Corrosion Behavior of API ۵L X۷۰ Carbon Steels in Hydrogen Sulfide Environments

As we all know, corrosion of pipelines by hydrogen sulfide is the most worrying factor in the production and transport of oil and gas. In this work the corrosion behavior of API ۵L X۷۰MS and X۷۰MO low carbon steels in hydrogen sulfide environments was investigated. Hydrogen induced cracking and sulfide stress cracking tests were carried out according to NACE TM۰۱۷۷ standard. After testing, blisters and cracks were observed only in X۷۰MO steel, probably due to its lower grain refinement and banded microstructure. Internal cracks seem to be initiated in elongated MnS inclusions. Corrosion process was studied by obtaining potentiodynamic polarization curves, which were registered after open circuit potential measurements, at room temperature. Both steels showed general corrosion in NACE ۱۷۷A solutions, but the corrosion rate values ​​in H۲S-saturated solution were about an order of magnitude higher than those ones in deaerated solution. Hydrogen permeation w::as char::acterized in accordance with ASTM G۱۴۸ standard. In deaerated H۲SO۴ solution, permeation measurements were similar for both steels. In H۲S-saturated solution, X۷۰ MO exhibited higher hydrogen oxidation current values ​​than X۷۰ MS. H۲S seems to promote the reduction of protons and increase the concentration of hydrogen atoms in the solution/steel interface, favoring the diffusion process. As X۷۰MO has a coarse microstructure, it offers more pathways for hydrogen diffusion.

کلیدواژه ها:

X۷۰ steel ، Hydrogen induced cracking ، Sulfide stress cracking ، Sour corrosion ، Low carbon steel.