How Do Organic Coatings Control Corrosion?
Organic coatings provide corrosion protection by isolating the corrosive environment from the steel structure to which they are applied. In general, a coating needs to have good barrier properties to provide good protection, ie. a low permeability to water, ions, dissolves gases, and other corrosives. Field experience and laboratory research have shown that highly protective coatings with good barrier properties have a high electrical resistance. A general consensus is that coatings need to have a resistance of at least 106 to 107 Ω • cm2 to provide corrosion protection.
What is Electrochemical Impedance Spectroscopy (EIS) and how is it used?
EIS is a well-established laboratory technique for evaluating the corrosion protection of organic coatings. The technique uses AC (Alternating Current) electricity to measure the impedance of a coating, where impedance is analogous to DC (Direct Current) resistance in DC circuitry. The AC technique is used in preference to DC measurements as additional information can be obtained about the capacitive properties of the coating, which influence performance.
As shown in Figure 1 below, the corrosion protection from a coating increases as its impedance increases. Therefore, a newly applied, high performance coating will have a high impedance in the range of Log Z=9 to 11.
When the coating is exposed to real or simulated service conditions, its impedance gradually decreases. The rate of decrease is a function of the coating formulation and the severity of the service conditions. Gradually the impedance will drop to the point where coating permeability to water is very high and corrosion protection is lost. This behavior has been suggested as a method to monitor and predict coating service life. EIS is a non-destructive, highly sensitive technique in which measurements can be obtained rapidly.
EIS measurements can also be made on coatings that are in service on industrial equipment. On the research side, EIS can also provide kinetic and mechanistic data on coating physical-chemical behavior through analysis of the various equivalent resistance and capacitive elements that contribute to the electrical behavior of the coating, particularly thin film coatings. EIS does not measure coating adhesion. Early detection of coating delamination and blistering can only be obtained for very thin film coatings (<2 mils film thickness).
Figure 1. Coating Impedance, Log Z (Z in Ω • cm2 @ 0.1 Hz)
How is Coating Impedance Measured?
The impedance of a coating is measured with an electrochemical cell consisting of the coated panel (test electrode), a reference electrode, and an inert counter electrode. A wide variety of cell designs are available. The electronic measuring equipment consists of a potentiostat, either a lock-in amplifier or frequency response analyzer, and a PC with specialty EIS software (Figure 2).
Figure 2. EIS Software
Coating impedance is measured as a function of the frequency (0.001 Hz to 100 kHz) of an applied AC voltage (10 to 500 mV rms). The resulting spectrum (Bode Plot) displays impedance as Log Z versus frequency as Log F, where “Z” is impedance in Ω • cm2 and “F” is frequency in Hertz. Information about coating protection and barrier properties is obtained at the low frequency end of the plot.
The coating impedance at 0.1 Hz is read from the Bode Plot. This value is used to compare impedance measurements of different coatings, or to examine the change in impedance as a function of time of one coating in a specific environment. Selection of Log Z at 0.1 Hz represents a compromise between speed of analysis and selection of a frequency at which the performance of different coatings can be reliably discriminated.
Example of How Coating Performance Can be Determined with EIS
In the laboratory, coated test panels are subjected to a simulated industrial process environment. Increasing the temperature above the service temperature accelerates the test. The impedance of the coating is measured at intervals during exposure to the test environment. The initial impedance and the decrease in impedance as a function of time are used to assess coating protection and deterioration. The way the impedance changes with time enables a better prediction of long-term performance of the coating.
An example is shown in Figure 3 in which high performance epoxies are exposed to 5% NaCl solution at 65°C. Coating A initially has very high impedance, typical of high performance coatings. However, its impedance drops consistently with time, suggesting that the impedance will soon drop below Log Z=6, at which point the coating no longer provides significant corrosion protection in demanding service. This coating is presumably taking up increasing amounts of water, likely a result of physico-chemical deterioration.
Coatings E and G show a similar trend. In contrast, Coating D initially has an impedance well below that of Coating A; however, the impedance of Coating D drops slightly during the first week of exposure, after which it stabilizes at Log Z=9.3 with little further change.
Coatings C and F similarly stabilize, although at lower impedances than Coating D. Coating B has the lowest impedance, but it appears to stabilize. Based solely on impedance data, the ranking of the coatings for anticipated life would be, starting with the best:
- Coatings C > Coatings D
- Coatings F > Coatings A and Coatings E
- Coatings G > Coating B
Figure 3. Coating Impedance as a function of time at 65°C