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Galvanic corrosion of tri-metallic couples is more complicated than that of bi-metallic couples. In this study, the effect of the pH of corrosive media on the galvanic corrosion of 2024 Al alloy/Q235 mild steel/304 stainless steel tri-metallic couples was investigated using potentiodynamic polarization, scanning electron microscopy, scanning vibrating electrode technique and a multi-channel galvanic corrosion meter. The results show that 2024 always acts as the only anode in 3.5 wt% NaCl at pH 5.56, 9.72 and 12.0, while both Q235 and 2024 act as anodes at pH 2.39 in the initial stage and then the role of Q235 changes at longer coupling time, which can be attributed to the effect of pH on the surface film of 2024. It is also found that the galvanic current density of a tri-metallic couple is the superposition of two bi-metallic couples when cathodic reactions are controlled by the diffusion of oxygen, otherwise it is smaller than that of the sum of two bi-metallic couples. The localized corrosion instead of uniform corrosion of anodic metal is accelerated by galvanic corrosion.

Pitting corrosion of magnesium (Mg) alloys is greatly associated with their microstructure, especially second phases. The second phases in traditional Mg alloys such as AZ91 are electrochemically nobler than Mg matrix, while the second phases in Rare earth (RE) Mg alloy GW93 are more active than Mg matrix. As a result, the pitting corrosion mechanism of Mg alloy GW93 is different from the traditional ones. This paper aims to clarify the pitting corrosion mechanism of Mg alloy GW93 through the studies of Volta potential by Scanning Kelvin Probe Force Microscopy (SKPFM), corrosion morphology by Scanning Electron Microscope (SEM), and corrosion resistance by electrochemical tests. Results reveal that the pitting corrosion of GW93 includes three stages, first, dissolution of the second phases, followed by corrosion of Mg matrix adjacent to the dissolved second phases, and finally, propagation of corrosion pits along the depth direction of the dissolved second phases. Anodic second phases and enrichment of Cl− in the thick corrosion product films dominate the propagation of pitting corrosion.

In this study, an environment-friendly hydrotalcite film has been deposited on AZ31 Mg alloy by a two-step technique. To improve conversion film technique and control film properties, batch studies have been carried out to address various process parameters such as immersion time, pH value, and temperature of the treatment solution. The morphologies and chemical composition were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The corrosion resistance of the samples with the various final films was then further compared by polarization curves. It can be concluded from the results that, at low value of pH, temperature, or long deposition time, the precursor film is mainly composed of a cracked layer. The transformation of hydrotalcite film is largely influenced by the pH value. There are optimum values of pH, temperature of the coating bath and immersion time for the film formation process to achieve the best quality and corrosion resistance of the hydrotalcite film. The optimum process is as follows: the sample is first immersed in the pretreatment solution with a pH value of about 8 at a temperature of 60 °C for 30 min to form a precursor film and then this precursor film is immersed into the post treatment solution with a pH of 10.5 at 80 °C for 1.5 h to obtain the Mg–Al hydrotalcite conversion film.