SVET and SIET Study of Galvanic Corrosion of Al/MgZn₂in Aqueous Solutions at Different pH

Abstract

The galvanic corrosion behavior of synthesized MgZn 2 intermetallic particle coupled with Al in 0.1 M NaCl solutions across pH 2 to12 has been studied using the scanning vibrating electrode technique (SVET), the scanning ion-selective electrode technique (SIET) and X-ray photoelectron spectroscopy (XPS). Results indicate the galvanic dissolution of MgZn 2 depends on pH as well as exposure time. SVET current density maps reveal that MgZn 2 is anodic relative to Al at pH 2 but cathodic at pH 12, while self-dissolution of MgZn 2 prevails at pH 4 and 6. However, the galvanic coupling effect is more pronounced at pH 2 and 12. SIET maps of H + and Cl − ion distribution reveal evidence of local alkalization on MgZn 2 and migration of Cl − ions away from MgZn 2 surface at pH 4 and 6 due to significant accumulation of OH − ions. High strength 7xxx series Al alloys, which are parts of structural components in the aerospace and automotive industry, are vulnerable to localized corrosion in the presence of chloride ions. The localized corrosion manifests in the form of pitting, crevice, inter-granular and exfoliation corrosion, which increase stress corrosion cracking susceptibility.1,2 Localized corrosion is initiated at microstructural inhomogeneities such as grain boundaries or coarse intermetallic phases. Anodic dissolution of grain boundary precipitates (intermetallic particles) such as MgZn 2 is always considered a probable reason for stress corrosion cracking in Al 7xxx series alloys.3 Galvanic corrosion can occur between the intermetallic particles and the surrounding alloy matrix due to the difference between the electrochemical activity of the particles and the matrix, this leads to the deterioration of Al alloys in engineering materials. 4 The electrochemical behavior of MgZn 2 and its role in corrosion of 7xxx series alloys has been investigated by various researchers using various traditional methods.5-8 For instance, electrochemical testing carried out on MgZn 2 by Birbilis et al. 6,7 and Wloka et al. 9 using the microcell method indicates that MgZn 2 is anodic relative to the matrix and corrodes freely above their corrosion potential. Potentiodynamic polarization carried out by Ramgopal et al. 4 on a thin film analog of MgZn 2 indicates that it is anodically very active. The scanning Kelvin probe force microscopy (SKPFM) investigations on Al 7075 by Andreatta and coworkers 5,10,11 revealed that the initiation of the attack at the intermetallic sites is caused by strong potential differences between the matrix and intermetallics, in particular, MgZn 2 particles along grain boundaries. However, the kinetic parameters relating to the galvanic couples could not be provided. It then becomes obvious that some issues remain regarding the galvanic corrosion behavior of MgZn 2 due to lack of information on the kinetics of local processes occurring at the matrix/ solution interface. 6,7,12,13 These issues include the galvanic current density distribution as well as the pH and ion distributions especially in near neutral...

Green corrosion protection for mild steel in acidic media: saponins and crude extracts ofGongronema latifolium

“…Thermodynamic parameters provide useful insights when discussing the corrosion behavior of different systems (Ikeuba et al , 2018). The temperature of the system was varied across the inhibitor concentrations studied from which the activation energy for the system was evaluated using Arrhenius equation given by the following equation (3): where C R is the corrosion rate, E a is the apparent effective activation energy, R is the general gas constant and A is the Arrhenius pre-exponential factor (Oguzie et al , 2010; Oguzie and Okhagbuzo, 2010).…”

Insitu SVET studies on the current density distribution on dissolving of Mg, MgZn2, Mg2Si and Al4Cu2Mg8Si7 surfaces in NaCl solutions

“…The samples cell was transferred into a humidistat to reduce evaporation after each scan. The detailed sample-probe arrangement in the SVET experiment is described in our previous work (Ikeuba et al , 2018; Ikeuba et al , 2019c). All SVET tests were carried out in an aerated environment at ambient temperature (21 ± 2◦C).…”

“…MgZn 2 have been noted to active and exhibit extensive anodic dissolution compared to the aluminum alloy matrix. Studies on the electrochemical behavior of MgZn 2 indicates that it corrodes freely above its corrosion potential (Birbilis and Buchheit, 2005; Wloka and Virtanen, 2008; Ikeuba et al , 2019a, 2018).…”

“…From the constituent of these intermetallic compounds it can be seen that they contain an appreciable amount of Mg (Kairy et al , 2016a; Kairy et al , 2016b; Wloka and Virtanen, 2008; Ramgopal et al , 2001). Reports in the literatures reveal that the corrosion of Mg containing alloys occurs by initial preferential dissolution of Mg followed by the enrichment of a more noble metal constituent (Ikeuba et al , 2018; Kairy et al , 2016a; Li et al , 2017). In view of the appreciable amount of Mg in these compounds, the distribution of local current density on the surface will have great influence on their dissolution behavior considering the fact that Mg is a very electropositive element.…”

“…The current density distribution on an electroactive surface can be generated using insitu analytical techniques such as the scanning vibrating electrode technique (SVET). This technique can readily provide a current density map through the vibration of a probe over sample surface in solution which shows distinct the anodic and cathodic zones as well as the magnitude of the corrosion current density on the corroding surface on a micro scale (Ikeuba et al , 2018; Williams et al , 2013). Herein, we present insitu SVET studies on the current density distribution on dissolving of Mg, Mg 2 Si, Al 4 Cu 2 Mg 8 Si 7 and MgZn 2 surfaces in sodium chloride solutions under an ambient temperature.…”