Abstract
High strength low alloy (HSLA) steels are used for the construction of pipelines for oil and natural gas transportation. For such applications pipelines must exhibit mechanical resistance and resistance to corrosion and hydrogen induced cracking (HIC). API 5L X65 steels are the main materials used for this purpose. However, for economic reasons, the use of steels of superior grades would be of interest. This work presents a comparative study of the corrosion and HIC resistances of an API 5L X65 and an API 5L X80 steel in deaerated solution A of NACE TM0284 standard. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization experiments were performed in non-sour and sour (H2S-saturated) media, and HIC resistance tests were carried out in the sour medium. Scanning electron microscopy (SEM) and optical microscopy (OM) characterizations of polished and corroded samples were also done. Electrochemical tests showed that the API 5L X80 steel is slightly more susceptible to surface corrosion, which can be probably linked to its higher inclusion content and smaller grain sizes, it was also susceptible to HIC. Mn and S-rich inclusions found in the crack path indicate that this microstructural feature may play a key role in crack propagation and HIC susceptibility
Highlights
Oil and natural gas are the main sources of energy in the world and the demand and consumption of their derivatives are constantly growing
The following conclusions can be drawn: 1. The electrochemical tests showed that the two steels investigated showed low corrosion resistance in solution A (NACE TM0284-2011) (5% sodium chloride (NaCl) and 0.5% acetic acid (CH3COOH)), being the performance worsened in sour gas medium
The results showed that the corrosion resistance of the API 5L X80 steel is slightly inferior to that presented by the API 5L X65, which can be ascribed to the higher number of inclusions in its microstructure
Summary
Oil and natural gas are the main sources of energy in the world and the demand and consumption of their derivatives are constantly growing In this context, there is an increase in the safety requirements for extraction and transportation of these inputs, as failures can result in severe economic losses and environmental damages. Atomic hydrogen results from corrosion, and its diffusion into the material microstructure is favored by the presence of H2S in the electrolyte and by the existence of sulfide rich inclusions in the material microstructure.11-12In the presence of such chemicals and microstructural features, the surface recombination of atomic H into H2 is hampered[13,14], enhancing the diffusion to the metal microstructure and the occurrence of HIC failures
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