Thixomolded AZ91D and MRI153M magnesium alloys and their enhanced corrosion resistance. Issue 3 (12th January 2020)

• Record Type: Journal Article
• Authors: Buzolin, Ricardo H.; Volovitch, Polina; Maltseva, Alina; Lamaka, Sviatlana; Blawert, Carsten; Mendis, Chamini L.; Lohmüller, Andreas; Kainer, Karl U.; Hort, Norbert
• Is Part Of: Materials and corrosion. Volume 71:Issue 3(2020)
• Page Start: 339
• Page End: 351
• Publication Date: 2020-01-12
• Subjects: AZ91D -- corrosion resistance -- hydrogen evolution -- magnesium alloys -- MRI153M -- thixomolding; Materials -- Periodicals; Metals -- Periodicals; Corrosion and anti-corrosives -- Periodicals; 620.1122305
• DOI: 10.1002/maco.201911327
• Languages: English
• ISSNs: 0947-5117

Abstract

Abstract: AZ91D and MRI153M alloys were produced by thixomolding. Their corrosion resistance is significantly higher than that of similar materials produced by ingot or die‐casting. A corrosion rate smaller than 0.2 mm/year in 5 wt% NaCl solution is measured for the thixomolded AZ91D alloy. The corrosion behaviour was evaluated using immersion tests, electrochemical impedance spectroscopy, hydrogen evolution, glow discharge optical emission spectroscopy, and atomic emission spectroelectrochemistry. A bimodal microstructure is observed for both alloys, with the presence of coarse primary α‐Mg grains, fine secondary α‐Mg grains, β‐phase, and other phases with a minor volume fraction. The amount of coarse primary α‐Mg is significantly higher for the AZ91D compared with the MRI153M. The network of β‐phase around the fine secondary α‐Mg grains is better established in the thixomolded AZ91D alloy. A combination of several factors such as the ratio of primary to secondary α‐Mg grains, localised corrosion or barrier effect due to other phases, as well as regions of preferential dissolution of the α‐Mg due to chemical segregation, are thought to be responsible for the high corrosion resistance exhibited by the thixomolded AZ91D and MRI153M. Abstract : The mechanism of dissolution of the α‐Mg matrix in AZ91D and MRI153M is a complex interaction between microgralvanic corrosion, a barrier to the corrosion propagation of Al12 Mg17 phase and the difference in corrosion resistance between fine secondary and coarse primary α‐Mg. Their corrosion resistance is notably higher than that of similar materials produced by other casting techniques.