Accelerated Cyclic Electrochemical Technique
Accelerated methods for determining the efficacy of organic barrier coatings on metal surfaces have been of interest for many years. One of the first published techniques was the Rapid Electrochemical Assessment of Paint (REAP) developed by Martin Kendig et al. In that paper a method was developed to estimate the time-to-failure of a coated sample. The method involved both electrochemical methods and salt fog methods in the assessment.
More recently, a purely electrochemical method of making this assessment has been developed as a standard by ISO (International Organization of Standardization) and uses cathodic polarization of the sample as a stress mechanism to induce failure of the coated sample. The coating quality is assessed by Electrochemical Impedance Spectroscopy (EIS) after each polarization. The ISO standard is ISO 17463 in the Paints and Varnishes section called “Guidelines for the determination of anticorrosive properties of organic coatings by accelerated cyclic electrochemical technique (ACET)”. The exact procedure is as follows: an initial assessment of the coating is made by performing an electrochemical impedance spectroscopy (EIS) measurement at the open circuit potential (if no open circuit potential is measurable because of the barrier nature of the coating a measurement should be made at a fixed DC potential equal to the potential of the underlying metal in the electrolyte solution). After this the next step involves a cathodic polarization of the coated sample at -2 to - 4 V vs the saturated calomel reference electrode (SCE); the exact potential depends on the barrier nature of the coating; the better the coating the more negative the potential. A minimum potential of 1V more negative than the open circuit potential is recommended. The polarization lasts for between 20 and 60 minutes. Once the polarization is complete, the sample is allowed to relax (that is, no polarization of the surface) for a period of up to 3 hours in which the open circuit potential is measured. If the potential stabilizes prior to the end of the 3 hour period, the next phase is initiated involving measurement of the EIS spectrum again at the new open circuit potential. This sequence of 3 steps, polarization, relaxation and EIS measurement, is repeated up to 6 times.
Implementation in the Gamry Framework
Impedance measurements on barrier coatings are some of the most difficult to make because of the high impedances that are measured which require a potentiostat with the capability to measure very small currents. The Gamry Reference 600+ potentiostat is one of the most sensitive commercially available potentiostats which can measure currents in the femtoamp range (10-15 A) and below. This level of current equates to impedances of up to 1 Teraohm at low frequency. The potentiostat has an effective resolution of up to 22 bits after offsetting the DC current level and multiplying the resultant signal (Gain) up to 100 times.
The ACET technique can be performed without user interaction by use of the Gamry Sequence Wizard Three individual experimental steps are combined in conjunction with a loop function to build the sequence. The steps that to be used are potentiostatic EIS, potentiostatic and open circuit potential.
Figure 1: The Sequence for the ACET Measurement
The operation of the sequence is as follows: the open circuit potential is measured until a stable value is reached. Once this stable value is reached, the impedance spectrum of the initial condition of the coated sample is measured. Now the sequence enters the loop of measurements that will be repeated 6 times. The potentiostatic application of the stress voltage is the first step, followed by the measurement of the open circuit potential during the relaxation of the film and the final step is the remeasurement of the impedance spectrum.
The test cell was based on the Gamry Paint Test Cell (PTC1) with a coated metal coupon as the sample electrode and a Portholes mask attached to the surface to restrict the exposed area. An area of 10 cm2 was exposed. The reference electrode was a silver/silver chloride reference electrode and the counter electrode was a graphite rod. The whole sample and cell setup was placed inside a grounded Vistashield Faraday cage to shield the cell from external electrical influences. The test solution was a 3.5% solution of sodium chloride solution in deionized water as prescribed by the standard.
Figure 2. The Test Cell Setup – Gamry’s PTC1 cell kit.
Figure 3. The six potentiostatic scans overlaid on top of each other.
The results of the potentiostatic scans show that successive application of the cathodic potential has an effect on the coating. The cathodic potential chosen for these experiments was -4 V vs (Ag/AgCl) and was applied for 1200 secs. (20 mins.). Each subsequent application shows a negative current of an increasing magnitude indicating that the coating is breaking down, allowing more current to pass. Figure 4 shows the relaxation behavior of the coatings after each potential application.
Figure 4. Open Circuit Potential of 6 Experiments
At the end of each Open Circuit Measurement, the impedance spectrum of the coating was measured. The applied potential for each measurement was the final measured potential from the open circuit measurement. Figure 5 shows the overlaid impedance spectra for the 6 impedance measurements. A small decrease in the low frequency plateau region for the subsequent spectra shows that the cathodic application of the potential caused a degradation of the coating (as shown by the arrow).
Figure 5: Impedance Results for the 6 tests.
The Gamry Reference 600+ potentiostat combined with the Gamry Sequence Wizard were successfully used to implement the Accelerated Cyclic Electrochemical Technique.