Relationship between Microstructure and Hydrogen Permeation of a Plastic Deformed API 5L X52 Steel

The main objective of this work was to study the effect of plastic deformation (cold work) on the hydrogen permeation of an API 5L X52 steel, commonly used in the oil industry, considering the relationship between microstructure and hydrogen diffusion.
The steel used was a pipe grade that was rectified to eliminate its original curvature and then cut into small samples and the thickness was reduced. After that, some of these samples were cold worked to get different deformation percentages in terms of area reduction.
The steel samples were characterized, before and after the electrochemical experiments by means of Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) to establish the relationship between microstructure and defects on the hydrogen produced damage.
The steel samples were electrochemically characterized using potentiodynamic polarization curves. The test solutions employed were 0.1 M de NaOH and 0.1M de Na₂SO₄ (pH ≈2).
The hydrogen permeation tests were performed on samples of the API 5L X52 steel in the as received and cold worked conditions. The experiments were carried out following the Devanathan and Stachurski technique, using 0.1M de Na₂SO₄ (pH ≈2) as the electrolyte in the cathodic cell compartment and 0.1 M de NaOH as the electrolyte in the anodic one. The electrode potentials employed were -900 mV vs SCE (saturated calomel electrode), and +150 mV vs SCE, for the cathodic and anodic polarization respectively.
The obtained results showed that the hydrogen absorption capacity is higher when the steel has the larger deformation degree. This could be associated to the presence of substructures with small cell size range (0.01-0.15 μm), meaning a higher dislocation density, which could act as a preferential site for a greater hydrogen absorption and transportation. On the other hand, it is important to state that with the increase in the deformation percentage, the mean hydrogen flux values are greater to that one of the steel in the as received condition.
All the diffusion coefficients have the same order of magnitude, although the steels with the higher deformation have slightly greater values. Again, these results are a clear indication that the hydrogen adsorption, intake and diffusion are favored by the dislocation arrangements that formed the above mentioned substructures. Moreover, the largest time required to reach the hydrogen saturation in the steel with the higher deformation degree is a strong evidence of a higher dislocation density presence.
The main hydrogen damage found was blisters associated to inclusions.  The blisters diameter increased as plastic deformation was increased. The blisters growth was associated to a coalescence mechanism.