- 1 Chloride Penetration Profile Determination with ESEM
- 1.1 Discussion
- 1.2 Safety
- 1.3 Definitions
- 1.4 Apparatus
- 1.5 Materials
- 1.6 Preparation of Calibration Standards
- 1.7 ESEM Operating Parameters
- 1.8 EDAX Map Collection Parameters
- 1.9 Setup
- 1.10 Collecting Maps
- 1.11 Gathering Data
- 1.12 Converting Maps to X-ray Counts
- 1.13 Analysis and Presentation of Data
- 1.14 Additional Considerations
Chloride Penetration Profile Determination with ESEM
This method utilizes the environmental scanning electron microscope and x-ray spectroscopy to determine the chlorine concentration at various depths within suitably prepared concrete specimens. The ESEM is operated in low vacuum mode and specimens are not coated. X-ray elemental maps are collected at sufficiently high magnification to insure that only cement paste is analyzed. Since an area of paste is rastered and spectra collected at each pixel, a volume-averaged chlorine concentration can be determined by summing elemental map pixel intensities and calculating chlorine concentration based on calibration with mortars having known chlorine concentrations. This reduces chlorine concentration variability associated with spot analysis. Variations in beam current are accounted for by periodic collection of spectra on a copper target.
Elemental maps are used because the EDAX software presently installed is able to collect maps automatically after a relatively painless and quick setup process. This greatly reduces operator interaction, increasing overall efficiency of both the operator and the instrument. Post processing tasks have been automated with Photoshop scripts to convert images to counts and spreadsheets for data analysis and presentation.
- The ESEM lab is a relatively safe space. Be aware of the high pressure nitrogen cylinder secured to the wall.
- Only trained and qualified operators are allowed to operate the instrument.
ESEM - Environmental Scanning Electron Microscope
- Environmental scanning electron microscope with x-ray spectroscopy/elemental mapping capability
- Chloride mortar standards
- Concrete specimen prepared as a plane-ground billet on working glass
- Carbon double-sided tape
- Pin mounts
Preparation of Calibration Standards
Need guidance here:: Karl
ESEM Operating Parameters
- 15kV accelerating voltage
- 0.3 Torr water vapor
- Gun bias at lowest setting (1)
- 7 Spot size
- 10mm FWD
- 800x magnification
- LFD installed
- High vacuum bullet installed
- Backscatter electron detector in place on high vac bullet
- Select No PLA during vacuum conditions setup
- Select Automatic purge cycle
EDAX Map Collection Parameters
- 64x50 matrix
- 100ms dwell
- 4.0 amp
- Collect maps for Cl and Cu (other elements as desired)
- Adhere a small piece of copper tape at a logical point (middle of standards and on a coarse aggregate grain in concrete specimens) on each of the specimens to be analyzed.
- Mount specimens firmly on pin mounts with carbon tape.
- Place specimens on multiple specimen stage with the edges corresponding to the top of the specimens all facing the same way and make note of the position. in ESEM and close chamber CAREFULLY OBSERVING THAT NO DETECTOR WILL BE STRUCK UPON CLOSING THE CHAMBER DOOR.
- Ensure vacuum parameters are properly set (see above).
- Pump chamber and wait for conditions to stabilize.
- Ensure beam parameters are properly set and start beam current.
- Select the BSE detector, focus and train stage z to FWD with the beam on one of the specimens.
- Set FWD to 10mm and optimize the image. Retrain stage z to FWD if required.
- Since the SE detector is disabled, filament current is set by making adjustment to filament current and observing the effect on the BSE image: Increment filament current up or down and note the change in image brightness. Adjust to a current where there is only a slight change in brightness upon increasing filament current.
- While filament is saturating, locate the positions of copper tape on each of the specimens. Retrain stage z to FWD after optimizing the image and set the FWD to 10mm. This must be repeated for all of the specimens in the chamber. Once stage z is trained and FWD is at 10mm, store the stage location in microscope control with a logical name. Note names and associate with their respective specimens.
- Locate the top edge of each specimen by setting magnification to 800x and placing the top edge in the center of the screen. Save the coordinates with logical names and make note of the names.
- Use good judgement when selecting areas of paste to map. Recall that a concentration profile with depth is being generated so select points separated by some distance along the general profile path. Do not select points immediately adjacent cracks or aggregate. Locate relatively large fields of paste and select an area in the center of that field such that when magnification is set to 800x, the entire field of view is filled with just cement paste.
- To account for beam current fluctuations, every fifth map will be a map of the copper target. The sequence is one map copper, four maps paste, one map copper, four maps paste... ending with a map of copper.
- To facilitate calculating actual depth, maps of the top and bottom of the specimen will also be collected, but they are not used in the analysis. The top of the specimen will be the first location mapped and the bottom of the specimen the last location mapped.
- A total of 24 paste maps are collected for the top-most (nearest exposed surface) specimen and 12 paste maps are collected from subsequent specimens below it. A total of 33 maps are collected from the top-most specimen and 18 maps from lower specimens, including top, bottom, copper and paste locations.
- Start the EDAX image collection and mapping software and select the status bar that indicates CPS and dead time.
- Clear stage locations.
- Select elements for which maps will be collected: Clka and Cuka must be selected.
- At microscope control, translate stage to the top of the specimen to be analyzed.
- Store the location in the EDAX mapping software by clicking the "First" button, comparing coordinates to those shown in microscope control and then clicking the "Add" button if the coordinates match. If coordinates do not match, move the stage slightly, click the "First" button again and compare coordinates. Do not add the coordinates to the list if they fail to match.
- At microscope control, translate stage to the copper target of the specimen to be analyzed and store the position in EDAX mapping.
- At microscope control, translate back to the top of the specimen being analyzed and locate a suitable field of paste to be analyzed moving a small distance along the profile path. Set magnification to 800x and assure that the field of view is only cement paste. Store the location in EDAX mapping. Repeat this step three more times after translating the stage small increments along the profile path.
- Translate the stage to the copper target on the specimen being analyzed and store the coordinates in EDAX mapping.
- After the last paste map coordinates are stored in EDAX mapping, translate the stage to the copper target on the specimen being analyzed and store the coordinates in EDAX mapping.
- Locate the bottom of the specimen being analyzed and store those coordinates in EDAX mapping. There should be a total of 33 coordinate sets stored in EDAX mapping for the top-most specimen and 18 for each of the specimens located below it.
- Start automated collection of maps in the EDAX software and store maps in a logically named folder.
- At the EDAX workstation, select the stage coordinate pane and scroll the coordinate list to the top. Press ctrl>PrtScn to copy the screen image to the clipboard. Open Paint and paste the contents of the clipboard, selecting resize when requested. Save the screen bitmap to the same folder where the maps were saved. If the coordinates are for the top-most specimen, you need to repeat the process after having scrolled to the bottom of the coordinate list. These screen shots will serve as the list of coordinates along the profile path so that concentration by depth can be determined.
- Copy the entire folder containing maps, screen shots and stage location table to only an approved storage device. USE OF AN UNAPPROVED STORAGE DEVICE WILL RESULT IN THE LOSS OF INSTRUMENT PRIVILEGES. This is necessary to insure that the workstations are not infected by viruses.
Converting Maps to X-ray Counts
- A script has been prepared for Photoshop that will interrogate text files written by the EDAX mapping software and sum x-ray counts based upon pixel intensities for each elemental map collected. It is available here (in a bit).
- In the absence of the Photoshop script, the following is required:
- Open the text file for the map file (each set of maps is stored in a folder named fld#### and each area mapped has three files associated with it: .bmp, .IPR and .txt - open the .txt file).
- Note the _DataRange values - these are the minimum and maximum pixel x-ray counts for the given element.
- Pixel intensities in the elemental map are linearly related to minimum and maximum x-ray counts with 0 intensity corresponding to the minimum count and 255 intensity corresponding to the maximum count. Calculate the x-ray count for each pixel in the map image and sum the result to get the total x-ray count for the area analyzed.
- This procedure is performed on all of the Cl maps collected from the concrete specimen and all of the Cu maps collected from the Cu target on said specimen.
ImageJ can be used to perform this task either manually or a macro can be prepared. ImageJ macro language has the ability to open and manipulate text files. ImageJ can also save images in text format, with pixel intensity values in a comma delimited list.
Analysis and Presentation of Data
The Photoshop script writes a comma delimited count summary text file for the elemental maps it analyzes. The file contains the field ID, file name and total x-ray count for each map in order of field ID. The first field will therefore be the top of the specimen, the next the copper target and then the first cement paste map.
For generation of the chloride profile in the cement paste, chlorine x-ray counts for the top, bottom and copper target maps are discarded and only those x-ray counts corresponding to maps of paste are used.
Likewise, for the copper x-ray counts, only counts from maps taken of the copper target are used, the rest are discarded. Beam current variation is assumed to be temporally related and Cl map x-ray counts are adjusted up or down based upon the Cu counts that they are bounded by. The amount of time required to collect a map and then translate the stage to the next map location is assumed to be constant. Once the Cl counts have been normalized to a specimen-constant beam current based upon Cu counts, the counts are then adjusted to a constant Cu count based upon the calibration standards used to calculate chlorine concentration. An Excel spreadsheet has been prepared that handles these adjustments and prepares the data for fitting to Fick's second law.