Chemistry
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Imaging Corrosion at the Metal-Paint Interface Using Time-of-Flight Secondary Ion Mass Spectrometry
Summary May 6th, 2019
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.
Transcript
This approach reveals the corrosion process at the metal-paint interface, providing insight into mechanical and chemical changes at the interface with high surface sensitivity. Time-of-flight secondary ion mass spectrometry, or ToF-SIMS, is a powerful surface tool. It provides chemical maps with high lateral and mass resolution and allows effective characterization at the metal-paint interface.
So an important tip that a new practitioner should know is to ensure that the sample does not touch the extraction cone to avoid potential damage to the instrument. The visual demonstration of this method is critical for researchers who are new to ToF-SIMS and will help them with the fundamental analysis process. To begin, load the prepared salt-exposed and air-exposed samples into the instrument load block.
Pump down the load block, transfer the samples to the main chamber, and wait until the chamber is at or below 10 to the minus eight millibars. Then, power up the liquid metal ion gun, or LMIG, the analyzer, and the light source. Set the primary gun as LMIG with the preferred metal, bismuth, and start the LMIG using predefined spectrometry.
Next, use either software or manual controls to move the sample stage to the Faraday cup. Then, auto-align the ion beam. After that, start measuring the target current at the Faraday cup, and select Direct Current.
Click X Blanking, and adjust it until the target current is maximized. Then, repeat the process with Y Blanking. Stop the measurement when finished.
Next, guided by the view through the main chamber window, slowly lower the sample stage until the top of the sample is lower than the bottom of the extractor cone. Then, position the stage under the cone so that the interface assembly is visible in the macro view in the software. After that, set the instrument to detect negative ions.
Load the desired analyzer settings, and activate the analyzer. Next, switch to the micro scale view, and set the raster field of view to 300 by 300 micrometers. Then, set the signal to secondary ion, the raster size to 128 by 128 pixels, and the raster type to random.
Adjust the secondary ion image of the ROI by slowly moving the sample stage vertically until the image is centered on the crosshair in the Navigator GUI. Do not move the joystick handle down too quickly while adjusting the Z direction, otherwise the extraction cone will hit the stage and get damaged. After that, use DC cleaning to remove the gold coating and surface contaminants.
Once the sample surface is clean, enable charge compensation, and load the desired flood gun settings. Then, refocus the secondary ion image on the ROI. Once it is focused, increase the reflector voltage until the secondary ion image disappears.
Then, decrease the voltage by 20 volts, and stop the adjustment. Next, open the mass spectrum in the imaging windows, and display the ROI of the metal-paint interface. Start a quick scan, and stop the scan once a spectrum appears.
Then, in the mass spectrum window, select the known peaks in the mass spectrum from the quick scan, and fill in the formulas. After that, add the peaks of interest to the peak list. Open the measurement window, set the raster type to random, the size to 128 by 128 pixels, and the rate to one shot per pixel.
Set the instrument to perform 60 scans, and begin the measurement. Save the completed spectrum afterwards. Then, name and save the ROI location.
Move the stage to locate new ROIs to analyze. Next, load the desired high-resolution SIMS imaging settings for the LMIG. Move the sample stage to the Faraday cup, and realign and refocus the ion beam for imaging.
Then, move the stage back to the saved ROI position. Adjust the reflector voltage, acquire a quick spectrum, and perform mass calibration. Then, set the raster type to random, the size to 256 by 256 pixels, and the rate to one shot per pixel.
Set the number of scans to 150, and run the image acquisition. When finished, export the data, remove the sample, and shut down the instrument. Secondary ion mass spectrometry showed small aluminum oxide and oxyhydroxide peaks at the aluminum-paint interface of a sample exposed only to air, indicating mild corrosion.
In contrast, a sample treated with salt water had much larger peaks and additional oxyhydroxide species. This was consistent with the salt water-treated sample having experienced more severe corrosion than the sample exposed only to air. 2D molecular images confirm that the aluminum oxide and oxyhydroxide species were much more prevalent in the sample that had been treated with salt water.
Understanding surface damage and corrosion development is very challenging. ToF-SIMS is a perfect tool for this application, as illustrated in this procedure. In addition to studying the corrosion process, ToF-SIMS has been widely used in material surface characterization in radiological, biological, and environmental samples.
Please be mindful that the settings of the mass spectra and image acquisition will vary depending on the types of the LMIG, remaining life of the LMIG, and other factors. We illustrate in this method that ToF-SIMS is very powerful in revealing the interfacial chemistry at the micro scale and providing chemical mapping with high lateral distribution and high mass accuracy. ToF-SIMS is a surface-sensitive technique.
Please always wear gloves, and protect samples that you are handling.
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