Elucidation of the Initiation of Pitting Corrosion and the Growth of Filiform Corrosion of AZ91D in Aqueous NaCl Electrolyte
Authors: Shao Zheng , Muto Izumi , Sugawara Yu 
Published: December 2020

1 Introduction
Mg-Al-Zn alloys such as AZ91 (Mg-9% Al-1% Zn) are very lightweight, and expected to be used in transportation equipment (e.g.: automotive and aerospace). Mathieu et al. studied the effect of elements and phase compositions of AZ91 alloy on corrosion behavior 1)-4), and they revealed that the content of aluminum and the distribution of Mg17Al12 have the significant effect on the corrosion resistance of the alloy. However, the corrosion mechanism is still not perfectly figured out. In this study, the aim is to elucidate the initiation of localized corrosion of die-cast AZ91D.
2 Experimental
2.1 Specimen
Die-cast AZ91D was cut into 25 mm× 30 mm × 5 mm sheets, mechanically ground with SiC paper through a 2000 grit, finished with 0.25 µm diamond paste, and finally cleaned ultrasonically with ethanol. To ascertain the grain structure, the specimens were etched in a mixed reagent of 3 mL HNO3 and 80 mL ethanol.
2.2 Macro-scale electrochemical measurements
Open circuit potentials and potentiodynamic polarization curves were measured at 298 K. The specimen surfaces were coated with an epoxy resin and subsequently with paraffin. Measurements were performed in a conventional three-electrode cell: the counter electrode was a Pt wire, and the reference electrode was Ag/AgCl (3.33 M KCl). The electrolyte used were naturally aerated 0.1 M and 0.01M NaCl (pH 8.0, adjusted with NaOH). The size of the electrode area was ca. 1cm2.
2.3 Micro-scale electrochemical measurements
To elucidate the pit initiation sites, potentiodynamic polarization curves for a small electrode area for AZ91D die-cast were measured. The electrode area was ca. 0.25 mm2 or ca. 0.01 mm2. The specimen surfaces, with the exception of the electrode area, were masked with a silicone sealant, and the sealant was dried for 24 h. To preserve the electrolyte, a small acrylic cell was put on the specimen. Before the potentiodynamic polarization, the reference and counter electrodes were connected with a small droplet of the electrolyte. The electrolyte used were naturally aerated 0.1 M NaCl (pH 5.46) and 0.01M NaCl (pH 8.0, adjusted with NaOH). After polarization, corrosion morphology was characterized using a laser microscope, optical microscope, and FE-SEM.
3 Results
In macro-scale measurements, the open circuit potential was approximately -1.5 V but oscillating during the immersion. The oscillations were due to the competition of corrosion initiation and repassivation. After immersion, it was observed that filiform corrosion occurred.
In micro-scale polarization, regardless of the size of the sample area, pitting tended to occur underneath the coating. The discoloration occurred on the eutectics. Some eutectics discolored, and some did not transform. The discolored eutectics might be close to the grain boundaries. The initiation of pitting was seen at the eutectic phase adjacent of β phase. Pitting caused the initiation of filiform corrosion. After the pit initiation, hydrogen gas bubbles were generated. During the growth of filiform corrosion, hydrogen was also continuously produced.
References
[1] S. Mathieu, C. Rapin, J. Steinmetz, and P. Steinmetz, “A corrosion study of the main constituent phases of AZ91 magnesium alloys” Corrosion Science, 45, pp. 2741–2755, 2003
[2] Y. Li, T. Zhang, and F. Wang, “Effect of micro crystallization on corrosion resistance of AZ91D alloy,” Electrochimica Acta, 51, pp. 2845–2850, 2006,
[3] R. Ambat, N. N. Aung, and W. Zhou, “Evaluation of microstructural effects on corrosion behavior of AZ91D magnesium alloy” Corrosion Science, 42, p. 23, 2000.
[4] L. Wang, B.-P. Zhang, and T. Shinohara, “Corrosion behavior of AZ91 magnesium alloy in dilute NaCl solutions,” Materials & Design, pp. 857–863, 2010

Figure 1