Conditions of Hydrogen-Induced Corrosion Occurrence of X65 Grade Line Pipe Steels in Sour Environments Available to Purchase

Hydrogen-induced cracking (HIC) tests were performed on fourteen types of X65 line pipe steels in various environments in which the hydrogen sulfide (H₂S) partial pressures $(PH2S)$ were in the range from 1 × 10^-5 MPa to 0.1 MPa and pH was in the range from 2 to 5. The conditions governing HIC occurrence were investigated and the following results were obtained:

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  Hydrogen permeability in steel ([Per_Fe]) was formulated as a function of $PH2S$ and $pH([PerFe])=7.1+0.96×(1.4logPH2S−0.51pH)$⁠, where $0.1MPa≥PH2S≥1×10−3MPa$ and $5≥pH;[PerFe]=3.3+0.75×(0.3logPH2S−0.51pH)$⁠, where $1×10−3MPa>PH2S≥1×10−5$ MPa and 5 ≥ pH.

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  Critical hydrogen permeability in steel ([Per_crit]) was expressed as a function of the critical length of inclusions (a_crit) and the maximum hardness of the center-segregation zone (Hv_max) for the investigated thermomechanical-controlled process (TMCP) steels and the controlled rolled and air-cooled (CR) steels (log [Per_crit] = [1.7 - 0.0030 Hv] – 1/4log[a_crit]).

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  If the maximum hardness of the center-segregation zone (Hv_max) and the size of inclusions—such as manganese sulfide (MnS) in TMCP steels and CR steels—are given, the occurrence of HIC can be predicted for a given environmental condition. The prediction model indicates how the sour-resistant line pipe steels should be produced, reducing the elongated inclusion length and the hardness in the center-segregation zone.