Generally speaking, some low alloy steels can meet the requirements for corrosive oil and gas environment containing H2S, but the corrosive environment containing CO2 or H2S, CO2, Cl – coexistence where the Martensitic stainless steel need, duplex stainless steel or even nickel-based alloy. The 1988 version of API 5CT added corrosion-resistant tubing steel grades, specified the C75 steel grade with Martensitic stainless steel grades of 9Cr and 13Cr
High strength Martensitic stainless steel pipe for oil well
In the wet environment with CO2 as the main gas, local corrosion damage of oil well pipe often occurs, such as pitting corrosion and intergranular corrosion, etc. If Cl – exists, the local corrosion will be intensified. It is generally considered that the corrosion can be ignored when the Carbon dioxide pressure is lower than 0.021MPa, and the corrosion will occur when the carbon dioxide pressure reaches 0.021MPa. When the pCO2 is higher than 0.021MPa, appropriate anti-corrosion measures should be taken. Generally, there is no damage caused by pitting when the co2 fraction is lower than 0.05Mpa.
It has been proved that the effect of using a sustained-release agent to prevent CO2 corrosion is limited, and the effect of using high chromium steel such as 9%-13%Cr steel is better. Since the 1970s, some natural gas Wells have used 9%Cr and 13Cr% stainless steel tubing to prevent CO2 corrosion. The American Petroleum Institute (API) recommends 9Cr and 13Cr martensitic stainless steel tubes (API L80-9Cr and L80-13Cr) for standardized use. 13Cr steel has better resistance to CO2 corrosion, while 9Cr-1Mo steel has better resistance to H2S stress corrosion cracking. In principle, neither steel is suitable if H2S is present in a CO2 atmosphere. When H2S exists in CO2 oil well, the SSCC resistance of the oil well pipe should be improved as far as possible, and the quenching and tempering heat treatment should be adopted to obtain uniform martensite and the hardness should be controlled below HRC22 as far as possible.
The stainless steel grade of oil well
However, API 13Cr steel tubes have significantly reduced CO2 resistance and shortened service life when the oil well temperature reaches 150℃ or higher. In order to improve the CORROSION resistance of API 13Cr steel tubes to CO2 and SSC (sulfide stress cracking), low carbon SUP13Cr steel tubes with Ni and Mo added were developed. The steel tube can be used in wet environments with high temperatures, high CO2 concentrations and a small amount of hydrogen sulfide. The structure of these tubes is tempered martensite and less than 5% ferrite. The corrosion resistance to CO2 can be improved by reducing carbon or adding Cr and Ni, and the corrosion resistance to pitting can be improved by adding Mo. Compared with API 13Cr steel pipe, the corrosion resistance to CO2 and SSC is greatly improved. For example, in the same corrosive environment, the corrosion rate of API 13Cr steel pipe is more than 1mm/a, while the corrosion rate of SUP13Cr steel pipe is reduced to 0.125mm/a. With the development of deep and ultra-deep wells, the oil well temperature continues to increase. If the oil well temperature is further increased to more than 180℃, the corrosion resistance of SUP13Cr oil well pipe also begins to decline, which cannot meet the requirements of long-term use. According to the traditional material selection principle, duplex stainless steel or Nickel base alloy should be selected.
Martensitic stainless steel pipe for oil pipeline
The pipeline pipe conveying corrosive oil and gas requires the same corrosion-resistant material as the oil well pipe. Previously, the pipe was usually injected with sustained-release agents or corrosion-resistant materials such as dual-phase stainless steel. The former is unstable in anticorrosion effect at high temperature and may cause environmental pollution. Although dual-phase stainless steel has good corrosion resistance, the cost is high, and welding heat input is difficult to control, welding preheating and post-welding heat treatment to the construction of the site brings difficulties. The martensitic 11Cr pipe for CO2 environment and the martensitic 12Cr pipe for CO2+ trace H2S environment are put into use. The column has good weldability, without preheating and post-weld heat treatment, its mechanical properties can be equal to X80 steel grade, and its corrosion resistance is better than that of the pipeline with retarded release agent or dual-phase stainless steel pipe.
Stainless steel pipe for pipeline
Duplex stainless steel pipe for the petroleum industry
The martensitic stainless steel SUP 15Cr cannot meet the corrosion resistance requirements when the temperature of the oil (gas) well containing CO2 exceeds 200℃, and duplex stainless steel with good resistance to CO2 and Cl — stress corrosion cracks is required. Currently, 22Cr and 25Cr duplex (Austenitic and Ferrite) stainless steels are suitable for CO2 Wells above 200℃, while manufacturers adjust Cr and Ni content to adjust corrosion resistance. Duplex steel is composed of ferrite plus the Austenitic phase. Besides Cr and Ni, Mo and N can be added to improve the corrosion resistance. In addition to the duplex stainless steel has good high-temperature corrosion resistance, compared with martensite stainless steel, it has better H2S stress corrosion cracking resistance, at room temperature NACE TM 0177-A test, in A solution, 85%SMYS loading environment, martensite stainless steel can only pass the 10kPa H2S partial pressure test, Duplex stainless steel 25Cr can pass 100kPa H2S partial pressure test.
In general, in the coexistence of CO2 and H2S environments, or H2S partial pressure does not reach critical but Cl- is very high, 13Cr steel (including super 13Cr steel) can not meet the requirements, 22Cr duplex stainless steel (ASF 2205) or super duplex stainless steel 25Cr, Even high Ni, Cr stainless steel and Ni-based and Fe-Ni based alloys such as G3, alloy 825 containing more than 20% Cr, Ni30% are required.