Characterization of the Alloy-Solution Interface

Published

October 16, 2008

Author(s)

H J. Mueller

Abstract

For metals undergoing corrosion, a layer of water molecules becomes charged due to electrons accumulating on the outer metal surface and hydrated positive ions in solution adjacent to the surface. The layer has properties of capacitance (C_dl) and resistance (R_ct) to electronic charge transport. This study characterized the adsorbed layer on metals and determined how metal composition, solution concentration, exposure time, and polarizing potential affected the layer.Methods: Complex impedances (EIS) were simulated with an equivalent electric circuit (EEC) and tested for goodness of fit, with nonlinear least square methods. Reflection-absorption infrared spectrometry (FT-IR) analyzed surfaces during solution exposure (in-situ), and also analyzed dried surfaces after exposure (ex-situ). An acid-base model determined surface energies (CAA) before and after solution exposure. For EIS, Cu and Ag were exposed to an artificial saliva of NaCl, KCl, CaCl₂, NaH₂PO₄, and urea, at component concentrations of (6.8, 5.4, 5.4, 5.0, 16.7) mmol/L, respectively, (pH = 4.8). For FT-IR and CAA, Co, Ni, and Ag-Cu-In, Cu-Al, Co-Cr, and Ni-Cr-Be alloys were exposed to NaCl solutions at mass fractions of (0.1 or 1.0) %.Results: An EEC of R_soln(C_dl(R_ct(RQ)))} was modeled. R_soln is a solution resistance, C_dl, and R_ct, defined above, and τ = (R Q)^1/n is a time constant with Q a constant phase element and n for dispersion (0 < or equal} n < or equal} 1). For Cu and Ag, C_dl, R_ct, τ, and n were (3 and 15) nF, (22 and 65) Ω}, (10 and 7) ms, and 0.56 and 0.77, respectively. In-situ IR initially revealed a (3600 to 3100) cm^-1 intense band that split (t > 15 min) into 3638 cm^-1 and 3145 cm^-1 bands. Ex-situ IR revealed a moderate band at 3436 cm^-1. In situ IR also revealed (5190 and 1650) cm^-1 bands. Band intensities for most interfaces increased by up to 3.6 times at 24 h. For both in-situ and ex-situ experiments, unidentified peaks occurred for Co, Ni, Co-Cr, Ni-Cr-Be, Ag-Cu-In, and Cu-Al alloys at 750 cm^-1, (782 and 610) cm^-1, (1120 and 905) cm^-1, (1128 and 863) cm^-1, (995 and 550) cm^-1, and (1368, 1157, 1096, and 644) cm^-1, respectively. Peak intensities increased after exposure to 1.0 % NaCl, with exposure time, and by anodic applied potentials. For 1.0 % NaCl exposures, Cu-Al and Co-Cr increased in surface energy from the as-polished condition of (35-45) dyne/cm by up to (0.6 and 0.4) times at 24 h after exposure. Polarized surfaces increased by up to 1.1 times. The increases were dominated by a basic polar component.Discussion: Adsorption and diffusion models correspond to n = 1 and n = 0.5, respectively. Hence, Cu was mainly under diffusion control, while Ag showed increased tendency for adsorption. The IR band positions agree with the stretch vibrations of free O-H (3638 cm^-1) and of hydrogen bonded O-H from water of crystallization (3145 cm^-1). Absorption due to intermolecular O-H bonding (3590 to 3230) cm^-1 was in-between 3638 cm_-1 and 3145 cm_-1, hence no metal hydroxides were assessed by in-situ FT-IR. However, the 3436 cm^-1 ex-situ band agreed with the O-H stretch from intermolecular bonding, suggesting metal hydroxide films. The basic energy component of the surface energies also suggested hydroxide films. Unidentified bands occurring below 1200 cm^-1 were attributed to inorganic compound formation.Conclusions: Characterization of metal-solution interfaces by combined EIS, FT-IR, and CAA provides insight into interfacial processes and mechanisms not attainable from each technique alone. Supported by the American Dental Association Health Foundation, National Institute of Standards and Technology, and by the National Institute of Dental Research Grant No P50-DE09322.

Proceedings Title

Proceedings of Surface Analysis '99, the 21st Annual Symposium on Applied Surface Analysis; sponsored by the American Vacuum Society

Conference Dates

June 14-17, 1999

Conference Location

Undefined

Conference Title

Symposium on Applied Surface Analysis

Pub Type

Conferences

Keywords

acid-base model, charge transfer resistance, contact angle analysis, double layer capacitance, electrochemical impedance spectroscopy, equivalent electrical circuit, grazing angle Fourier, surface films, transform infrared spectroscopy

Citation

Mueller, H. (2008), Characterization of the Alloy-Solution Interface, Proceedings of Surface Analysis '99, the 21st Annual Symposium on Applied Surface Analysis; sponsored by the American Vacuum Society, Undefined (Accessed November 8, 2024)