The microstructure and corrosion performance of AZ31B-H24 magnesium alloy sheet

Abstract

AZ31 magnesium alloy sheet offers a promising candidate for the weight reduction of large-scale automotive components in order to save fuel and reduce the carbon dioxide emission. Despite its good corrosion performance in air, high humidity and the presence of aggressive ions such as chlorides significantly reduce the corrosion resistance leading to localised and fast dissolution. Consequently, the use of magnesium-based alloys is limited, in particular in external automotive applications. A comprehensive understanding of the corrosion behaviour of AZ31 sheet, taking into account the alloy's microstructure, is needed in order to improve its corrosion protection.This research programme elaborately investigated the commercial AZ31B-H24 magnesium alloy sheet with regards to its microstructure and corrosion performance in concentrated and dilute sodium chloride solutions. The grain structure and typical AlMn intermetallic phases were characterized in addition to grain boundary particles of γ-Mg17(Al,Zn)12. Possessing nobler electrochemical potentials with respect to the magnesium matrix, all the intermetallic phases provided cathodically active sites corroding at significantly slower rates. During corrosion, coarse AlMn intermetallic particles were found to dealloy primarily with selective dissolution of aluminium. High-resolution microscopy revealed that zinc and aluminium segregation on the grain boundaries led to intergranular corrosion.The corrosion in AZ31B-H24 alloy proceeded in three well-defined sequential stages upon immersion in sodium chloride solutions, namely corrosion initiation (stage I), shallow filiform-like propagation (stage II) and localised in-depth corrosion (stage III). It was concluded that the corrosion stages were mainly determined by the formation of resistive corrosion products and defects in the alloy. The comprehensive ex-situ microscopic study of the three corrosion stages in combination with real-time measurement of hydrogen evolution and conventional electrochemical investigations provided new insights such as the formation of crystallographically oriented pores (COP) indicating corrosion propagation mechanism. The COP preferentially developed parallel to the {10-10} and the {11-20} families of planes as identified by electron backscatter diffraction. Mechanistic corrosion aspects including the cathodic activation of the intact surface were discussed and explained. The intact surface was covered by a bilayer, consisting of an outer layer of crystalline Mg(OH)2 platelets and an inner layer of hydrated and amorphous Mg(OH)2, that grew with immersion time. Zinc enrichment formed under the bilayer and its thickness increased with increasing exposure reaching 7 ± 1 nm after 4 days immersion. Nano-sized particles in the enrichment were detected and identified as MgZn2. Enrichment in zinc and aluminium under the corrosion products seemed to have minor influences on the overall cathodic activity.

Date of Award	1 Aug 2017
Original language	English
Awarding Institution	The University of Manchester
Supervisor	George Thompson (Supervisor) & Xiaorong Zhou (Supervisor)