Here, we present a protocol to measure the tribocorrosion rate and wear-corrosion synergy of thin film and bulk Al alloys in simulated sea water at room temperature.
This method can help answer key questions in the tribocorrosion field about how to quantify the tribocorrosion rate or how to measure wear corrosion synergy of methods and coatings. The main advantage of this technique is that it can be used for the tribocorrosion studies of few micrometer thick metallic thin films and coatings. Demonstrating the procedure will be Jia Chen, a grad student from the Tsai Research Laboratory.
Begin by using a computer numerical control machine to cut as-received aluminum 3003 alloys into several 1.5 by 2-square centimeter coupons. Using 180-grit sandpaper, mechanically grind one side of each sample for 30 seconds along one arbitrary direction. Rotate the sample 90 degrees and grind the alloy with 240-grit sandpaper until the scratch lines from the previous step are completely removed as confirmed by optical microscopy.
Then use a soft brush and running water to clean the sample surface and rotate and scratch the sample as just demonstrated using 400, 600, and 1200-grain sandpapers. After the last grinding, pour one ounce of a one-micrometer alumina suspension onto a clean microfiber cloth pad and polish the sample in one direction until the scratch lines have been removed. Then polish the surface with 0.3 and 0.05-micrometer polishing suspensions until a mirror finish has been achieved.
Place the polished specimen in a beaker with 40 milliliters of deionized water and transfer the beaker to an ultrasonic cleaner for one to two minutes to remove any surface particles. Then use compressed gas to completely dry the surface. To prepare the working electrode, cut a five-centimeter long piece from a one to two-millimeter diameter electrical wire and strip away about one centimeter of the protective plastic cover on both ends to expose the interior copper wire to air.
Use an appropriate conductive adhesive to electrically connect one end of the wire to the unpolished side of the bulk sample and use electrochemical stop-off lacquer to paint a one by one-square centimeter window on the polished side and to cover the entire unpolished side of the sample, including the exposed copper wire. Then dry the painted sample completely in a well-ventilated fume hood for at least 24 hours before the experiments. To perform a tribocorrosion test, collect the prepared aluminum sample, a standard silver-silver chloride electrode, and an activated titanium mesh for use as the working, reference, and counter electrodes respectively.
Place the working electrode centrally at the bottom of the corrosion cell and secure the electrode to the bottom of the cell with Super Glue, placing the exposed copper wire tip above the expected electrolyte surface height. Place the reference electrode about one centimeter above the working electrode and loosely bend the counter electrode to wrap around the working electrode. Connect the electrodes with a potentiostat, taking care that the electrodes are not touching, and open the electrochemical software package that interfaces with the USB-controlled potentiostat to turn on the potentiostat.
To measure the wear rate from cathodic protection, move the indenter probe to one millimeter away from the surface of the sample. After confirming the location of the indenter near the center of the sample and no electrical contact between the electrodes, the probe, and the sample surface, move the indenter forward 200 millimeters and pour 3.5 weight aqueous sodium chloride solution into the corrosion cell until the electrodes, probe, and sample surfaces are immersed. Then move the indenter probe as close as possible to the sample surface and select Experiment to view the DC Corrosion.
Then select Potentiostatic Mode and apply a cathodic potential of 350 millivolts below the open circuit potential. To set up the wear test in the Universal Mechanical Tester software, apply a one-hertz scratch frequency, five-millimeter scratch length, and 0.5-newton normal load. Then click Run to begin the tribocorrosion.
To measure the tribocorrosion rate from the tribocorrosion test at the open circuit potential, repeat the test as demonstrated but with the applied potential set to the open circuit potential. Following the testing protocol as demonstrated, the tribocorrosion rate and the material loss obtained for the aluminum thin film sample after tribocorrosion can be measured at the cathodic, open circuit, and anodic potentials. Here a summary of the tribocorrosion rate, wear rate, corrosion rate, and wear corrosion synergy of the aluminum thin film sample components used to calculate the total material loss are shown.
While attempting this procedure, it's important to remember to achieve a mirror finish of the sample before the test and to ensure good electrical connection between the sample and the connecting wire. Visual demonstration of this method is critical as the accurate execution of multiple steps of the procedure are critical for achieving reliable and repeatable tribocorrosion test results and are difficult to master by text alone. Don't forget to use all appropriate safety practices when performing experiments, including the use of engineering controls and personal protective equipment.
