Benzyltriphenylphosphonium chloride (BPP), an intermediate that forms [BPP][NTf2] with the same cation, also inhibits corrosion. Previous work has demonstrated that the cations of BPP cover the magnesium surface to repel corrosive substances [22]. However, the inhibitory efficiency of BPP is unsatisfactory due to the presence of Cl− anions in BPP. Therefore, it is necessary to find other compounds to form "coupling inhibitors" with BPP. L-Histidine (L-His) is non-toxic, cheap, easy to produce, and has a purity greater than 99%. In previous literature, L-His has been used as a corrosion inhibitor for steel, copper and nickel [[23], [24], [25]]. Maybe it's a potential corrosion inhibitor for magnesium alloys. Moreover, L-His contains an imidazole ring, which can be adsorbed on the metal surface by donating electrons and giving back, reducing the contact between the metal and corrosive substances.

The inhibitory behavior of benzyltriphenylphosphonium chloride mixed inorganic and organic inhibitors on magnesium alloys has been reported, including zinc nitrate as well as sodium aminopropyltriethoxysilicate (APTS-Na) [26] and sodium phosphate and sodium dodecylbenzene sulfonate (SDBS)[27]. BPP can not only be adsorbed on the surface of magnesium alloy, but also activate the porous passivation layer on the surface of magnesium alloy to form a protective film and inhibit corrosion. The protective effect of uneven films is limited. The adsorbed L-His repels Cl− anions and BPP in the NaCl solution, further reducing corrosion. In addition, the cations of BPP will act as a bridge between magnesium and L-His to form a stable complex, which is conducive to the formation of a uniform and dense film on the surface of magnesium. The synergistic effect between BPP and L-His changes the interface properties between the magnesium alloy and the electrolyte and reduces hydrogen evolution. Based on the above analysis, the combination of BPP and L-His may be a good performance magnesium synergistic corrosion inhibitor, but it has not been reported yet.

This work attempts to study whether a single BPP or L-His can alleviate the corrosion of AZ33 M magnesium alloy in 0.05 wt% and 0.5 wt% NaCl solutions. Further study of their synergistic effects aims to explore efficient synergistic inhibitors. The inhibitory behavior is discussed based on electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. Visually detect corrosion results with scanning electron microscopy (SEM) and atomic force microscopy (AFM). Furthermore, the inhibition mechanism was elucidated through X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) analysis and theoretical calculations. To the best of our knowledge, there are few studies on the inhibitory behavior of BPP and L-His on magnesium alloys, especially their synergistic effects. Reports on its inhibitory mechanism are even rarer. The central goal of this work is not only to find efficient inhibitors for magnesium alloys but also to elucidate their inhibition mechanisms. A clear inhibitory mechanism will facilitate the exploration of other robust inhibitors. The inhibitors developed in this work will also be used in combination with other anti-corrosion technologies to achieve better performance for potential large-scale applications in industry.