Potentiodynamic Corrosion Testing

The overall goal of this procedure is to provide a reliable setup used to carry out potentiodynamic corrosion on small metallic samples. This method can answer key questions in the biomedical field, such as being able to deduce the corrosion susceptibility of implant or materials. The main advantage of this technique is that it is a relatively inexpensive method of corroding small metallic samples.

Construct the sample holder from stainless steel spacers and an M3 stainless-steel threaded screw held in place with an M3 hexagonal nut. Remove the head of the threaded screw using pliers, and polish the cut segment to maintain the thread pattern. When all individual components are ready, assemble the electrode holders.

Each electrode holder contains three spacers joined together by the M3 screws, resulting in an 11.5 centimeter handle. Place the hexagonal nuts at the junction of the screw and spacers to lock the connection. Once the electrode holders are assembled, apply multiple coats of stop-off lacquer to prevent the stainless steel rods from corroding while immersed in the corrosion chamber.

To do this, quickly coat half of each electrode holder and allow it to dry before coating the other half. This will help obtain a complete well-sealed coat without damaging the areas to be coated. Place the electrodes in an elevated position in the fume hood to dry.

Ensure the freshly coated regions do not touch other surfaces, as this will ruin the applied coat. Clean the corrosion vessel before every corrosion run. Scrub the vessel with household detergent and rinse thoroughly with tap water.

Repeat this step three times. Then, rinse the corrosion vessel three times with deionized water to remove potential contaminants found in tap water. Once rinsing with deionized water is completed, pour 300 milliliters of 95%ethanol into the corrosion vessel and swirl around to contact all internal surfaces.

Pour out the ethanol and repeat this step three times. Leave the corrosion vessel under a fume hood for 30 minutes to allow all of the ethanol to completely evaporate before rinsing with the electrolyte to be used for the corrosion run, as described in the text protocol. The working electrode is the stainless steel screw, the specimen under analysis.

Before inserting the electrode into the corrosion vessel, wipe it with an 80%ethanol soaked wipe, and place it in a glass beaker filled with 100 milliliters of phosphate buffered saline. Use a connection pin to attach the electrode holders onto the electrode suspenders, Fit the electric suspenders into the entry points of the corrosion vessel's lid. Seal the corrosion vessel with a glass reaction lid and clamp to ensure a waterproof seal.

The lid of the chamber provides five entry points for the experimental and measurement apparatus. Use a three-electrode configuration, consisting of a reference, counter, and working electrode. For the reference electrode, use a standard silver-silver chloride electrode.

For the counter electrode, use a platinum mesh that is loosely bent to wrap around the specimen under test. Once the chamber is sealed with all electrodes placed inside the corrosion vessel, set the temperature to 37 degrees celsius, and open the nitrogen valve with a flow rate of 150 cubic centimeters per minute. Leave the temperature and nitrogen running for 60 minutes before conducting a run.

Keep the nitrogen running for the duration of the experiment. Make electrical connections between the potentia stat and the three electrodes, and then turn the potentia stat on. Leave the sample to equilibrate and stabilize within the corrosion vessel environment.

The time duration for this varies and is dependent on material. Monitor the potential using measurement view to determine if stabilized conditions are reached. The potentia will be constant with no fluctuations when stable conditions are achieved.

Select the cyclic voltammetry potentia stat procedure within the setup view from the procedure tab. Select the option to automate the current range. Set the highest current in the range to be 10 milliamps and the lowest current in the range to be 10 nanoamps for the working electrode.

Ensure the final cut off selection is controlled through the potential, by setting the cycle back parameter to 800 millivolts to allow the hysteresis loop to complete. Record the open circuit potential, or OCP, from the measurement view into the OCP parameter text box. Set the start potential 100 millivolts below the recorded OCP value.

Set the upper vertex potential to 800 millivolts, the lower vertex to 100 millivolts below the start potential, and the stop potential to 100 millivolts below the lower vertex potential. Then, set the scan rate to 0.001 volts per second, and the step potential to 0.0024 volts per second. Now, press start.

To remove the sample from the electrode holder, prepare three small jars of 50 milliliters of dichloromethane under the fume hood. Remove the tested samples from the electrode holders by immersing the lower end of the holder in dichloromethane for 30 minutes inside the fume hood. Once detached, place the specimen into the next jar of dichloromethane and leave for 15 minutes.

Repeat this process with the third and final rinse to get rid of any excess coating on the attachment sections of the sample. Shown here, is the representative results of a potentiodynamic polarization scan from a corrosion run for stainless steel 316, which can be further used to extract important corrosion parameters that provide corrosion characterization of the test specimen. This polarization scan shows the direction of the forward and reverse scans.

and specifies the point at which the protection potential and pitting potential are observed. The hysteresis loop is reversed at the set potential and returns to intercept the anodic curve, identifying the protection potential. It is critical to have a system that provides high-quality scans, which are reproducible and reliable, since potentiodynamic polarization plots are used to calculate the corrosion current and corrosion potential using tafel extrapolation.

Once mastered, this technique can be done in approximately two hours if performed properly. This does not include the time for the sample to corrode, as this time is material dependent. While attempting this procedure, it is important to remember not to contaminate any electrodes and any glassware prior to the corrosion run.

Limiting any impurities is critical. Following this procedure, other methods like scanning electron microscopies, surface analysis, or other imaging techniques, or quantification of corrosion techniques, can be performed in order to answer additional questions. After watching this video, you should have a good understanding of how to conduct a reliable electro-dynamic corrosion system.

Don't forget that working with microstop laquer can be extremely hazardous, and precautions such as double gloves and working under a film hood, should always be taken while performing this procedure.