**1. Introduction**

The 5xxx series Al-Mg alloys are widely used materials in the automotive industry due to their high strength-to-weight ratios, weldability, and good corrosion resistance [1–3]. Strength and ductility can be improved by adding solute Mg to the alloys because of the solid solution strengthening mechanism [4,5]. Nevertheless, as the amount of Mg increases, a selective oxidation prevails and causes a formation of oxide inclusions at elevated temperatures [6]. The rapid oxidation of magnesium alloys can be suppressed by additions of Ca [7]. With this background, a new alloying strategy has been developed. The strategy uses an Mg + Al2Ca master alloy instead of pure Mg during casting. The Mg + Al2Ca master alloy improves the oxidation resistance of Al-Mg alloys by forming a protective CaO/MgO mixed layer on the surface [8,9]

Previous studies have shown that anodic β-phase (β-Mg2Al3) precipitation along grain boundaries is an important factor affecting the intergranular corrosion (IGC) susceptibility of Al-Mg alloy [6–8]. The Mg segregation leads to anodic β-phase formation at 50–200 ◦C. The β-phase precipitation is referred to as sensitization [10–13]. The β-phase formation is usually observed at grain boundaries (GB), sub-grains, intermetallics, dislocations and other defects. Generally, the sequences of β precipitation have been reported as follows [14–16]:

Solid solution α → Guinier–Peston zones → β" → β' → β precipitation The β phases are electrochemically active compared to the Al matrix which leads to galvanic corrosion. The distribution and morphology of β precipitates affect the susceptibility of the alloy to IGC. To evaluate the IGC susceptibility, a nitric acid mass-loss test (NAMLT, ASTM G67) can be used to measure the corrosion rate.

**Citation:** Choi, K.-H.; Kim, B.-H.; Lee, D.-B.; Yang, S.-Y.; Kim, N.-S.; Ha, S.-H.; Yoon, Y.-O.; Lim, H.-K.; Kim, S.-K. Intergranular Corrosion and Microstructural Evolution in a Newly Designed Al-6Mg Alloy. *Materials* **2021**, *14*, 3314. https://doi.org/ 10.3390/ma14123314

Academic Editor: Marián Palcut

Received: 11 May 2021 Accepted: 9 June 2021 Published: 15 June 2021

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In this research, the IGC susceptibility of sensitized Al-6Mg alloy with two different cold rolling conditions was studied. It is not yet clearly understood how the cold rolling process affects the corrosion behavior [17]. D'Antuono et al. observed that an increased rolling reduction increased the growth rate of β precipitation due to lowering nucleation temperature [18]. Additionally, an initial β precipitation was observed preferentially at low-angle grain boundaries rather than high-angle grain boundaries [19]. On the contrary, Wang et al. found that the maximum corrosion depth decreases with increasing cold rolling reduction ratio. They also stated that larger thickness reductions are attributable to an increased number of small-sized grains formed at the grain boundary, which can eventually break off the continuity of corrosion.

This paper aims to figure out the effect of cold rolling and sensitization treatment on the IGC susceptibility of a newly designed Al-6Mg alloy. In this study, a microstructure evolution of the alloy was analyzed by electron backscattered diffraction (EBSD). The continuity of β-phase precipitation at grain boundary was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of the cold rolling reduction ratio and sensitization heat treatment on the IGC susceptibility are discussed.
