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 H218O or D216O, followed by a 20 h immersion in a 0.01 wt% NaCl H218O or D216O solution. Sputter depth profiles were obtained for 16O, 18O, 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.
| Original language | English |
|---|---|
| Pages (from-to) | C367-C375 |
| Journal | Journal of the Electrochemical Society |
| Volume | 164 |
| Issue number | 7 |
| DOIs | |
| State | Published - 2017 |
Funding
This paper is dedicated to the memory of J. Henry who performed the profilometry measurements in this study. The authors thank R. Sharpe of the University of Manitoba and D.W. Coffey, T.M. Lowe, T. Geer, and T.L. Jordan of Oak Ridge National Laboratory for their assistance with the experimental work. M.G. Frith, J. Qu, and B.A. Pint provided helpful comments and suggestions on the results and the manuscript. This research was sponsored by the U. S. Department of Energy, Energy Efficiency and Renewable Energy Vehicle Technologies Office. A portion of this research conducted via the Center for Nanophase Materials Sciences, which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U. S. Department of Energy.