Hydrogen-related attack is a specific mechanism that can contribute to SCC (Stress Corrosion Cracking), through its influence on internal stress and the microstructure due to atomic hydrogen concentration diffused and accumulated during operation conditions due to the formation of atomic hydrogen in presence and absence of polarization conditions (cathodic protection). In this work, an API X-65 grade steel pipeline is exposed to NS4 synthetic groundwater solution in hydrogen permeation cell type experiments in order to follow the hydrogen reaction in the cathodic compartment and to characterize the total current between anodic and cathodic compartments and the parameters obtained from polarization tests. A theoretical model based on adsorption coverage is used to characterize atomic hydrogen that forms and diffuses within the steel, as a competition of the hydrogen and bicarbonate ions for the available active sites at the interface. Electrochemical Impedance Spectroscopy is used to characterize the interfacial electrode-electrolyte interface reactions at different sodium bicarbonate concentrations at room temperature under static conditions in order to follow the influence of hydrogen ion for the surface active sites to form atomic hydrogen. For SCC conditions the AC impedance shows the behavior of the interface when molecular hydrogen is induced by polarization, qualitative analysis includes DC electrochemical testing associated with AC impedance measurements. Experimental analysis considers the quantification of atomic hydrogen by considering interfacial reactions and magnitudes obtained by AC impedance during potential bias experiments and associated with covering factors obtained from DC techniques. Steel pipelines samples were mechanical pretreated to influence the surface and increase the hydrogen sensitivity into the metallic structure SCC testing under different polarization conditions is presented as the consequence analysis for hydrogen entry mechanism under near neutral solution.
Hydrogen reaction is considered one of the most studied ones by different areas of science and technology, cathodic reaction from hydrogen ion in aqueous solution converted to hydrogen molecular gas, or recombination reaction that considers atomic to molecular formation of hydrogen Stress Corrosion Cracking is a form of corrosion that considers hydrogen under specific situations, like NNSCC (Near neutral stress corrosion cracking). Near Neutral solutions are simulated with NS4 solution ((g/L: 0.122 potassium chloride (KCl), 0.483 sodium bicarbonate, (NaHCO3), 0.181 calcium chloride (CaCl2.H2O) and 0.131 magnesium sulfate (MgSO4.7H2O)) [1], this environment considers groundwater conditions similar to the pipelines that are exposed to underground environments while used for transmission lines. Neutral environments can form atomic and/or molecular hydrogen as a product of chemical or electrochemical reactions [2-4]. Hydrogen ion in aqueous solution is transported through the bulk solution and the corrosion products formed thermodynamically mainly of iron carbonate and iron oxides depending on the pH and temperature conditions [5]. Mass transport phenomenon of the ionic species considers migration and diffusion within the pores of the corrosion product layer. This physical barrier decreases the flux of the ionic species to the metallic surface. Some ions will accumulate within the porous layer and others will react at the metal surface-layer interface forming different corrosion products that might integrate in the solid structure.
