Transmission Electron Microscopy Study of Aqueous Film Formation and Evolution on Magnesium Alloys

Abstract

The films formed on ultrahigh purity Mg, Elektron 717 (ZE10A), and AZ31B in water at room temperature were characterized by TEM, XPS, and SIMS. The films consisted primarily of MgO, with surface regions also containing Mg(OH) 2 and MgCO 3 . SIMS suggested H throughout the films and into the underlying metal. Segregation of Zn to the metal/film interface and Al in the film was observed for AZ31B. Similar Zn film segregation was also detected for Elektron 717, along with Nd at the alloy/film interface and nano-size Zn 2 Zr 3 precipitates throughout the film. Implications of these findings on film growth are discussed. Magnesium and its alloys are lightweight structural materials of great interest for reduced vehicle weight and improved fuel efficiency in the automotive industry due to their good strength, low density, amenability to casting, and ease of recycling.1-3 A major drawback, however, is the poor corrosion resistance of Mg under many conditions. 4 Magnesium is considered the most reactive structural material and is susceptible to multiple forms of corrosion; including general dissolution, galvanic coupling, localized corrosion, and stress corrosion cracking. 2,3,5,6 Surface treatments/coatings are needed for many applications which result in increased cost and can be a source of component durability issues. 1The inability of Mg alloys to establish a continuous and fully protective surface film under many exposure conditions is a key factor underlying their susceptibility to corrosive attack. Films formed on Mg alloys under humid air or aqueous corrosion conditions are typically based on mixtures of MgO and Mg(OH) 2 .4,7-12 Although MgO can offer a degree of protection under ambient dry atmospheric corrosion conditions, it can be readily hydrated in the presence of water or water vapor to form Mg(OH) 2 , which offers limited stability and reduced protectiveness. 4 Further, considerations based on Pilling-Bedworth (P-B) ratio have also been invoked for explaining the inability of Mg to establish a fully protective oxide film. 13 Magnesium has a P-B ratio of ∼0.8, which places surface MgO in tension, and can result in a porous, cracked, nonprotective oxide layer. (Typically a P-B ratio of 1-2 is considered a minimum, but not sufficient prerequisite for protective film formation). Corrosion can be further significantly accelerated when aggressive electrolyte species such as chloride are present. 5,14 Alloying has been shown to modify surface film performance; 15 however, a detailed mechanistic understanding of how and why is frequently lacking.Extensive studies based on electrochemical assessment and surface film chemistry depth profiling of Mg alloys have been pursued by techniques such as Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), etc. [16][17][18][19][20][21][22][23] However, what is lacking is a detailed nano/micro structural level picture of the film nature and alloy/film interface regions, particularly with regards to segregation of...

Tracer Film Growth Study of Hydrogen and Oxygen from the Corrosion of Magnesium in Water

“…The MgO-based film formed on AZ31B in water contained Al distributed throughout the film thickness. 31 The film formed on E717, as well as the underlying alloy, also contained nanoscale Zn 2 Zr 3 precipitates (small bright particles in Fig. 1d).…”

“…1d). 31 The enrichment of Zn at the film-meal interface was also observed for both the E717 and AZ31B. 31 The thinner films formed on AZ31B and E717 in ambient water from 4 to 24 h, relative to UHP Mg, are attributed to effects of segregation of Al and Zn (AZ31B) and Zr and Zn (E717) to the films and film-metal interface regions (details in reference 31).…”

“…intermixed MgO + Mg(OH) 2 or H dissolved in MgO is not known). 31 The film formed on UHP Mg (Fig. 1a) varied from ∼40 nm to over 1 micron, the film on AZ31B was relatively uniform in thickness, on the order of ∼100 nm (Fig.…”

“…In addition to MgO, the film near surface regions formed in 4 h and 24 h water contained small amounts of Mg(OH) 2 and MgCO 3 , based on X-ray photoelectron spectroscopy (XPS). 31 The films are considered "partially hydrated" as hydrogen was detected throughout the MgO-base films by SIMS, with decreasing intensity moving from the film surface to the underlying alloy (the form of this hydrogen, i.e. intermixed MgO + Mg(OH) 2 or H dissolved in MgO is not known).…”

“…A companion study 31 determined film microstructures formed on the UHP Mg, AZ31B, and E717 after 4, 24, and 48 h exposure at room temperature in H 2 O, and was used to guide selection of tracer water exposure conditions in the present work. The tracer studies were conducted using either H 2 18 18 O water.…”