Localized Corrosion Resistance of UNS S32750 and UNS S33207 Duplex Umbilical Tubes in Synthetic Seawater

Localized Corrosion Resistance of UNS S32750 and UNS S33207 Duplex Umbilical Tubes in Synthetic Seawater

Authors: Q.J.Meng and A.Adeleke, Honeywell Corrosion Solutions; S.Derrickson, Oceaneering Multiflex

Source: CORROSION 2010, March 14 - 18, 2010, San Antonio, TX

Keywords: duplex stainless steel, localized corrosion, umbilical, synthetic seawater

Preview ABSRACT

The resistance to localized corrosion of girth-welded UNS S32750 and UNS S33207 duplex stainless steel umbilical tubes in synthetic seawater at different temperatures was investigated using electrochemical methods, such as cyclic potentiodynamic polarization (CPP) and potentiostatic polarization, and long-term immersion tests. Repassivation potentials as determined from the CPP tests were compared with the open-circuit potential (OCP) obtained from the long-term immersion tests to address the long-term localized corrosion resistance of umbilical tubes. Electrochemical results indicate that the critical pitting temperature (CPT) for the welded S32750 and S33207 tubes were 53°C and 68°C, respectively. The critical crevice temperature (CCT) for both welded tubes was similar to the CPT values. Interestingly, despite the critical temperatures mentioned above, the 45-day immersion tests show no pitting or crevice corrosion for the welded S32750 tubes at temperatures of 50~70°C and the welded S33207 tubes at temperatures of 60~80°C in aerated synthetic seawater.

INTRODUCTION

Subsea umbilicals are used to control the subsea structures such as wellheads from the platform control stations by supplying control signals and injecting chemicals. UNS S32750 super duplex stainless steel has been widely used as umbilical material due to its excellent corrosion resistance and high fatigue properties since it was introduced in 1993. Recently, UNS S33207 hyper duplex was also introduced for umbilical applications, which exhibits higher mechanical properties and better corrosion resistance [1].

Recently, due to the worldwide demand for more oil and gas, some oil and gas reservoirs have been explored in deeper waters with water depth often greater than 1500 meters and pressure rating above 103.4 MPa. In addition, several factors have lead to increased temperatures within the umbilical, which can exceed the critical crevice temperature of the super duplex. With these increasingly hostile environments and operators seeking ever greater design lives for field components, it becomes important to better characterize materials for service. This is also important because, despite the excellent corrosion resistance of the high alloy steels used in umbilicals, they often limit the temperature service range for the entire umbilical. This temperature limitation can seriously influence the design of the umbilical, and can amount to tens of thousands of dollars per degree when considering the total cost of the umbilical.

This study seeks to demonstrate the temperature limits for using UNS S32750 super-duplex and UNS S33207 hyper-duplex stainless steels as umbilical materials due to localized corrosion such as pitting and crevice corrosion. It will also compare the results of tests designed to simulate the actual conditions in service with critical pitting temperature tests.

EXPERIMENTAL PROCEDURES

Materials and Specimens

The girth-welded UNS S32750 and UNS S33207 duplex stainless steel (DSS) tubes were used in this work. The size of the S32750 and S33207 DSS tubes was 1.27 and 3.18 cm inside diameter, respectively. The nominal chemical composition and pitting resistance equivalent number (PREN) of these two materials are listed in Table 1.

The tubes were tested with the as-formed surface finish.