In most conditions when the partial pressure of CO2 is above 30psi, carbon steels present high corrosion rates. Steel grades alloyed with high contents of Chromium (Cr), Molybdenum (Mo) and Nickel (Ni) are able to effectively resist corrosion. 13Cr Steel OCTG Pipe made with 13% Chromium (13Cr) are the least expensive corrosion-resistant alloy (CRA) tubulars that can be used to combat C02 corrosion and H2S environmental cracking without inhibitor treatment. These stainless steel tubulars cost only half as much as duplex stainless steel and one-fourth to one-tenth as much as high-nickel CRA materials. Using 13Cr Steel OCTG Pipe might also be less expensive and more cost-effective than using carbon/low-alloy steel with inhibition. The 13Cr Steel OCTG Pipes are especially attractive for tubing applications in offshore production operations where working space is limited and work over costs are high. Tubulars made with 13% Cr have been widely used in North America, the North Sea, Gulf of Mexico and North Slope of Alaska.
13Cr steel is susceptible to corrosion at high temperature, to sulfide stress cracking (SSC) in moderate to high H2S environments and to crevice corrosion. Therefore, application of 13Cr Steel OCTG Pipe does require precautions in tubular design, procurement, handling, storage, and field operations.
Of all the corrosion resistant alloys (CRA) used in oil and gas developments, martensitic stainless steels (MSS) are some of the most common. These steels not only have the advantages of normal steels: high strength, cost effective, easy to manufacture, available in a wide variety of product forms, but they also have good CO2 corrosion resistance at higher temperatures as a result of their 11% minimum chromium content. One of the most common types of MSS used in the oil and gas industry is 13% chromium steel. NACE and API standards both reference several different varieties of 13 chromium steels for use in oil and gas projects.
All 13 chromium steels have about 12 to 14% chromium and other minor alloying additions, and can be placed into two general groups: conventional and modified. Conventional 13Cr steels (13Cr) are those most common in oil and gas, some of which are CA6NM (UNS J91150), AISI 410(UNS S41000), and AISI 420 (UNS S42000). These alloys have excellent corrosion resistance when compared to carbon steels up to about 300F (150C) and are used for a variety of wellhead, tubular, and down hole components, but they have very limited resistance to cracking in H2S. Modified 13Cr (M13Cr) steels came from the need for increased corrosion resistance at higher temperatures, increased H2S cracking resistance, and increased yield strength above 85 ksi (586 MPa) primarily for tubular goods and other down hole equipment. These were developed by taking a 13Cr and adding nickel (Ni), molybdenum (Mo), copper (Cu), and other alloying elements. M13Cr provide better corrosion resistance than 13Cr up to about 350F (177C), yield strengths up to 110 ksi (758 MPa), and improved H2S cracking resistance.
13 chromium steels do have limitations. As stated, they are sensitive to cracking in H2S containing environments, but they are also susceptible to cracking at very low pHs. NACE practices limit the hardness and heat treatments of many 13 chromium steels and provide environmental limits to prevent sulfide stress cracking (SSC). The environmental limits require H2S partial pressure, chloride concentration, and pH be considered when determining if a 13 chromium steel is acceptable for use. The H2S partial pressure becomes a more interesting issue in high pressure/high temperature (HP/HT) completions. At the anticipated high reservoir pressures, only a few ppm of H2S are required to exceed the provided NACE environmental limits. Additionally, 13 chromium steels are susceptible to environmentally assisted cracking if the pH drops to significantly less than 3.5 or very rapidly without H2S being present.
Another limitation to 13Cr steels is pitting. When these steels are exposed to oxygenated fluids, pits form rapidly, and the surface will not be able to re-passivate in these locations. Given sufficient time, the pits can propagate in to severe cracks or become pronounced localized corrosion, and the steel will no longer be resistant to corrosion in those locations. Oxygenated fluids can be as benign appearing as condensed water on the steel or rain water, and will cause pitting if not readily dried.
The selection of which 13Cr or M13Cr steel to use can be challenging depending on the service environment. Identified several important environmental conditions that should be considered when selecting 13Cr steels for oil and gas use: acidizing and acid flow back, flowing production environment, and shut-in environment. Each of these environmental conditions presents different factors that determine the feasibility of using a MSS, which alloys should be considered, and how selected ones can be further evaluated if required. 13Cr and M13Cr are very beneficial to oil and gas due to their cost, strength, corrosion resistance, and availability, and will remain beneficial for the foreseeable future.
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