Experimental investigations on corrosion performance of heat-treated AZ91 Mg alloy

Corrosion Behaviour of High-Pressure Die-Cast AZ91 Alloy in NaCl Solution: Effects of Friction Stir Process at High Rotational Speed

“…In the former case (Figure 2a), the CR exponentially decreases from 39.1 ± 2.3 mm/year at 1 h of immersion time to 6.9 ± 0.2 mm/year at 20 h, and then it tends to settle at around 2.5 ± 0.1 mm/year from 40 h to 160 h. In the latter case, i.e., considering the BM-smooth sample (Figure 2b), CR starts from similarly high values, 36.0 ± 2.7 mm/year at 1 h, but it reaches 3.0 ± 0.2 mm/year just after an immersion time of 10 h. The CR continues to be lower than BM-rough conditions up to 160 h of corrosion time, where it is already stabilized at 0.6 ± 0.1 mm/year. This rapid corrosion attack was also reported by Sahu et al [3] who analysed the CR of polished AZ91 samples in 3.5% NaCl solution. The same authors reported CR values of the AZ91 alloy in the range of 0.02-0.22 mm/year due to the low surface roughness values conferred by the polishing process and due to the protective behaviour of the hydroxide compounds formed during the corrosion process.…”

“…Simultaneously, they helped the corrosion behaviour analysis in FSPed regions (areas indicated by the black arrows in Figure 1b) due to the intensification of the corrosion phenomena. The corrosion rate (CR, (mm/year)) was evaluated, according to both the ASTM NACE TM0169/G31 standard [38] and other studies [3,13,27], by the following equation:…”

“…where the constant k (-) is equal to 87.6 (W) to obtain a CR in mm/year, ∆W (mg) is the weight loss, t (h) is the immersion time, A (cm 2 ) is the area of the sample, and δ is the density of magnesium alloy (1.81 g/cm 3 ). The weights of unprocessed and FSPed samples were measured before and after each immersion test by an analytical balance.…”

“…Magnesium alloys are demonstrated to be attractive to the automotive sector, in which the AZ91 alloy covers 90% of all Mg cast products, and aerospace applications thanks to their high specific strength and low density which promote, for example, a decrease in oil consumption [1]. Simultaneously, their excellent biocompatibility and good biomechanical compatibility make them suitable in the biomedical field, e.g., for orthopaedic implants or the treatment of cardiovascular disease [2][3][4][5]. Magnesium-based alloys are, however, susceptible to corrosion attacks in humid or wet atmospheres after an oxide layer rupture.…”