Application of Side-View Imaging and Real-Time Hydrogen Measurement to the Investigation of Magnesium Corrosion

Abstract

This work illustrates how side-view optical imaging, anodic potentiodynamic polarization and realtime hydrogen evolution measurements can be applied and complementary used to study the processes occurring on a corroding magnesium surface. Side-view imaging reveals that hydrogen evolution takes three different forms: i) large and stable bubbles on the uncorroded regions, ii) a stream of fine hydrogen bubbles at the corrosion front and iii) medium-sized bubbles behind the corrosion front. The observation suggests that the relatively large bubbles ahead and behind the corrosion front are associated with a purely cathodic reaction that provides the 'remote current'. This current, combined with the depassivating role of chloride ions, maintains the corrosion front active, generating regions where either the metal is locally directly exposed to the electrolyte or it is only covered by a poorly protective chloride-rich film. Regardless of the precise nature or morphology of the poorly protective (or absent) surface film at the corrosion front, additional hydrogen evolution occurs due to the large overpotential available. Overall, the anodic current associated with magnesium oxidation is the sum of the 'remote current', which is manifested with hydrogen evolution ahead and behind the corrosion front, and the 'local' current associated with hydrogen evolution at the corrosion front. From the electrical viewpoint, the corrosion front acts as a current amplifier where the current amplification effect can be measured directly by real-time gravimetric hydrogen measurement performed simultaneously with anodic potentiodynamic polarization.

The contribution of hydrogen evolution processes during corrosion of aluminium and aluminium alloys investigated by potentiodynamic polarisation coupled with real-time hydrogen measurement

“…3,4,12,16,[21][22][23][24][25][26][27] On these metals, with relatively low electrochemical potential for oxidation, hydrogen evolution is always thermodynamically possible in the presence of water, but may be hindered by the presence of an oxide, hydroxide, or mixed film that physically separates the metal surface from the electrolyte. In some conditions, when the film is locally disrupted, such as for example during anodic polarisation in chloride-containing environment or at active corrosion sites (pits) during free corrosion, hydrogen evolution might occur locally, providing additional current sustaining corrosion propagation.…”

“…2-5, 21, 34 Thus, when cathodic activation takes place, the overall rate of hydrogen evolution at the free corrosion potential increases since there is an increase in hydrogen evolution occurring on the cathodically activated corroded regions, which result in increased availability of anodic current at the corrosion front which, in turn, results in further local hydrogen evolution. 12,21,23 Based on the previous considerations, it is clear that understanding the details of hydrogen evolution can provide insight into the corrosion behaviour in simple and more complex alloys. For aluminium, however, the experimental investigation at the open circuit potential is not trivial since, during free corrosion, the amount of hydrogen evolved is relatively low, and difficult to measure directly.…”

“…local acidification and/or presence of aggressive ion such as chlorides. 12,22,23,28 A model was proposed, where the superfluous hydrogen evolution was generated at the active corrosion front, due to the local disruption of passivity, resulting either in the direct exposure of the metal to the electrolyte or in the formation of only a poorly protective chloriderich film. 12,28 At those locations, due to the large potential difference available locally, hydrogen evolution can take place and, overall, the corrosion front acts as a current amplifier, where the anodic current produced at some locations far from the corrosion front, i.e.…”

The Role of Surface Films and Dissolution Products on the Negative Difference Effect for Magnesium: Comparison of Cl−versus Cl−Free Solutions

“…21 Moreover, the majority of hydrogen evolved is visibly seen to originate at the head of most actively advancing corrosion fronts and is not uniform across the blackened areas which may suggest that the 'older' and net cathodic regions are not the primary source of anodic hydrogen evolution e . [21][22][23] Further electrochemical evidence for cathodic activation was shown through a series of alternating anodic and cathodic polarization scans which revealed increasing cathodic kinetics with increasing anodic polarization of Mg surfaces. 24 Additional cathodic potentiodynamic polarization scans performed after various anodic treatments also revealed an increase in the corrosion potential, in addition to increased cathodic kinetics.…”

“…It has been argued by many that the growing anode sites specifically possess the highest reaction rate for HER. [21][22][23] Another important aspect of the SVET work is that the measured net cathodic current density at these cathodic sites increased as the net anodic current density became greater to maintain charge balance (∫ I anodic dt −|∫ I cathodic dt| = ∫ I applied dt). However, the integrated net cathodic current density with time from the SVET at steady state is much less than the volumetric hydrogen evolution rate d .…”