Hydrogen evolution is one of the common cathodic reactions involved in corrosion of metals and alloys in aqueous solutions. The reduction of hydrogen ions is a source of atomic hydrogen on the surfaces of metals. Corrosion reactions, the application of cathodic protection and electroplating, etc., can introduce hydrogen into metals via diffusion. The presence of hydrogen in metals can have a significant influence on the mechanical properties and electrochemical behavior of metals depending on the environments. Stainless steels generally corrode at very low rates in high temperature aqueous environments because of their tendency to form a protective oxide film. Hydrogen is incorporated into the structural alloys during production, fabrication, processing, from in service conditions, and from the electrochemical reaction during corrosion. The transport of hydrogen through passive films is involved in many corrosion processes such as pitting, stress corrosion cracking and corrosion fatigue.

The nature of passive films on metals and alloys is a decisive factor that controls their corrosion behavior. Mmuch work has been devoted to the study of the electronic properties and chemical composites of passive films on metals and alloys. Based on the fact that the potential-dependent transpassive dissolution varies with the electronic properties of the passive film, Sato proposed a breakdown mechanism of the passive film on metals, the electrochemical stability of a passive film strongly depends on the electron energy band structure in the film. Bianchiet al. concluded that the high susceptibility of stainless steel to pitting nucleation was connected to n-type conductivity of the oxide film. Cr-bearing stainless steels have been widely used in power generation systems and other industrial systems. The effects of hydrogen on the surface film and electrochemical behavior of 13Cr stainless steel in a chloride solution were investigated in the present work. Hydrogen was introduced into the samples by cathodic charging under galvanostatic condition at 25 ^oC in sulfuric acid solution with thiourea. The specimens were charged at a current density 2 mA/cm² for 2 h. Electrochemical measurements were carried out on both uncharged and hydrogen charged specimens for evaluating their corrosion resistance. Potentiodynamic anodic polarization curves were carried out in 200 ppm wt.% NaCl solution at 25 ^oC, as shown in Figure. 1. Pre-charged hydrogen specimen showed a significantly lower open-circuit potential of about -0.67 V(SCE),  and the non-charged specimen showed OCP of about -0.11V (SCE). Anodic current density in the anodic polarization curves of the hydrogen-charged specimen was significantly higher than that of the non-charged specimen. These results indicate that the corrosion resistance of 13Cr steel was significantly reduced by charged-hydrogen.

Figure 2 show the electrochemical impedance spectra (EIS) for hydrogen-charged and non-charged specimens after different immersion periods in 200ppm wt.% NaCl solution at 25 ^oC. EIS diagrams of hydrogen-charged and non-charged specimens exhibited two parts: a capacitive loop in the high-frequency region and a capacitive loop in the low-frequency region. The second capacitive arc radius for the hydrogen-charged specimen was significantly less than that of the non-charged specimen.

The scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) was used for the element and surface morphology analysis of the specimens after immersion tests in 200ppm wt.% NaCl solution at 25 ^oC for 68 h. Little corrosion product could be found on the non-charged specimen surface while more corrosion product appeared on the hydrogen-charged specimen surface, as shown in Fig. 3. Cr content in the film formed on hydrogen-charged specimen was lower than that on non-charged specimen, indicating the effect of hydrogen on the chemical composition of surface film.