Research on the Steel for Oil and Gas Pipelines in Sour Environment

Research on the Steel for Oil and Gas Pipelines in Sour Environment

Xiaodong Shao 1, 2*

1State Key Laboratory for Performance and Structure Safety of Petroleum Tubular Goods and Equipment Materials, No. 89 Jinyeer Road, Xi’an 710077, China

2CNPC Tubular Goods Research Institute, No. 89 Jinyeer Road, Xi’an 710077, China

Abstract. In recent years, there are more and more oil and gas fields containing H2S corrosive media, and it is imperative to develop oil and gas transmission pipes in acidic environment. The research progress of steel for oil and gas pipelines in acidic environment is reviewed from the aspects of H2S corrosion mechanism and influencing factors in pipeline service under acidic service conditions. At the same time, the development status of sour-resistance corrosion oil and gas transmission steel tubes at home and abroad is introduced. Pipeline steel is further explored in terms of SSCC-resistance research and improvement of corresponding standards.

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1. Introduction

Oil and gas pipeline transportation is characterized by high efficiency, economy and safety, and is the main form of long-distance transportation of oil and gas [1]. The development of pipelines tends to be large-caliber and high-pressure, the service conditions of pipelines are becoming more and more demanding, the transportation medium is complex, and many pipelines need to cross densely populated areas or deserts, cold regions, etc., which puts forward the performance of pipelines. High technical requirements, especially the requirements of corrosion resistance, Academician Li Helin pointed out that anti-hydrogen induced cracking (HIC) pipeline steel and steel pipe is one of the development directions of high performance oil and gas transmission steel pipe. Corrosion is a key factor affecting the reliability and service life of pipeline transportation systems. It can not only cause perforation, but also cause oil and gas leakage. It may also cause an explosion, threaten personal safety, pollute the environment, and the consequences are extremely serious. According to the National Transportation Safety Administration, 74% of the leaks in the US gas pipelines and gas gathering pipelines are caused by corrosion, and H2S corrosion is one of the main forms of pipeline corrosion. According to the American Society of Corrosion Engineers (NACE), in natural gas containing water and H2S, when the partial pressure of H2S in the gas is equal to or greater than 0.0003 MPa, it is called an acidic environment.

Since the 1970s, the development conditions of oil and natural gas have changed significantly in various countries. At present, although the purification process has been carried out before the natural gas transmission, the natural gas to be transported into the gas pipeline does not meet the requirements of dry and purified natural gas. H2S in the pipeline. In-pipe corrosion caused by the presence of water is still inevitable. In recent years, accidents caused by pipe H2S corrosion have occurred, resulting in huge economic losses and environmental pollution. Therefore, solving or mitigating H2S corrosion of oil and gas pipelines has very important economic and social benefits. According to the development needs of Chinese oil and gas industry, it is imperative to develop Chinese own acidic service conditions to transport steel pipes. Foreign countries have started research in this area earlier, and there are specialized agencies to study the transportation of steel pipes in an acidic environment. Although domestic work in this area started late, it also carried out a series of research work. The author mainly reviews the research progress of steel for acid and oil pipelines from the corrosion mechanism and influencing factors of H2S in acidic environment pipelines, and introduces the development of oil and gas pipelines in acidic environment at home and abroad. Research direction.

2. H2S corrosion mechanism of pipeline steel

The corrosion of pipeline steel in acidic environment mainly includes hydrogen induced cracking and sulfide stress corrosion cracking (SSCC). At present, the research on H2S corrosion of pipeline steel is mainly focused on HIC. Pipeline steel in the oil-rich environment rich in H2S in the state of no stress or non- tensile stress, the hydrogen generated by the corrosion into the steel to form a crack called HIC, generally refers to hydrogen-induced bubbling (surface crack) and hydrogen-induced step cracking ( Internal crack). SSCC means that the hydrogen atoms generated by H2S penetrate into the interior of the steel, dissolve in the crystal lattice, cause brittleness, and form cracks under the action of external tensile stress or residual stress. SSCC is in sensitive materials, acidic environment and stretching. The three conditions of stress occur together [2]. When transporting acidic oil and gas, the inside of the pipeline is exposed to H2S. Under the joint action of stress and H2S and other corrosive media, SSCC often occurs in the pipeline.

3. Research on the HIC of pipeline steel 

3.1 Influence of the environment on HIC

The results show that when the partial pressure of H2S is 0.1~0.5MPa, the corresponding hydrogen permeability is higher, which means that the sensitivity of HIC is greater [3]. As the pH decreases, the HIC sensitivity of the pipeline steel also increases. When CO2 is dissolved in water to form carbonic acid, hydrogen ions are released, which lowers the pH of the environment and increases the sensitivity of HIC. Cl- accelerates the corrosion rate in the range of pH 3.5~4.5, and the HIC sensitivity increases [4]. When the temperature is around 30°C, the additive effect of hydrogen adsorption and diffusion is the largest, that is, the HIC sensitivity is the strongest, below 30°C, the hydrogen diffusion rate and activity decrease, and it is difficult to aggregate active hydrogen above 30°C.

3.2 Influence of composition and structure on HIC

The influence of pipeline steel composition and structure on HIC is the most in-depth study, and the results are also the most, which can be summarized into three aspects.

(1) Chemical composition. The elements that are more sensitive to HIC are carbon, manganese, sulfur, phosphorus, calcium, copper, and molybdenum. As the carbon content increases, the HIC sensitivity increases, and the increase in carbon content and carbon equivalent in the pipeline steel causes the steel to form the martensite structure most sensitive to hydrogen bubbling during hot rolling, thus reducing the carbon content and carbon equivalent. It can improve the H2S-resistance corrosion performance of pipeline steel. Sulfur can promote the occurrence of HIC, which is the most easily nucleated site of HIC with manganese-generated MnS. Baosteel's research indicates that the addition of calcium can change the form of sulfide inclusions, making it a dispersed spheroid, thereby improving the HIC- resistance ability of steel. The test found that the best effect is when the calcium-sulfur content ratio is greater than 1.5. Adding an appropriate amount of manganese to the pipeline steel can improve the hardenability of the steel, cause solid solution strengthening, and compensate for the decrease in strength caused by low carbon. The segregation of manganese and phosphorus can cause the formation of band-like structures sensitive to HIC, thus increasing the manganese content., will lead to more banded tissue generation, which increases HIC sensitivity. The effect of copper on HIC is obvious. In NACE B solution, HIC sensitivity decreases significantly with increasing copper content. However, in the H2S environment with pH<4.5, the passivation film of copper is no longer formed, and at this time, the copper prevents the effect of HIC from disappearing. The addition of molybdenum can lower the phase transition temperature, inhibit the formation of massive ferrite, promote the transformation of acicular ferrite, improve the strength of steel, reduce the ductile-brittle transition temperature, and improve the HIC-resistance ability. The addition of microalloying elements such as bismuth, vanadium and titanium to pipeline steel can effectively prevent the growth of austenite grains, refine grains and enhance the corrosion resistance of pipeline steels [5,6]. Due to the influence of chemical composition on the HIC resistance of pipeline steel, the chemical composition of different steel grade HIC- resistance oil and gas transmission steel pipes has clear technical requirements in ISO 3183.

(2) Microstructure. Sodium thiocyanate was selected as the chromogenic reagent in the proposed method. The effect of the chromogenic reagent was examined by measuring the absorbance of solution containing certain amounts of molybdenum and variable amounts of sodium thiocyanate. It was found that 10ml of 100.0mg·mL-1 sodium thiocyanate solution sufficed to complex the amounts of molybdenum taken, with higher concentrations the absorbance was essentially constant. Ten millilitres of 100.0mg·mL-1 sodium thiocyanate solution were recommended as a suitable amount of chromogenic reagent. The thermodynamically balanced and stable fine grain structure is the ideal structure for HIC-resistance. For medium and low-strength pipeline steels, HIC is prone to low temperature conversion of manganese, phosphorus and other elements in the banded pearlite structure and the center of the plate thickness. In particular, in order to improve the strength of pipeline steel, the addition of many alloying elements promotes the formation of low temperature conversion microstructure, which provides a convenient place for the occurrence of HIC [7]. With the increase of the microstructure of steel, the HIC-resistance performance decreases gradually, and the HIC-resistance performance decreases with the increase of martensite structure. With the increase of the band structure level, the HIC- resistance performance of pipeline steel is gradually reduced. Yin Guanghong et al found that the HIC- resistance performance of pipeline steel will be significantly enhanced after normalizing heat treatment.

(3) Non-metallic inclusions. The morphology and distribution of non-metallic inclusions affect the HIC resistance of pipeline steel. The non-metallic inclusion interface is a strong hydrogen trap [8]. On the interface between the pipeline steel and H2S, the hydrogen ions generated by the electrochemical reaction are trapped in the hydrogen trap after the electrons are converted into hydrogen atoms at the anode. When the hydrogen pressure rises to a certain value, it is in the non-metal.

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