Abstract
The paper reports on the influence of alternating current (AC) on the hydrogen uptake of X80 pipeline steel cathodically polarized with a constant current density of -100 A/m2 in different media (0.2 M Na2SO4, 0.2 M and 0.8 M NaOH, 0.2 M NaCl/MgCl2/CaCl2, artificial soil solution) at room temperature. Under these conditions AC current densities of 30, 100 and 500 A/m2 were applied. While the Devanathan-Stachurski method was not successful to indicate AC effects on hydrogen permeation, the volumetric quantification of hydrogen permeation proved its feasibility.
In the absence of promoters (e.g. arsenite) no hydrogen permeation was measured in all media regardless of the presence of AC (30 to 500 A/m2). Hydrogen permeation started only after addition of a promotor. In NaOH solution containing arsenite as promotor a small AC effect could be detected which, however, led to a small reduction of the hydrogen permeation rate. The effect was more visible at high AC current densities (e.g. 500 A/m2). As a side effect, intergranular 'cathodic' iron dissolution with formation of intergranular pits was found in promotor-free 0.2 M NaCl/MgCl2/CaCl2 solution. Within 24 h pit depths of 50 µm (some grain layers) could be formed in combination with Fe-, Ca- and Mg-containing deposits exhibiting filigree structures. In the presence of AC the formation of such deposits and scales was retarded. The mechanism of this 'cathodic' iron dissolution is discussed. During the experimental investigations no indications were found that AC can favor hydrogen related failure mechanisms at high strength pipeline steels under cathodic corrosion protection.
