Film Breakdown and Nano-Porous Mg(OH)₂Formation from Corrosion of Magnesium Alloys in Salt Solutions

Abstract

Small angle neutron scattering (SANS) and scanning transmission electron microscopy (STEM) were used to study film formation by magnesium alloys AZ31B (Mg-3Al-1Zn base) and ZE10A (Elektron 717, E717: Mg-1Zn + Nd, Zr) in H 2 O and D 2 O with and without 1 or 5 wt% NaCl. No SANS scattering changes were observed after 24 h D 2 O or H 2 O exposures compared with as-received (unreacted) alloy, consistent with relatively dense MgO-base film formation. However, exposure to 5 wt% NaCl resulted in accelerated corrosion, with resultant SANS scattering changes detected. The SANS data indicated both particle and rough surface (internal and external) scattering, but with no preferential size features. The films formed in 5 wt% NaCl consisted of a thin, inner MgO-base layer, and a nano-porous and filamentous Mg(OH) 2 outer region tens of microns thick. Chlorine was detected extending to the inner MgO-base film region, with segregation of select alloying elements also observed in the inner MgO, but not the outer Mg(OH) 2 . Modeling of the SANS data suggested that the outer Mg(OH) 2 films had very high surface areas, consistent with loss of film protectiveness. Implications for the NaCl corrosion mechanism, and the potential utility of SANS for Mg corrosion, are discussed. Magnesium and its alloys are of great interest for a wide range of structural and functional applications, including lightweight automotive and aircraft structural materials, biomedical implants, fuel cells and hydrogen storage, batteries, etc.1-7 However, the high reactivity and rapid corrosion of Mg in many aqueous and humid environments is a key issue. [8][9][10][11] Of particular challenge and interest is the acceleration of Mg corrosion in the presence of salt species. Mechanisms of accelerated attack in salt solutions include enhanced local micro-galvanic coupling at alloy second phases and/or impurities (e.g. Fe, Ni, Cu etc.) and disruption of the MgO and/or Mg(OH) 2 base surface films by Cl-. 3,9,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Film formation by Mg alloys under aqueous conditions can be quite complex, involving MgO, Mg(OH) 2 , and MgCO 3 base phases, as well as complexes of those phases when alloy additions (e.g. Al) or environmental species (e.g. Cl-) become incorporated into parts or all of the film structure. [15][16][17]21,[27][28][29][30][31][32][33][34] In some cases, porous and even filiform-like corrosion films have been reported on bare or surface-modified Mg alloys. 11,14,16,18,24,[35][36][37][38] Small angle neutron scattering (SANS) has emerged as a powerful technique to assess structural and morphological features in the ∼1 to up to ∼200-300 nm feature size range (specific range accessed depends on the instrument capabilities and settings, as well as the nature of neutron scattering from the test samples). 39,40 Information can be obtained regarding size, morphology, and number density of scatterers (e.g. 2 nd phase precipitates, phase separation, grain structure, voids, pores, etc.), as well as surface area a...

Tracer Film Growth Study of the Corrosion of Magnesium Alloys AZ31B and ZE10A in 0.01% NaCl Solution

“…The film was non-uniform in thickness, typical of the aqueous corrosion of Mg alloys (especially in the presence of NaCl). 16,[20][21][22]28,[31][32][33]35,36,45,47,66 The film structure was complex, but generally morphologically duplex, with an overall "convex appearing" outer film region that consisted of a fine layered/striated morphology varying in total thickness on the order of ∼10-100 nm thick, overlying an inner, nodular-like film extending inward ∼0.5 to 1 μm for the location imaged. The outer convex film likely corresponded with the smooth, globular features evident in the secondary electron surface image shown in Fig.…”

“…Refs. 16,22,24,35,44). The rough surface and undulating, nonuniform nature of the films formed in the 20 h 2 nd step 0.01 wt% NaCl exposure resulted in very diffuse transitions between film and metal in the SIMS data (Figs.…”

“…Refs. 35,36). Penetration of Cl through the porous film and to the film/metal interface has been reported, with film growth mechanism(s) dominated by dissolution-reprecipitation and deposition of porous Mg(OH) 2 20,35,44 The use of only 0.01 wt% NaCl in the present work in the 20 h 2 nd step of the sequential tracer exposure sequence was necessary to limit film thickness to a range that could potentially be analyzed by SIMS depth sputtering in a comparative manner to the earlier pure water tracer study.…”

Corrosion Behavior of Pure Magnesium with Low Iron Content in 3.5 wt% NaCl Solution

“…The theoretically calculated surface energies of Mg (0001), and (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) 33 A higher surface energy can result in an increased dissolution rate. The theoretical dissolution rates of Mg crystallographic planes (10-10) and (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) are predicted to be about 18-20 times higher than that of the basal plane (0001). 34 A corrosion study on a hcp Zn single crystal showed that the corrosion resistance of the (0001) basal plane is about 1.4 times that of the (1120) and (1010) planes in 1 M (NH 4 ) 2 SO 4 .…”

Exploring the Effects of Intermetallic Particle Size and Spacing on the Corrosion of Mg-Al Alloys Using Model Electrodes

“…19,[22][23][24][25][26] Cathodically active sites have been examined through the use of scanning vibrating electrode technique (SVET) on 99.9% 25 and 99.99% 23 pure Mg, Mg-Nd binary alloys 24 and AZ31. 22,27 To date, mechanisms have been proposed, which explain this behavior, including: (i) transition metal enrichment from impurities, 28,29 (ii) particle enrichment to the oxide layer 26,30,31 and (iii) Al redeposition. 30 Evidence of cathodic activation has been noted to also be influenced by cathodic sites upon the Mg sample surface, namely intermetallic particles (IMPs) and impurities.…”