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Time-of-flight secondary ion mass spectrometry is applied to demonstrate the chemical mapping and corrosion morphology at the metal-paint interface of an aluminum alloy after being exposed to a salt solution compared with a specimen exposed to air.
Corrosion developed at the paint and aluminum (Al) metal-paint interface of an aluminum alloy is analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS), illustrating that SIMS is a suitable technique to study the chemical distribution at a metal-paint interface. The painted Al alloy coupons are immersed in a salt solution or exposed to air only. SIMS provides chemical mapping and 2D molecular imaging of the interface, allowing direct visualization of the morphology of the corrosion products formed at the metal-paint interface and mapping of the chemical after corrosion occurs. The experimental procedure of this method is presented to provide technical details to facilitate similar research and highlight pitfalls that may be encountered during such experiments.
Al alloys have wide applications in engineering structures, such as in marine technology or military automotive, attributable to their high strength-to-weight ratio, excellent formability, and resistance to corrosion. However, localized corrosion of Al alloys is still a common phenomenon which affects their long-term reliability, durability, and integrity in various environmental conditions1. Paint coating is the most common means to prevent corrosion. Illustration of the corrosion developed at the interface between metal and paint coating can provide insights in determining the appropriate remedy for corrosion prevention.
The corrosion of Al alloys may take place via several different pathways. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) are two commonly applied surface microanalysis techniques in investigating corrosion. XPS can provide elemental mapping but not a holist molecular view of the surface chemical information2,3, while SEM/EDX provides morphological information and elemental mapping but with relatively low sensitivity.
ToF-SIMS is another surface tool for chemical mapping with high mass accuracy and lateral resolution. It has a low limit of detection (LOD) and is capable of revealing the distribution of the corrosion species formed at the metal-paint interface. Typically, SIMS mass resolution can reach 5,000-15,000, sufficient to differentiate the isobaric ions4. With its submicron spatial resolution, ToF-SIMS can chemically image and characterize the metal-paint interface. It provides not only morphological information but also the lateral distribution of molecular corrosion species at the top few nanometers of the surface. ToF-SIMS offers complementary information to XPS and SEM/EDX.
To demonstrate the capability of ToF-SIMS in surface characterization and imaging of the corrosion interface, two painted Al alloy (7075) coupons, one exposed to air only and one to a salt solution, are analyzed (Figure 1 and Figure 2). Understanding the corrosion behavior at the metal-paint interface exposed to the saline condition is critical to understand the performance of the Al alloy in a marine environment, for example. It is known that the formation of Al(OH)3 occurs during Al’s exposure to seawater5, but the study of Al corrosion still lacks comprehensive molecular identification of the corrosion and coating interface. In this study, the fragments of Al(OH)3, including Al oxides (e.g., Al3O5-) and oxyhydroxide species (e.g., Al3O6H2-), are observed and identified. The comparisons of SIMS mass spectra (Figure 3) and molecular images (Figure 4) of the negative ions Al3O5- and Al3O6H2- provide the molecular evidence of the corrosion products formed at the metal-paint interface of the salt solution-treated Al alloy coupon. SIMS offers the possibility to elucidate the complicated chemistry occurring at the metal-paint interface, which can help shed light on the efficacy of surface treatments in Al alloys. In this detailed protocol, we demonstrate this effective approach in probing the metal-paint interface to help new practitioners in corrosion research using ToF-SIMS.
1. Corrosion sample preparation
2. Analysis of the metal-paint corrosion interface using ToF-SIMS
3. Analysis of the ToF-SIMS data
Figure 3 presents the comparison of mass spectra between the metal-paint interface treated with salt solution and the interface exposed to air. The mass spectra of the two samples were acquired using a 25 kV Bi3+ ion beam scanning in 300 µm x 300 µm ROIs. The mass resolution (m/∆m) of the salt solution-treated sample was approximately 5,600 at the peak of m/z- 26. The raw data of the mass spectra were exported after binning 10 channels. A grap...
ToF-SIMS differentiates the ions according to their time of flight between two scintillators. The topography or sample roughness affects the flight time of the ions from different starting positions, which usually leads to a poor mass resolution with an increased width of peaks. Therefore, it is critical that the ROIs being analyzed are very flat, to ensure good signal collection8.
Another pitfall to avoid is charging. Since the Al-paint interface was fixed with the ins...
The authors have nothing to disclose.
This work was funded by the QuickStarter Program supported by Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the U.S. DOE. This work was performed using the IONTOF ToF-SIMS V, located in the Biological Sciences Facility (BSF) at PNNL. JY and X-Y Yu also acknowledged the support from the Atmospheric Sciences & Global Change (ASGC) Division and Physical and Computational Sciences Directorate (PCSD) at PNNL
Name | Company | Catalog Number | Comments |
0.05 µm Colloidal Silica polishing Solution | LECO | 812-121-300 | Final polishing solution |
1 µm polishing solution | Pace Technologies | PC-1001-GLB | Water based polishing solution |
15 µm polishing solution | Pace Technologies | PC-1015-GLBR | Water based polishing solution |
3 µm polishing solution | Pace Technologies | PC-1003-GLG | Water based polishing solution |
6 µm polishing solution | Pace Technologies | PC-1006-GLY | Water based polishing solution |
Balance | Mettler Toledo | 11106015 | It is used for measuring the chemicals. |
Epothin 2 epoxy hardener | Buehler | 20-3442-064 | Used for casting sample mounts |
Epothin 2 epoxy resin | Buehler | 20-3440-128 | Used for casting sample mounts |
Fast protein liquid chromatography (FPLC) conductivity sensor | Amersham | AKTA FPLC | Used to measure the conductivity of the salt solution. |
Final B pad | Allied | 90-150-235 | Used for 1 µm and 0.05 µm polishing steps |
KCl | Sigma-Aldrich | P9333 | Used to make the salt solution. |
Low speed saw | Buehler Isomet | 11-1280-160 | Used to cut the Al coupons that are fixed in the epoxy resin. |
MgCl2 | Sigma-Aldrich | 63042 | Used to make the salt solution. |
MgSO4 | Sigma-Aldrich | M7506 | It is used to make the salt solution. |
NaCl | Sigma-Aldrich | S7653 | It is used to make the salt solution. |
NaOH | Sigma-Aldrich | 306576 | It is used for adjusting pH of the salt solution. |
Paint | Rust-Oleum | 245217 | Universal General Purpose Gloss Black Hammered Spray Paint. It is used to spray on the Al coupons. |
Pan-W polishing pad | LECO | 809-505 | Used for 15, 6, and 3 µm polishing steps |
pH meter | Fisher Scientific | 13-636-AP72 | It is used for measuring the pH of the salt solution. |
Pipette | Thermo Fisher | Scientific | Range: 10 to 1,000 µL |
Pipette tip 1 | Neptune | 2112.96.BS | 1,000 µL |
Pipette tip 2 | Rainin | 17001865 | 20 µL |
Silicon carbide paper | LECO | 810-251-PRM | Grinding paper, 240 grit |
Sputter coater | Cressington | 108 sputter coater | It is used for coating the sample. |
Tegramin-30 Semi-automatic polisher | Struers | 6036127 | Coarse/fine polishing/grinding |
ToF-SIMS | IONTOF GmbH, Münster, Germany | ToF-SIMS V, equipped with Bi liquid metal ion gun and flood gun | It is used to acquire mass spectra and images of a specimen. |
Vibromet 2 vibratory polisher | Buehler | 67-1635-160 | Final polishing step |
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