Potentiodynamic Corrosion Testing

Summary

Here, we present a protocol to set up and run an in vitro potentiodynamic corrosion system to analyze pitting corrosion for small metallic medical devices.

Abstract

Different metallic materials have different polarization characteristics as dictated by the open circuit potential, breakdown potential, and passivation potential of the material. The detection of these electrochemical parameters identifies the corrosion factors of a material. A reliable and well-functioning corrosion system is required to achieve this.

Corrosion of the samples was achieved via a potentiodynamic polarization technique employing a three-electrode configuration, consisting of reference, counter, and working electrodes. Prior to commencement a baseline potential is obtained. Following the stabilization of the corrosion potential (E_corr), the applied potential is ramped at a slow rate in the positive direction relative to the reference electrode. The working electrode was a stainless steel screw. The reference electrode was a standard Ag/AgCl. The counter electrode used was a platinum mesh. Having a reliable and well-functioning in vitro corrosion system to test biomaterials provides an in-expensive technique that allows for the systematic characterization of the material by determining the breakdown potential, to further understand the material's response to corrosion. The goal of the protocol is to set up and run an in vitro potentiodynamic corrosion system to analyze pitting corrosion for small metallic medical devices.

Introduction

Electrochemical techniques provide a quick and relatively inexpensive method to obtain the electrochemical properties of a material. These techniques are based predominately on the ability to detect corrosion of a metal by observing the response of the charge-transfer process to a controlled electrochemical disturbance ^1-5. Corrosion of metal implants within a body environment is critical due to the adverse implications on biocompatibility and material integrity ⁶. The main factor contributing to corrosion of implants within the body is the dissolution of the surface oxide leading to an increased release of metallic ions ^7-11. This results in adverse biological reactions, which can be found locally, but with potentially systemic effects leading to the premature failure of the implant ^10,12-28.

The corrosion characteristics of a test specimen are predicted from the polarization scan produced by a potentiostat. A polarization scan allows for the extrapolation of the kinetic and corrosion parameters of a metal substrate. During a scan, the oxidation or reduction of an electro-active species can be limited by charge transfer and the movement of reactants or products. These factors are all encapsulated by the polarization scan; therefore the importance of having a system that produces a reliable and repeatable polarization scan across multiple cycles is of great importance. The main focus of this manuscript is to provide a protocol identifying the rationale and steps taken to obtain a well-functioning potentiodynamic corrosion system.

Protocol

1. Construction of the Sample Holder

1. Construct the sample holder from stainless steel spacers and a M3 stainless steel threaded screw, held in place with a M3 hexagonal nut.
2. Remove the head of the threaded screw using pliers and polish the cut segment to maintain the thread pattern.
3. 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 cm handle. Place the hexagonal nuts at the junction of the screw and spacers to lock the connection.
4. Solder (60/40% Sn/Pb) a toothless alligator clip onto the screw at the end of the rod. This will ensure a firm hold to later attach the electrode during analysis.
5. Once the electrode holders are assembled, apply multiple coats of stop-off lacquer (electrical sealant) to prevent the stainless steel rods from corroding whilst immersed in the corrosion chamber.
   1. Place all electrode holders with the samples attached to the alligator clip into a fume hood prior to coating. Place a 20 ml syringe into the fume hood. Use the syringe to collect the stop-off lacquer.
   2. Turn on the fume hood and pour the stop-off lacquer into a small glass jar. Pull 10 ml of stop-off lacquer into the syringe and coat the surface of the electrode holders. Make sure not to cover the test sample, which is going to be analyzed for corrosion.
   3. Coat half of each electrode holder and place in the fume hood to dry before coating the other half. This will help obtain a complete well-sealed coat without damaging the areas to be coated. Ensure that during the drying phase, the freshly coated regions do not touch other surfaces, as this will ruin the applied coat.
   4. Place the electrode holders in an elevated position during drying with no contact to any surfaces. Coat the electrodes quickly due to the quick solidification of the stop-off lacquer. This completes the first layer.
   5. Once dry, repeat the process to obtain 3 coats along the entire area.
6. Before commencing the corrosion run, leave the holders to dry for 24 hr after the completion of the final coat. All coating processes occur at RT, no heating or cooling steps are required although they may accelerate/decelerate the curing process.
7. Making a faraday cage
   1. Construct a Faraday cage by coating two plastic containers of the same size with 4 layers of aluminum foil to cover all sides.
   2. Cut two small holes out at the rim of upper plastic container to allow the electrode connection to the potentiostat and the nitrogen line to the nitrogen tank to pass through. A split design of the Faraday cage allows the upper component to be removed at the end of a run without needing to replace the lower section housing the tank.
   3. Fit the outer compartment (water chamber) into the Faraday cage. Leave the second half to the side and place on top of the lower compartment only when the corrosion vessel has been sealed (later in the procedure).

2. Cleaning of Glassware

1. Clean the corrosion vessel (700 ml cylindrical flask) before every corrosion run. Scrub the vessel with household detergent and rinse thoroughly with tap water. Repeat this step 3 times.
2. Rinse the corrosion vessel 3 times with de-ionized water (DI) water to remove potential contaminants found in tap water.
3. Once rinsing with DI water is completed pour 300 ml of 95% ethanol into the corrosion vessel and swirl around to contact all internal surfaces. Pour out the ethanol and repeat this step 3 times.
4. Leave the corrosion vessel under a fume-hood for 30 min to allow all of the ethanol to completely evaporate.
5. Take the clean, dry corrosion vessel and rinse it with the electrolyte that will be used for the corrosion run. For each rinse fill the corrosion vessel with 200 ml of the electrolyte and repeat this procedure 3 times. For this study, rinse the corrosion vessel with phosphate buffered saline (PBS). The chemical make-up of the PBS (10 L) electrolyte used throughout is 80 g NaCl, 11.5 g Na₂HPO₄, 2 g KCl and 2 g KHPO₄.
6. Following the rinse, fill the corrosion vessel with the required volume of PBS ready for the reaction.

3. Setup of Apparatus

1. Clamp a heater with an inbuilt circulation system to the side of the outer compartment using a clamp. The size of the outer compartment needs to be approximately 30 cm x 20 cm x 20 cm and made of either glass or polymeric to be capable of housing the smaller corrosion vessel and the heater system.
2. Fill the outer compartment with tap water until the level of water is higher than the height of the suspended electrodes within the corrosion vessel. The smaller compartment is the corrosion vessel (previously described in section 2).
3. Seal the corrosion vessel with a glass reaction lid and clamp to ensure a waterproof seal. The lid of the chamber provides six entry points for experimental and measurement apparatus.
4. Suspend a thermometer from one of the entry points of the reaction lid to provide a reading of the temperature within the corrosion cell. Suspend all three electrodes from the lid using the other 3 entry points. Use polytetrafluoroethylene (PTFE) tape to secure the seal of each connection.
5. Use a three-electrode configuration consisting of a reference, counter, and working electrode. The working electrode is the stainless steel screw (specimen under analysis). Before inserting the electrode into the corrosion vessel, wipe with an 80% ethanol soaked wipe and place in a glass beaker filled with 100 ml of PBS.
6. Use a connection pin to attach the electrode holders onto the electrode suspenders. Fit the electrode suspenders into the entry points of the corrosion vessel's lid.
7. Place the working electrode centrally with the counter and reference electrode being suspended from either side. Seal the glass entry point and the corrosion suspenders using PTFE tape.
8. For the reference electrode, use a standard Ag/AgCl. For the counter electrode, use a platinum mesh that was loosely bent to wrap around the specimen under test (working electrode).
9. Fill the Ag/AgCl electrode with 3 M KCl using a pipette. Following extensive use, change and refill the Ag Ag/Cl. To do this release the tip of the electrode to empty out the fluid into a small glass collection vessel (beaker). Once all the solution is removed insert the tip and fill with 3 M KCl.
10. Use tape on all junctions to ensure the whole chamber is sealed.
11. Once the chamber is sealed with all electrodes placed inside the corrosion vessel, set the temperature to 37 °C and open the nitrogen valve with a flow rate of 150 cm³/min. Leave the temperature and nitrogen running for 60 min before conducting a run. Keep nitrogen running for the duration of the experiment.

4. Running Corrosion Test

1. Open the electrochemical software package, which interfaces with the USB controlled potentiostat.
2. Make electrical connections between the potentiostat and the 3 electrodes and then turn the potentiostat on.
3. Open and use the "measurement view" to view the potential and current readings of corrosion environment. During the open circuit potential (OCP) phase where no ramp potential is yet applied the current reading between the working (positive potential) and counter (negative) electrode is around (0 ± 0.01) µA. The improper sealing of the chamber with PTFE tape can cause fluctuations in the current reading due to the chamber being aerated with nitrogen gas to remove oxygen molecules.
4. Leave the sample to equilibrate and stabilize within the corrosion vessel environment. The time duration for this varies (1 to 6 hr) and is dependent on material. Monitor the potential using measurement view to determine if stabilized conditions are reached. The potential will be constant with no fluctuations when stable conditions are reached.
5. After stable conditions are reached, start the corrosion run. However before this can be done, fill in the "corrosion program" and "cyclic voltammetry (CV)" conditions using the skeleton template provided by analytical software.
6. Select the cyclic voltammetry potentiostat procedure within the setup view from the procedure tab.
7. Enable the following parameters to be sampled for the corrosion run: the time, working electrode (WE) potential, and current for the corrosion run.
8. Select the option to automate the current range. Set the highest current in the range to be 10 mA, and the lowest current in the range to be 10 nA for the WE.
9. Ensure the final cut-off selection is controlled through the potential by setting the 'cycle back' parameter to 0.8 mV to allow the hysteresis loop to complete.
10. Record the OCP from the measurement view into the OCP parameter text box. Set the start potential 100 mV below the recorded OCP value. Set the upper vertex potential to 800 mV, the lower vertex to 100 mV below the start potential and the stop potential to 100 mV below the lower vertex potential. Set the scan rate to 0.001 V/sec and the step potential to 0.0024 V/sec. Now Press start.

5. After the Completion of the Corrosion Run

Note: After the completion of the corrosion run the polarization scan is shown within the analysis view of the software. For each polarization run the presenter view lists the OCP, the plot for E vs. t and the CV staircase which is a plot of E vs. Log (i).

1. Within each plot link, determine internal filtration of the data points, Tafel extrapolation, and plot options. Expand each link presented to show various parameters of interest, which collectively form the electrochemical parameters. The polarization scan (current density vs. potential), determines the open circuit potential, pitting potential (E_pit) and the protection potential (E_pro).
2. Tabulate the anodic and cathodic Tafel constants, the corrosion rate, the corrosion current, corrosion current density, the start potential, and end potential under the corrosion rate using the Tafel slope link.

6. Removing the Sample from the Electrode Holder

1. Prepare 3 small jars of 50 ml with dichloromethane under the fume hood.
2. Remove tested samples from the electrode holders by immersing the lower end of the holder in dichloromethane for 30 min inside a fume-hood.
3. Once detached, place the specimen into the next jar of dichloromethane and leave for 15 min. Repeat this process with the third and final rinse to get rid of any excess coating on the attachment sections of the sample.
4. Wipe the remaining sealant from the clip and sample and finally rinse with DI water.

Results

At the conclusion of the procedure an in vitro corrosion system is setup to conduct corrosion studies. Specific procedures such as the cleaning of the corrosion vessel and the Faraday cage were introduced into the protocol to improve noise performance. The fundamental concept of a good polarization scan is to identify the electro-physical conditions of the material providing valuable information in order to understand the corrosion susceptibility of a metallic material. The proce...

Discussion

Polarization scans produced from the stainless steel samples showed clean continuous plots correlating with scans seen in literature indicative of a well functioning corrosion system which is both reliable and reproducible ²⁹. Poor reproducibility of potentiodynamic pitting potentials is identified with a spread of a few hundred millivolts, with pitting potential being characterized by a stochastic process ²⁹. This is commonly due to the variables of temperature, halide content and potential (V); th...

Materials

Name	Company	Catalog Number	Comments
Potentiostat	Metrohm	PGSTAT101
Ag/AgCl reference electrode, shielded	Metrohm	6.0729.100
Electrode shaft	Metrohm	6.1241.060
Polisher Forcipol 1v	Metkon	3602
Clindrical flask 700 ml	SciLabware	FR700F
Reaction lid	SciLabware	MAF2/41
Dichloromethane	Sigma-Aldrich	MKBR7629V	Use under a fumehood. Wear protective clothing.
Thermo / HAAKE D Series Immersion Circulators	Haake