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 graphical software was applied to plot the mass spectra for presentation. It is known that the protective layers containing Al(OH)3 are formed after the Al corrosion starts6. The oxide (Al3O5–) and oxyhydroxide species (Al2O4H–, Al2O5H3–, Al3O6H2–) of Al(OH)3 fragments7 were observed in the metal-paint interface of the salt solution-exposed Al coupon (Figure 3a) and were more prominent when compared to the same peaks in the air-exposed sample (Figure 3b). This indicates that the Al coupon exposed to the salt solution had experienced more severe corrosion compared to the air-exposed one. The result is consistent with the known knowledge that solutions containing salts, such as seawater, are chemically aggressive and contribute to the corrosion process of an Al alloy.
Figure 4 depicts 2D molecular images of selected Al species m/z– 161 Al3O5– and 179 Al3O6H2– acquired from the metal-paint interface treated with a salt solution (Figure 4a) and the interface exposed to air (Figure 4b). The depicted ion intensities of m/z– 161 and 179 were both normalized to the intensities of total ions. The images of the same peak were adjusted to the identical color scale. The images were obtained from 100 scans of 256 x 256 pixels of the 300 µm x 300 µm ROI. The 2D images provide the distribution of the chemical species of the Al corrosion products in two different samples. The peaks m/z– 161 and 179 were more prevalent in the metal-paint interface treated with the salt solution, displaying stronger intensities than the ones shown in the air-exposed sample. This result agrees with the mass spectra results and further demonstrates ToF-SIMS’s analytical capabilities of chemical identification and molecular imaging.
Figure 1: Photos showing the metal-paint interface preparation process. Figure 1 depicts the metal-paint interface preparation process. After the Al coupons were fixed in the epoxy resin (a), they were sprayed with the commercial paint product and set for 24 h till they were completely dry (b). Four lines were scribed on the paint on top of the Al coupon cylinders (c). The carved Al coupon cylinders were exposed to air or a salt solution for 3 weeks in Petri dishes (d). The Al coupon cylinders were cut and trimmed to expose the metal-paint interfaces (e) and coated with gold layers prior to ToF-SIMS analysis (f). Please click here to view a larger version of this figure.
Figure 2: The schematic of the metal-paint interface analysis by ToF-SIMS and a photo of the IONTOF V instrument. Figure 2 illustrates the analysis process of the metal-paint interface using ToF-SIMS. The metal-paint interface (a) was bombarded by a Bi3+ primary ion beam and generated the secondary ions, resulting in mass spectra (b) and a SIMS image (c). The ToF-SIMS V instrument (d) used for the metal-paint interface analysis described in this work is displayed. Please click here to view a larger version of this figure.
Figure 3: Comparison of mass spectra of the metal-paint interfaces of Al coupons. The figure shows the spectral difference between the interface treated with a salt solution and the one treated with air. Please click here to view a larger version of this figure.
Figure 4: Molecular images of chemical species at the metal-paint interface of Al coupons. This comparison shows the difference in 2D distribution of species formed in corrosion by salt solution and by air. Please click here to view a larger version of this figure.
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 |
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.
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.
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.