Study of the effect of cyclic load on hydrogen permeation in high strength steels under cathodic protection

Introduction: Pipelines and offshore applications, operating under cathodic protection to prevent general corrosion, are usually realized with High Strength Low Alloy (HSLA) steels. The risk of hydrogen embrittlement (HE) in pipeline steels due to cathodic overprotection are described in literature. HE is related to absorption and subsequent diffusion of atomic hydrogen through the lattice. On HSLA steels, HE occurs only in dynamic load conditions in the plastic field, such as slow strain rate or corrosion fatigue. Many studies were carried out on the effect of applied stress on hydrogen diffusion and HE mechanism but, at present, the phenomena are not fully understood. In this work, the effect of cyclic load in the elastic field on passivity current (iP) and hydrogen steady state permeation current (iHss) is reported. Materials and Methods: The permeation tests were carried out on API 5L X65 steel, according to ISO 17081:2014. The specimen acts as bi-electrode between the two compartments of the Devanathan-Stachurski cell. In the anodic compartment, it is polarized at +340 mV vs Ag/AgCl in a 0.1 M NaOH solution. The cathodic compartment is filled with a borate solution (pH 8.5) and a cathodic current density of 2 mA/cm2 was applied once passivity current reached values below 0.05 µA/cm2. Tests in unloaded conditions and under cycling load up to 80% TYS in the frequency range between 10-2 Hz and 1 Hz were performed. Results and Discussion: Tests realized in unloaded conditions show iHss around 1 µA/cm2 and hydrogen diffusion coefficients around 7·10-12 m2/s. Such values are comparable to literature data collected on X65 steels. Preliminary tests under cyclic load show that iP modifies with load, but the response is shifted in phase. The amplitude of ∆iP increases with the load amplitude and frequency. Such behavior can be ascribed to the rupture and reformation of passive film, which is more evident at the highest strain rates (high frequencies). Similar behavior was noticed also for iHss, but values of ∆iHss are of the same order of magnitude of ∆iP. The average value of the hydrogen stationary permeation flux remains almost constant as the load frequency increases