The ability of Mg2Ge crystals to behave as ‘smart release’ inhibitors of the aqueous corrosion of Zn-Al-Mg alloys

Mechanistic Study on the Corrosion Behaviour of Zinc and Zinc-Calcium Alloys Designed for Enhanced Metallic Coatings in the Presence of Chloride and Phosphate Ions

“…This occurrence of re-solubility and precipitation was due to the reduction in local pH at the anodic front. This phenomenon of change in local pH was also observed for ZMA and ZMA-Ge alloys [5,12]. It is also worth noting that the cathodic site (opposite side to the anodic site) where oxygen Supplementary material related to this article can be found online at doi:10.1016/j.corsci.2022.110956.…”

“…All experiments were performed in 0.17 M NaCl at pH 7. The dissolved oxygen concentration within the bulk electrolyte was presumed to be equal to the equilibrium concentration value for air-saturated water i.e., 2.8 × 10 − mol dm − 3 [12,24]. SVET has been used comprehensively to understand the surface and cut-edge corrosion of metallic coatings and also the effectiveness of phosphate inhibitors [4,[25][26][27][28][29].…”

“…The probe vibrates perpendicularly at a constant frequency of 140 Hz and an amplitude of 25 µm at 100 µm above the corroding surface. The equipment detail description, operating procedure and calibration procedure can be found in detail elsewhere [12,24,[31][32][33]. During corrosion, potential fields are set up in the electrolyte due to the movement of ionic current flux between anodes and cathodes.…”

“…A Meiji Techno 7100 optical microscope together with an Infinity 2 digital camera was used to capture images under immersion conditions. The detailed methodology has been described previously [5,12,24]. TLM is a simple and effective technique that enables capturing images of corroding surfaces at a microstructural level under immersion conditions at room temperature.…”

“…The latter has proven to provide improved corrosion resistance over standard Zn and binary Zn-Al due to pH changes induced by the initial attack on MgZn 2 rich phases promoting alkaline pH values [4,5] that stabilises zinc phases [6,7] and promote the formation of stable double layered hydroxide corrosion products [8][9][10][11]. We have previously shown that further quaternary additions of Germanium (Ge) [12] can enhance the performance further through the preferential release of Mg 2+ from the microstructure without the concurrent release of hydrolysable Zn 2+ thus maintaining alkaline pH conditions across the surface of the alloy. These findings provide further opportunities for other elemental additions that may modify the surface electrolyte pH.…”