Microelectrochemical Property of Precipitates in Al-Mg Alloy

Al-Mg alloys are widely used for many applications, such as marine and land vehicles, construction industry, and chemical plants. Mg is added to Al to strengthen via solid solution hardening and to improve corrosion resistance. However, β phases (Al₃Mg₂) precipitates along grain boundaries after heat-treatment at elevated temperatures from 323 K to 573 K, and several kind of precipitates also exist in Al-Mg alloys. Localized corrosion is thought to be caused by local cell between the matrix and the precipitates. It is important to analyze the microelectrochemical property of precipitates and the matrix. And also, corrosion initiation mechanism is still unclear. In this research, microelectrochemical measurements were carried out to observe initiation site and growth of pitting.

Specimen was AA5182 alloy. The chemical composition was shown in Table 1. No heat treatment was performed in this study. At first, the characterization of the precipitates on the specimen was conducted. Precipitates were classified into two types (Types A and B) by optical microscopic observation of the polished surface using Ar⁺ ion milling. The colors of Types A and B were blue and gray, respectively. From the result of energy dispersive X-ray analysis (EDS), Types A and B were determined to be Mg₂Si and Fe-rich intermetallic. To ascertain the existence of β phase, the specimens were immersed in 40 mass% H₃PO₄for 180 s. No selective dissolution along grain boundaries was observed. It was considered that β phase did not precipitated in as-received conditions (mill-annealed).

Microscopic potentiodynamic polarization was performed in a naturally aerated 0.1 M NaCl solution at 298 K to observe initiation site and growth of pitting. The sample was mechanically ground with SiC paper through a 4000 grit, and then polished down to 1 µm using diamond paste. Electrode area was about 0.1 mm². Fig.1 (a) shows the microscopic polarization curve. Several current spikes were generated during the polarization, and these were thought to be related to pit initiation. Fig. 1 (b) exhibits an image of the electrode area by scanning electron microscope (SEM). As shown in Fig. 2 (a) and (b), initiation site of stable pit was related to the precipitate. Other dissolution sites were also observed: Mg₂Si became Si-oxide by selective dissolution of Mg, and the matrix around Fe-rich intermetallic dissolved since the intermetallic became cathodic site.