Martensitic and duplex steels are increasingly used in the oil and gas industries when high C02 environments are encountered. Nevertheless, very often the field conditions make traditional 13% Cr steel insufficient due to its limited corrosion resistance and the duplex being too expensive. This paper is aimed at evaluating the performances of new 95 ksi grade and new 110 ksi grade supermartensitic 13Cr-5Ni-2Mo steel for sweet and slightly sour environments. Mechanical properties at room temperature and high temperatures, Sulfide Stress Cracking (SSC) and Stress Corrosion Cracking (SCC) resistance, and localized and weight loss corrosion resistance have been investigated on both moditled 13% Cr, traditional 13% Cr and duplex 22% Cr and 25% Cr. Suparmrtensitic 13Cr-5Ni-2Mo proved to be an interesting alternative to the traditional CRAS for use as Oil Country Tubular Goods (OCTG) in typical North Sea environments (high chloride, high temperature, high C02 wells containing H2S) with improved corrosion resistance properties.
The application field of API-13% Cr martensitic stainless steel (AISI 420, hereafter referred to as 13Cr) is steadily increasing due to its good C02 corrosion resistance and cost effectiveness [1, 2]. From 1980 to 1993, more than 2400 km of 13Cr OCTG were reported to be installed in oil and gas wells [3]. However, 13Cr steel is susceptible to sulfide stress cracking (SSC) in the C02 environment with H2S partial pressure higher than 0.045 psi (0.003 bar) in laboratory tests and less resistant to general and localized corrosion at elevated temperatures [4]. Based on test results and field experiences, it was described in NACE MR0175-94 that API 5CT grade L80 type 13Cr tubing and casing are acceptable up to a maximum H2S partial pressure of 1.5 psi (0.1 bar) in production environments with a produced water pH greater than or equal to 3.5 [5, 6].
Even in sour environments where SSC is the major concern, pitting corrosion resistance is of great importance as it is well known that cracks may nucleate from pit tips [7]. 13Cr steel is generally used in C02 containing oil field environments at temperatures lower than 150°C and salinity lower than 4% [8, 9]. Above those limits, martensitic stainless steel suffers horn pitting corrosion and exhibits no advantages over low alloy steels in terms of general corrosion. In order to expand application limits of 13Cr steels OCTG to higher temperature and higher H2S partial pressure regions, but also in order to provide higher mechanical grade products, a new type of 13% Cr martensitic stainless steel has been developed and a commercial supply of it is available.
The aim of this paper is to describe the mechanical properties, both at room and high temperatures, and corrosion performances of that new supemrartensitic stainless steel for OCTG suitable for a slightly sour environment and to compare with that of 13Cr steel (low reference) and duplex stainless steels DSS 22% Cr and DSS 25% Cr (high reference).
