosion failures of duplex stainless steels, and most of these failures started with pitting attacks and then developed to stress corrosion cracking 9-10. Substantial efforts have been made in the past to investigate the service temperature conditions on DSS. Elsaady et al. 11found the resistance of the S31803 duplex stainless steel to pitting corrosion decreased with the aging temperature till 850°C. Tsai et al. 12 found the duplex stainless steels are susceptible to SCC at ambient and elevated temperatures with chloride/sulfide environments. Oltra et al. 13 found the cracking is not only dependent on the mechanical events but also on the effect of H2S upon the electrochemical conditions. A number of studies were devoted to the problem of high temperatures on pitting resistance of DSS. Deng et al. 14 studied the critical pitting temperature (CPT) of S31803 duplex stainless steel in NaCl solution. Geng 15 studied the pitting corrosion resistance in different zones of the S31803 duplex stainless steel’s weld joint and base metal was evaluated by the polarization technique in 3.5% NaCl solution at 25°C. Matteo Gastaldi 16 investigated the effect of temperature in the range 20~60°C on the resistance to chloride-induced corrosion of low-nickel duplex stainless steel. M. Barteri 17 studied the engineering stability diagrams of duplex stainless steels for down-hole simulated environments and investigated the sulphide stress corrosion cracking (SSCC) morphology. The critical partial pressure for the extruded and quenched materials was around 0.070.35 bar (1-5 psi) at 80°C. The standard of NACE MR0175 18 gave recommendations and requirements for the selection and qualification of CRAs (corrosion-resistant alloys) for service in equipment used in oil and natural gas production in H2S-containing environments. In this standard the application boundary for S31803 was T<232°C and PH2S<10 kPa.

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