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

Abstract

A sequential isotopic tracer study of corrosion film growth for Mg-3Al-1Zn-0.25Mn (AZ31B) and Mg-1.2Zn-0.25Zr-<0.5Nd (ZE10A) was conducted by 4 h immersion in H 2 18 O or D 2 16 O, followed by a 20 h immersion in a 0.01 wt% NaCl H 2 18 O or D 2 16 O solution. Sputter depth profiles were obtained for 16 O, 18 O, H, and D using secondary ion mass spectrometry (SIMS). Compared to the previous tracer study for these alloys in salt-free water, the addition of 0.01 wt% NaCl resulted in a transition from oxygen inward-dominated film growth to a component of mixed inward/outward film growth for both alloys. The hydrogen tracer behavior remained inward growing for AZ31B, and short-circuit, inward growing for ZE10A, in both pure water and in 0.01 wt% NaCl solution, with extensive penetration of D beyond the film and into the underlying alloy also observed for ZE10A. Analysis of the films by X-ray photoelectron spectroscopy (XPS) and cross-section scanning transmission electron microscopy (STEM) indicated intermixed Mg(OH) 2 and MgO, with the relative fraction of Mg(OH) 2 peaking near the center of the film. These findings suggest a decoupled film growth mechanism, with initial formation of oxide followed by NaCl-accelerated conversion to hydroxide, likely by both solid-state and dissolution-precipitation processes. Low density Mg alloys are of great interest for the production of lightweight vehicle parts to achieve increased fuel efficiencies and better performance.1-6 Key challenges to achieve widespread adoption in automotive vehicle applications include the cost of primary Mg metal, the formability of wrought Mg alloys (to expand application beyond current Mg cast parts), and the rapid corrosion of Mg in many aqueous and humid environments.6 From a corrosion perspective, the high reactivity of Mg, which leads to micro-and/or macro-galvanic corrosion coupling, and the inability of Mg alloys to sufficiently establish and/or maintain a protective surface film are major challenges in many environments. [7][8][9][10][11][12][13][14] Of particular detriment is the acceleration of Mg corrosion in salt-containing environments, which has been linked to increased local micro-galvanic coupling at second phases/metallic impurity sites and enhanced Mg dissolution, as well as disruption of the Mg(OH) 2 /MgO based surface film. The successful application of isotopic tracer studies employing secondary ion mass spectrometry (SIMS) to gain insights into the film growth mechanism of Mg alloys in room-temperature water and 85• C humid air was recently demonstrated. 45,46 Isotopic tracer studies have been widely used in oxidation studies of Al-, Fe-, Ni-, and Zr-base alloys, as well as SiC, with significant new insights gained regarding the growth mechanism of the oxide films. 45,[47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65] However, application of this experimental approach to Mg is particularly challenging due to the formation of both MgO and Mg(OH) 2 products in the film, necessitating the use of both ...

The Influence of Chloride Ion Concentration on the Localized Corrosion of E717 Magnesium Alloy

“…From experiments carried out in pure water, it was concluded that the Fe-containing Zr particles acted as local cathode sites during the corrosion of E717 alloy. Others have studied the influence of chloride ions concentration on the properties of the surface film formed on E717 under immersion conditions 19,20 .In comparison to a dense bilayered film, consisting of an inner MgO layer and an outer Mg(OH)2 film formed in water, the presence of high Clconcentration produces a significantly thicker, nano porous outer layer 19 . Chloride ion penetration into the inner MgO-base layer was detected at high concentrations of ≥ 1% w/v by using a combination of SEM and STEM, but not after immersion in more dilute concentration of 0.01 % w/v 20 , although the authors report a different growth mechanism within the dense MgO base layer.…”

Peridynamic Modeling of Intergranular Corrosion Damage

“…21 The authors considered a two-phase medium: solid metal and liquid electrolyte. Inspired by experimental observations that show gradual changes in the alloy composition and mechanical properties from the bulk to the surface, in a partially corroded layer at corrosion front, [29][30][31][32][33] they introduced an effective diffusion in the solid that represents both the dissolution of metal and the out-diffusion of ions into the electrolyte. Assuming that regular diffusion in the liquid governs the movement of ions in the electrolyte, they solved a single diffusion equation over the two-phase material, with different diffusion coefficients depending on the phase.…”

Corrosion Resistance and Apatite-Forming Ability of Composite Coatings formed on Mg–Al–Zn–Ca Alloys

“…The alloying element Ca might have different effect on the microstructure, and hence the resulting PEO film of the Mg–Al–Zn alloys as compared to that of the Mg–Al–Mn alloys. The development of an Mg based-biodegradable implant included Zn as alloying element to improve its mechanical strength [25,26,27]. This work aims to throw light on explaining the corrosion behavior and apatite-forming ability of the coatings formed on the Mg–6Al–1Zn–xCa alloys as a result of alteration in the microstructure with a variation of Ca content.…”