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 &#38; Global Change (ASGC) Division and Physical and Computational Sciences Directorate (PCSD) at PNNL

<ol>
	<li>Szklarska-Smialowska, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pitting+corrosion+of+aluminum.">Pitting corrosion of aluminum.</a> Corrosion Science. 41, 1743-1767 (1999).</li><li>Liu, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+first+quantitative+XPS+study+of+the+surface+films+formed,+by+exposure+to+water+on+Mg+and+on+the+Mg-Al+intermetallics:+Al3Mg2+and+Mg17Al12.">A first quantitative XPS study of the surface films formed, by exposure to water on Mg and on the Mg-Al intermetallics: Al3Mg2 and Mg17Al12.</a> Corrosion Science. 51 (5), 1115-1127 (2009).</li><li>Linford, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+introduction+to+time-of-flight+secondary+ion+mass+spectrometry+(ToF-SIMS).">An introduction to time-of-flight secondary ion mass spectrometry (ToF-SIMS).</a> Vacuum Technology &amp; Coating. , (2014).</li><li>Cushman, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+pictorial+view+of+LEIS+and+ToF-SIMS+instrumentation.">A pictorial view of LEIS and ToF-SIMS instrumentation.</a> Vacuum Technology &amp; Coating. , 27-35 (2016).</li><li>Soler, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hydrogen+generation+by+aluminum+corrosion+in+seawater+promoted+by+suspensions+of+aluminum+hydroxide.">Hydrogen generation by aluminum corrosion in seawater promoted by suspensions of aluminum hydroxide.</a> International Journal of Hydrogen Energy. 34 (20), 8511-8518 (2009).</li><li>Ahmad, Z., Abdul Aleem, B. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Degradation+of+aluminum+metal+matrix+composites+in+salt+water+and+its+control.">Degradation of aluminum metal matrix composites in salt water and its control.</a> Materials &amp; Design. 23 (2), 173-180 (2002).</li><li>Verdier, S., Metson, J. B., Dunlop, H. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Static+SIMS+studies+of+the+oxides+and+hydroxides+of+aluminium.">Static SIMS studies of the oxides and hydroxides of aluminium.</a> Journal of Mass Spectrometry. 42 (1), 11-19 (2007).</li><li>Esmaily, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+ToF-SIMS+investigation+of+the+corrosion+behavior+of+Mg+alloy+AM50+in+atmospheric+environments.">A ToF-SIMS investigation of the corrosion behavior of Mg alloy AM50 in atmospheric environments.</a> Applied Surface Science. 360, 98-106 (2016).</li><li>Hunt, C. P., Stoddart, C. T. H., Seah, M. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+surface+analysis+of+insulators+by+SIMS:+Charge+neutralization+and+stabilization+of+the+surface+potential.">The surface analysis of insulators by SIMS: Charge neutralization and stabilization of the surface potential.</a> Surface and Interface Analysis. 3 (4), 157-160 (1981).</li><li>Stingeder, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantitative+distribution+analysis+of+B,+As+and+Sb+in+the+layer+system+SiO2/Si+with+SIMS:+elimination+of+matrix+and+charging+effects.">Quantitative distribution analysis of B, As and Sb in the layer system SiO2/Si with SIMS: elimination of matrix and charging effects.</a> Fresenius' Zeitschrift f&#252;r analytische Chemie. 327 (2), 225-232 (1987).</li><li>Cushman, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sample+Charging+in+ToF-SIMS:+How+it+Affects+the+Data+that+are+Collected+and+How+to+Reduce+it.">Sample Charging in ToF-SIMS: How it Affects the Data that are Collected and How to Reduce it.</a> Vacuum Technology &amp; Coating. , (2018).</li><li>Dubey, M., Brison, J., Grainger, D. W., Castner, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparison+of+Bi(1),+Bi(3)+and+C(60)+primary+ion+sources+for+ToF-SIMS+imaging+of+patterned+protein+samples.">Comparison of Bi(1), Bi(3) and C(60) primary ion sources for ToF-SIMS imaging of patterned protein samples.</a> Surface and Interface Analysis: SIA. 43 (1-2), 261-264 (2011).</li><li>Kozole, J., Winograd, N., Smentkowski, V. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cluster+Secondary+Ion+Mass+Spectrometry.">Cluster Secondary Ion Mass Spectrometry.</a> Surface Analysis and Techniques in Biology. , 71-98 (2014).</li><li>Tyler, B. J., Rayal, G., Castner, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multivariate+analysis+strategies+for+processing+ToF-SIMS+images+of+biomaterials.">Multivariate analysis strategies for processing ToF-SIMS images of biomaterials.</a> Biomaterials. 28 (15), 2412-2423 (2007).</li><li>Song, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Corrosion+behaviour+of+extruded+AM30+magnesium+alloy+under+salt-spray+and+immersion+environments.">Corrosion behaviour of extruded AM30 magnesium alloy under salt-spray and immersion environments.</a> Corrosion Science. 78, 353-368 (2014).</li><li>Esmaily, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+capability+of+in-situ+exposure+in+an+environmental+scanning+electron+microscope+for+investigating+the+atmospheric+corrosion+of+magnesium.">On the capability of in-situ exposure in an environmental scanning electron microscope for investigating the atmospheric corrosion of magnesium.</a> Ultramicroscopy. 153, 45-54 (2015).</li><li>Liao, J., Hotta, M., Motoda, S. -. i., Shinohara, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Atmospheric+corrosion+of+two+field-exposed+AZ31B+magnesium+alloys+with+different+grain+size.">Atmospheric corrosion of two field-exposed AZ31B magnesium alloys with different grain size.</a> Corrosion Science. 71, 53-61 (2013).</li><li>deVries, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface+characterization+methods-+XPS,TOF-SIMS,+and+SAM+a+complimentary+ensemble+of+tools.">Surface characterization methods- XPS,TOF-SIMS, and SAM a complimentary ensemble of tools.</a> Journal of Materials Engineering and Performance. 7 (3), 303-311 (1998).</li><li>Zhang, H., Cooper, S. L., Guan, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface+characterization+techniques+for+polyurethane+biomaterials.">Surface characterization techniques for polyurethane biomaterials.</a> Advances in Polyurethane Biomaterials. , 23-73 (2016).</li></ol>

The authors have nothing to disclose.

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 insulating resin, charging was expected. Charge accumulates on the sample surface as the ROI is bombarded with the primary ion beam, affecting the kinetic energies of ions that are emitted from the surface. Charging results in wide peaks and a decreased mass resolution. To avoid the negative influence of this effect, 10 nm of gold was sputtered on the interface surface to form the conductive path prior to SIMS analysis. Other methods can be applied to reduce the charging effect, including applying the flood gun, optimizing the voltage of the reflector, and selecting Random mode as the beam raster pattern. The flood gun generates a stable electron current with low energy. It is commonly used for charge compensation during the SIMS analysis9,10,11. In addition, the voltage of the reflector, an ion optic that enhances the mass resolution, needs to be adjusted, depending on the degree of charging. ToF-SIMS software provides an efficient way to optimize the reflector as described in step 2.5.5 of the protocol. The selection of Random mode as the beam raster pattern before acquiring SIMS data further reduces the charging effect. This mode mitigates the problem occurring in the row-by-row scanning mode, allowing the accumulated charge more time to dissipate9,11.
ToF-SIMS can be equipped with multiple ion sources, including - but not limited to - Cs+, C60+, and Bin+. Polyatomic ion sources (e.g., Bi3+ and C60+) produce higher yields of secondary ions emitted from the sample surface compared to atomic ion beams (e.g., Cs+ and Bi1+)12,13. Furthermore, comparing Bi3+ to C60+, Bi3+ is more surface sensitive to low-mass fragments and, thus, has a higher lateral resolution with better images12. Therefore, Bi3+ was selected as the analysis beam in this work since we focused on the low-mass peaks related to aluminum corrosion species.
ToF-SIMS is a sensitive surface technique that can provide chemical specificity with a high spatial resolution14. Other surface tools applied in corrosion studies include XPS and SEM/EDX2,15,16,17. XPS can provide quantitative measurements of the chemical state and electronic state of the elements that exist within a sample but with a higher LOD (0.1%) than SIMS (parts-per-billion - parts-per-million level)18,19. SEM/EDX is not as sensitive as ToF-SIMS, although SEM is often used to obtain morphological features of surfaces. In addition, the chemical mapping of SIMS makes it possible to visualize the molecular ion distribution at the corrosion interface, while SEM/EDX only provides the elemental ion mapping. Thus, SIMS&#8217;s molecular mapping is more informative in investigating the interfacial corrosion process.
This work demonstrates that ToF-SIMS is a powerful tool in deciphering the corrosion speciation at the interface due to its low LOD, high mass resolution, and high spatial resolution. Furthermore, SIMS offers multimodal microanalysis, attributable to its quasi-nondestructive nature. Thus, the same sample can be analyzed by other analytical tools and provide comprehensive information. Ideally, the integration of SIMS, XPS, and SEM can provide more comprehensive insights into corrosion behavior at the metal-paint interface.

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&#8217;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.

<table><tbody><tr><td>0.05 &amp;micro;m Colloidal Silica polishing Solution</td><td>LECO</td><td>812-121-300</td><td>Final polishing solution</td></tr><tr><td>1 &amp;micro;m polishing solution</td><td>Pace Technologies</td><td>PC-1001-GLB</td><td>Water based polishing solution</td></tr><tr><td>15 &amp;micro;m polishing solution</td><td>Pace Technologies</td><td>PC-1015-GLBR</td><td>Water based polishing solution</td></tr><tr><td>3 &amp;micro;m polishing solution</td><td>Pace Technologies</td><td>PC-1003-GLG</td><td>Water based polishing solution</td></tr><tr><td>6 &amp;micro;m polishing solution</td><td>Pace Technologies</td><td>PC-1006-GLY</td><td>Water based polishing solution</td></tr><tr><td>Balance</td><td>Mettler Toledo</td><td>11106015</td><td>It is used for measuring the chemicals.</td></tr><tr><td>Epothin 2 epoxy hardener</td><td>Buehler</td><td>20-3442-064</td><td>Used for casting sample mounts</td></tr><tr><td>Epothin 2 epoxy resin</td><td>Buehler</td><td>20-3440-128</td><td>Used for casting sample mounts</td></tr><tr><td>Fast protein liquid chromatography (FPLC) conductivity sensor</td><td>Amersham&amp;nbsp;</td><td>AKTA FPLC</td><td>Used to measure the conductivity of the salt solution.</td></tr><tr><td>Final B pad</td><td>Allied</td><td>90-150-235</td><td>Used for 1 &amp;micro;m and 0.05 &amp;micro;m&amp;nbsp; polishing steps</td></tr><tr><td>KCl&amp;nbsp;</td><td>Sigma-Aldrich</td><td>P9333</td><td>Used to make the salt solution.</td></tr><tr><td>Low speed saw</td><td>Buehler Isomet</td><td>11-1280-160</td><td>Used to cut the Al coupons that are fixed in the epoxy resin.</td></tr><tr><td>MgCl&lt;sub&gt;2&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td>63042</td><td>Used to make the salt solution.</td></tr><tr><td>MgSO&lt;sub&gt;4&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td>M7506</td><td>It is used to make the salt solution.</td></tr><tr><td>NaCl</td><td>Sigma-Aldrich</td><td>S7653</td><td>It is used to make the salt solution.</td></tr><tr><td>NaOH</td><td>Sigma-Aldrich</td><td>306576</td><td>It is used for adjusting pH of the salt solution.</td></tr><tr><td>Paint</td><td>Rust-Oleum&amp;nbsp;</td><td>245217</td><td>Universal General Purpose Gloss Black Hammered Spray Paint. It is used to spray on the Al coupons.&amp;nbsp;</td></tr><tr><td>Pan-W polishing pad</td><td>LECO</td><td>809-505</td><td>Used for 15, 6, and 3 &amp;micro;m polishing steps</td></tr><tr><td>pH meter</td><td>Fisher Scientific</td><td>13-636-AP72</td><td>It is used for measuring the pH of the salt solution.</td></tr><tr><td>Pipette&amp;nbsp;</td><td>Thermo Fisher&amp;nbsp;</td><td>Scientific&amp;nbsp;</td><td>Range: 10 to 1,000 &amp;micro;L</td></tr><tr><td>Pipette tip 1</td><td>Neptune&amp;nbsp;</td><td>2112.96.BS&amp;nbsp;</td><td>1,000 &amp;micro;L</td></tr><tr><td>Pipette tip 2</td><td>Rainin</td><td>17001865</td><td>20 &amp;micro;L</td></tr><tr><td>Silicon carbide paper</td><td>LECO</td><td>810-251-PRM</td><td>Grinding paper, 240 grit</td></tr><tr><td>Sputter coater</td><td>Cressington</td><td>108 sputter coater</td><td>It is used for coating the sample.&amp;nbsp;&amp;nbsp;</td></tr><tr><td>Tegramin-30 Semi-automatic polisher</td><td>Struers</td><td>6036127</td><td>Coarse/fine polishing/grinding</td></tr><tr><td>ToF-SIMS</td><td>IONTOF GmbH, M&amp;uuml;nster, Germany</td><td>ToF-SIMS V, equipped with Bi liquid metal ion gun and flood gun</td><td>It is used to acquire mass spectra and images of a specimen.</td></tr><tr><td>Vibromet 2 vibratory polisher</td><td>Buehler</td><td>67-1635-160</td><td>Final polishing step</td></tr></tbody></table>

imaging corrosion at the metal-paint interface using time-of-flight secondary ion mass spectrometry

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.

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 &#181;m x 300 &#181;m ROIs. The mass resolution (m/&#8710;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 &#181;m x 300 &#181;m 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&#8217;s analytical capabilities of chemical identification and molecular imaging.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59523/59523fig1.jpg" /> 
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). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59523/59523fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59523/59523fig2.jpg" /> 
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. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59523/59523fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59523/59523fig3.jpg" /> 
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. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59523/59523fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/59523/59523fig4.jpg" /> 
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. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59523/59523fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Imaging Corrosion at the Metal-Paint Interface Using Time-of-Flight Secondary Ion Mass Spectrometry at JoVE.com

Imaging Corrosion at the Metal-Paint Interface Using Time-of-Flight Secondary Ion Mass Spectrometry

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 ...

imaging-corrosion-at-metal-paint-interface-using-time-flight

Nanyang Technological University

Research

JoVE Journal

Chemistry

8.2K Views.  Pacific Northwest National Laboratory. Deze aanpak onthult het corrosieproces op de metaalverfinterface en geeft inzicht in mechanische en chemische veranderingen op de interface met een hoge oppervlaktegevoeligheid. Time-of-flight secundaire ionenmassaspectrometrie, of ToF-SIMS, is een krachtig oppervlaktegereedschap. Het biedt chemische kaarten met een hoge laterale en massaresolutie en maakt effectieve karakterisering mogelijk op de metaalverfinterface.Dus een belangrijke tip die een nieuwe beoefenaar moet weten is om er...

Video: Imaging Corrosion at the Metal-Paint Interface Using Time-of-Flight Secondary Ion Mass Spectrometry

Characterization of Surface Modifications by White Light Interferometry: Applications in Ion Sputtering, Laser Ablation, and Tribology Experiments

In materials science and engineering it is often necessary to obtain quantitative measurements of surface topography with micrometer lateral resolution. From the measured surface, 3D topographic maps can be subsequently analyzed using a variety of software packages to extract the information that is needed.
In this article we describe how white light interferometry, and optical profilometry (OP) in general, combined with generic surface analysis software, can be used for materials science and engineering tasks. In this article, a number of applications of white light interferometry for investigation of surface modifications in mass spectrometry, and wear phenomena in tribology and lubrication are demonstrated. We characterize the products of the interaction of semiconductors and metals with energetic ions (sputtering), and laser irradiation (ablation), as well as ex situ measurements of wear of tribological test specimens.
Specifically, we will discuss:
<ol class="lroman">
	<li>Aspects of traditional ion sputtering-based mass spectrometry such as sputtering rates/yields measurements on Si and Cu and subsequent time-to-depth conversion.</li>
	<li>Results of quantitative characterization of the interaction of femtosecond laser irradiation with a semiconductor surface. These results are important for applications such as ablation mass spectrometry, where the quantities of evaporated material can be studied and controlled via pulse duration and energy per pulse. Thus, by determining the crater geometry one can define depth and lateral resolution versus experimental setup conditions.</li>
	<li>Measurements of surface roughness parameters in two dimensions, and quantitative measurements of the surface wear that occur as a result of friction and wear tests.</li>
</ol>
Some inherent drawbacks, possible artifacts, and uncertainty assessments of the white light interferometry approach will be discussed and explained.

White light microscope interferometry is an optical, noncontact and quick method for measuring the topography of surfaces. It is shown how the method can be applied toward mechanical wear analysis, where wear scars on tribological test samples are analyzed; and in materials science to determine ion beam sputtering or laser ablation volumes and depths.

In materials science and engineering it is often necessary to obtain quantitative measurements of surface topography with micrometer lateral resolution. ...

Engineering

Imaging of Biological Tissues by Desorption Electrospray Ionization Mass Spectrometry

Mass spectrometry imaging (MSI) provides untargeted molecular information with the highest specificity and spatial resolution for investigating biological tissues at the hundreds to tens of microns scale. When performed under ambient conditions, sample pre-treatment becomes unnecessary, thus simplifying the protocol while maintaining the high quality of information obtained. Desorption electrospray ionization (DESI) is a spray-based ambient MSI technique that allows for the direct sampling of surfaces in the open air, even in vivo. When used with a software-controlled sample stage, the sample is rastered underneath the DESI ionization probe, and through the time domain, m/z information is correlated with the chemical species' spatial distribution. The fidelity of the DESI-MSI output depends on the source orientation and positioning with respect to the sample surface and mass spectrometer inlet. Herein, we review how to prepare tissue sections for DESI imaging and additional experimental conditions that directly affect image quality. Specifically, we describe the protocol for the imaging of rat brain tissue sections by DESI-MSI.

Desorption electrospray ionization mass spectrometry (DESI-MS) is an ambient method by which samples, including biological tissues, can be imaged with minimal sample preparation. By rastering the sample below the ionization probe, this spray-based technique provides sufficient spatial resolution to discern molecular features of interest within tissue sections.

Mass spectrometry imaging (MSI) provides untargeted molecular information with the highest specificity and spatial resolution for investigating biological ...

Bioengineering

Determining the Chemical Composition of Corrosion Inhibitor/Metal Interfaces with XPS: Minimizing Post Immersion Oxidation

An approach for acquiring more reliable X-ray photoelectron spectroscopy data from corrosion inhibitor/metal interfaces is described. More specifically, the focus is on metallic substrates immersed in acidic solutions containing organic corrosion inhibitors, as these systems can be particularly sensitive to oxidation following removal from solution. To minimize the likelihood of such degradation, samples are removed from solution within a glove box purged with inert gas, either N2 or Ar. The glove box is directly attached to the load-lock of the ultra-high vacuum X-ray photoelectron spectroscopy instrument, avoiding any exposure to the ambient laboratory atmosphere, and thus reducing the possibility of post immersion substrate oxidation. On this basis, one can be more certain that the X-ray photoelectron spectroscopy features observed are likely to be representative of the in situ submerged scenario, e.g. the oxidation state of the metal is not modified.

A protocol to avoid the oxidation of metallic substrates during sample transfer from an inhibited acidic solution to an X-ray photoelectron spectrometer is presented.

An approach for acquiring more reliable X-ray photoelectron spectroscopy data from corrosion inhibitor/metal interfaces is described. More specifically, ...

In Situ Characterization of Shewanella oneidensis MR1 Biofilms by SALVI and ToF-SIMS

Bacterial biofilms are surface-associated communities that are vastly studied to understand their self-produced extracellular polymeric substances (EPS) and their roles in environmental microbiology. This study outlines a method to cultivate biofilm attachment to the System for Analysis at the Liquid Vacuum Interface (SALVI) and achieve in situ chemical mapping of a living biofilm by time-of-flight secondary ion mass spectrometry (ToF-SIMS). This is done through the culturing of bacteria both outside and within the SALVI channel with our specialized setup, as well as through optical imaging techniques to detect the biofilm presence and thickness before ToF-SIMS analysis. Our results show the characteristic peaks of the Shewanella biofilm in its natural hydrated state, highlighting upon its localized water cluster environment, as well as EPS fragments, which are drastically different from the same biofilm's dehydrated state. These results demonstrate the breakthrough capability of SALVI that allows for in situ biofilm imaging with a vacuum-based chemical imaging instrument.

In Situ Characterization of Shewanella oneidensis MR1 Biofilms by SALVI and ToF-SIMS

This article presents a method for growing a biofilm for in situ time-of-flight secondary ion mass spectrometry for chemical mapping in its hydrated state, enabled by a microfluidic reactor, System for Analysis at the liquid Vacuum Interface. The Shewanella oneidensis MR-1 with green fluorescence protein was used as a model.

Bacterial biofilms are surface-associated communities that are vastly studied to understand their self-produced extracellular polymeric substances (EPS) ...

Atom Probe Tomography Analysis of Exsolved Mineral Phases

Element diffusion rates and temperature/pressure control a range of fundamental volcanic and metamorphic processes. Such processes are often recorded in lamellae exsolved from host mineral phases. Thus, the analysis of the orientation, size, morphology, composition and spacing of exsolution lamellae is an area of active research in the geosciences. The conventional study of these lamellae has been conducted by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and more recently with focused ion beam (FIB)-based nanotomography, yet with limited chemical information. Here, we explore the use of atom probe tomography (APT) for the nanoscale analysis of ilmenite exsolution lamellae in igneous titanomagnetite from ash deposits erupted from the active Soufri&#232;re Hills Volcano (Montserrat, British West Indies). APT allows the precise calculation of interlamellar spacings (14&#8211;29 &#177; 2 nm) and reveals smooth diffusion profiles with no sharp phase boundaries during the exchange of Fe and Ti/O between the exsolved lamellae and the host crystal. Our results suggest that this novel approach permits nanoscale measurements of lamellae composition and interlamellar spacing that may provide a means to estimate the lava dome temperatures necessary to model extrusion rates and lava dome failure, both of which play a key role in volcanic hazard mitigation efforts.

Analysis of the morphology, composition, and spacing of exsolution lamellae can provide essential information to understand geological processes related to volcanism and metamorphism. We present a novel application of APT for the characterization of such lamellae and compare this approach to the conventional use of electron microscopy and FIB-based nanotomography.

Element diffusion rates and temperature/pressure control a range of fundamental volcanic and metamorphic processes. Such processes are often recorded in ...

Environment

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F&#8722;

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound electrons with neutral molecules/atoms. State-of-the-art in vacuo anion generation techniques allow application to a broad range of atomic, molecular, and cluster anion systems. These are separated and selected using time-of-flight mass spectrometry. Electrons are removed by linearly polarized photons (photo detachment) using table-top laser sources which provide ready access to excitation energies from the infra-red to the near ultraviolet. Detecting the photoelectrons with a velocity mapped imaging lens and position sensitive detector means that, in principle, every photoelectron reaches the detector and the detection efficiency is uniform for all kinetic energies. Photoelectron spectra extracted from the images via mathematical reconstruction using an inverse Abel transformation reveal details of the anion internal energy state distribution and the resultant neutral energy states. At low electron kinetic energy, typical resolution is sufficient to reveal energy level differences on the order of a few millielectron-volts, i.e., different vibrational levels for molecular species or spin-orbit splitting in atoms. Photoelectron angular distributions extracted from the inverse Abel transformation represent the signatures of the bound electron orbital, allowing more detailed probing of electronic structure. The spectra and angular distributions also encode details of the interactions between the outgoing electron and the residual neutral species subsequent to excitation. The technique is illustrated by the application to an atomic anion (F&#8722;), but it can also be applied to the measurement of molecular anion spectroscopy, the study of low lying anion resonances (as an alternative to scattering experiments) and femtosecond (fs) time resolved studies of the dynamic evolution of anions.

Here, we present a protocol for photoelectron imaging of anionic species. Anions generated in vacuo and separated by mass spectrometry are probed using velocity mapped photoelectron imaging, providing details of anion and neutral energy levels, anion and neutral structure and the nature of the anion electronic state.

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound ...

3D Depth Profile Reconstruction of Segregated Impurities Using Secondary Ion Mass Spectrometry

The presented protocol combines excellent detection limits (1 ppm to 1 ppb) using secondary ion mass spectrometry (SIMS) with reasonable spatial resolution (~1 &#181;m). Furthermore, it describes how to obtain realistic three-dimensional (3D) distributions of segregated impurities/dopants in solid state materials. Direct 3D depth profile reconstruction is often difficult to achieve due to SIMS-related measurement artifacts. Presented here is a method to identify and solve this challenge. Three major issues are discussed, including the i) nonuniformity of the detector being compensated by flat-field correction; ii) vacuum background contribution (parasitic oxygen counts from residual gases present in the analysis chamber) being estimated and subtracted; and iii)&#160;performance of all steps within a stable timespan of the primary ion source. Wet chemical etching is used to reveal the position and types of dislocation in a material, then the SIMS result is superimposed on images obtained via scanning electron microscopy (SEM). Thus, the position of agglomerated impurities can be related to the position of certain defects. The method is fast and does not require sophisticated sample preparation stage; however, it requires a high-quality, stable ion source, and the entire measurement must be performed quickly to avoid deterioration of the primary beam parameters.

The presented method describes how to identify and solve measurement artifacts related to secondary ion mass spectrometry as well as obtain realistic 3D distributions of impurities/dopants in solid state materials.

The presented protocol combines excellent detection limits (1 ppm to 1 ppb) using secondary ion mass spectrometry (SIMS) with reasonable spatial ...

Preparation of Nanoparticles for ToF-SIMS and XPS Analysis

Nanoparticles have gained increasing attention in recent years due to their potential and application in different fields including medicine, cosmetics, chemistry, and their potential to enable advanced materials. To effectively understand and regulate the physico-chemical properties and potential adverse effects of nanoparticles, validated measurement procedures for the various properties of nanoparticles need to be developed. While procedures for measuring nanoparticle size and size distribution are already established, standardized methods for analysis of their surface chemistry are not yet in place, although the influence of the surface chemistry on nanoparticle properties is undisputed. In particular, storage and preparation of nanoparticles for surface analysis strongly influences the analytical results from various methods, and in order to obtain consistent results, sample preparation must be both optimized and standardized. In this contribution, we present, in detail, some standard procedures for preparing nanoparticles for surface analytics. In principle, nanoparticles can be deposited on a suitable substrate from suspension or as a powder. Silicon (Si) wafers are commonly used as substrate, however, their cleaning is critical to the process. For sample preparation from suspension, we will discuss drop-casting and spin-coating, where not only the cleanliness of the substrate and purity of the suspension but also its concentration play important roles for the success of the preparation methodology. For nanoparticles with sensitive ligand shells or coatings, deposition as powders is more suitable, although this method requires particular care in fixing the sample.

A number of different procedures for preparing nanoparticles for surface analysis are presented (drop casting, spin coating, deposition from powders, and cryofixation). We discuss the challenges, opportunities, and possible applications of each method, particularly regarding the changes in the surface properties caused by the different preparation methods.

Nanoparticles have gained increasing attention in recent years due to their potential and application in different fields including medicine, cosmetics, ...

Co-localizing Kelvin Probe Force Microscopy with Other Microscopies and Spectroscopies: Selected Applications in Corrosion Characterization of Alloys

Kelvin probe force microscopy (KPFM), sometimes referred to as surface potential microscopy, is the nanoscale version of the venerable scanning Kelvin probe, both of which measure the Volta potential difference (VPD) between an oscillating probe tip and a sample surface by applying a nulling voltage equal in magnitude but opposite in sign to the tip-sample potential difference. By scanning a conductive KPFM probe over a sample surface, nanoscale variations in surface topography and potential can be mapped, identifying likely anodic and cathodic regions, as well as quantifying the inherent material driving force for galvanic corrosion.
Subsequent co-localization of KPFM Volta potential maps with advanced scanning electron microscopy (SEM) techniques, including back scattered electron (BSE) images, energy dispersive spectroscopy (EDS) elemental composition maps, and electron backscattered diffraction (EBSD) inverse pole figures can provide further insight into structure-property-performance relationships. Here, the results of several studies co-localizing KPFM with SEM on a wide variety of alloys of technological interest are presented, demonstrating the utility of combining these techniques at the nanoscale to elucidate corrosion initiation and propagation.
Important points to consider and potential pitfalls to avoid in such investigations are also highlighted: in particular, probe calibration and the potential confounding effects on the measured VPDs of the testing environment and sample surface, including ambient humidity (i.e., adsorbed water), surface reactions/oxidation, and polishing debris or other contaminants. Additionally, an example is provided of co-localizing a third technique, scanning confocal Raman microscopy, to demonstrate the general applicability and utility of the co-localization method to provide further structural insight beyond that afforded by electron microscopy-based techniques.

Kelvin probe force microscopy (KPFM) measures surface topography and differences in surface potential, while scanning electron microscopy (SEM) and associated spectroscopies can elucidate surface morphology, composition, crystallinity, and crystallographic orientation. Accordingly, the co-localization of SEM with KPFM can provide insight into the effects of nanoscale composition and surface structure on corrosion.

Kelvin probe force microscopy (KPFM), sometimes referred to as surface potential microscopy, is the nanoscale version of the venerable scanning Kelvin ...

Screening of Coatings for an All-Solid-State Battery Using In Situ Transmission Electron Microscopy

With the ever-increasing use of Li-ion batteries, especially due to their adoption in electric vehicles, their safety is in prime focus. Thus, the all-solid-state batteries (ASSBs) that use solid electrolytes instead of liquid electrolytes, which reduce the risk of flammability, have been the center stage of battery research for the last few years. However, in the ASSB, the ion transportation through the solid-solid electrolyte-electrode interface poses a challenge due to contact and chemical/electrochemical stability issues. Applying a suitable coating around the electrode and/or electrolyte particles offers a convenient solution, leading to better performance. For this, researchers are screening potential electronic/ionic conductive and nonconductive coatings to find the best coatings with suitable thickness for long-term chemical, electrochemical, and mechanical stability. Operando transmission electron microscopy (TEM) couples high spatial resolution with high temporal resolution to allow visualization of dynamic processes, and thus is an ideal tool to evaluate electrode/electrolyte coatings via studying (de)lithiation at a single particle level in real-time. However, the accumulated electron dose during a typical high-resolution in situ work may affect the electrochemical pathways, evaluation of which can be time-consuming. The current protocol presents an alternative procedure in which the potential coatings are applied on Si nanoparticles and are subjected to (de)lithiation during operando TEM experiments. The high volume changes of Si nanoparticles during (de)lithiation allow monitoring of the coating behavior at a relatively low magnification. Thus, the whole process is very electron-dose efficient and offers quick screening of potential coatings.

Screening of Coatings for an All-Solid-State Battery Using In Situ Transmission Electron Microscopy

Utilizing the volume change of Si nanoparticles during (de)lithiation, the present protocol describes a screening method of potential coatings for all-solid-state batteries using in situ transmission electron microscopy.

Imaging Corrosion at the Metal-Paint Interface Using Time-of-Flight Secondary Ion Mass Spectrometry

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