Dr. Bernhard Wessling
Ormecon Chemie GmbH & Co. KG, Ammersbek
(a subsidiary of Zipperling Kessler & Co.)
The technique is used by us to predict the performance of the complete primer / top coat (or eventually including an intercoat) system. We developed this technique as a routine method together with G. Popkirov based on his new FFT-EIS technique[41]. Measurements are carried out with a three-electrode system using a cell as described in[42]. The experimental set-up for this Fast Fourier Transformation Electrochemical Impedance Spectrometer is given in Fig. 9. A frequency-rich perturbation signal with a small amplitude is applied to the electrochemical cell controlled by an EG&G Princeton Research potentiostat Model 263A. In the present investigation a computer programmed sum of 42 sine waves distributed over 4 decades was used to synthesize the perturbation signal of a home-built signal generator The peak-to-peak amplitude of the perturbation voltage was usually 150 mV. The perturbation and the response signal are amplified and filtered by a Stanford Research Systems Inc. Model SR 640 Dual Channel Low-Pass filter. A/D and D/A conversion, timing and controlling were carried out by a 16-bit, 100 kHz transient recorder PC-card from United Electronic Industries, Inc. Model Win- 30/3016. Impedance spectra were evaluated by fast Fourier transformation of the perturbation and the response signal (Fig. 10).
For
the experiments describing our procedure and representative examples,
the following lacquer systems were used on CORRPASSIVTM
primer:
The tests were made on sand blasted panels from Mercedes Benz and are from the same serie with those evaluated in 4.5 (cf. [30]).
Fig. 11a-c show experimental EIS data for the three different top coated steel panels as a function of immersion time. The measurements were carried out in 10% NaCl. Changes in the impedance spectra were observed over a six day period except for the 1-C acrylic top coat with a one day period.
The recording of the spectra takes about 5 - 20 seconds, the Nyquist and Bode plots are available for interpretation after another 15 seconds. The bottleneck for measuring more samples is only the number of available immersion cells, as they are placed and immersed in the same cell as they are measured. But as we are not measuring all of the systems under development,but only those which have passed the first screening, this is not a practically important limitation.
The performance of the complete system under evaluation can be assessed after 1 or 2 weeks at most. If their impedance spectra do not change significantly during this time (as shown by sample 1, which is CORRPASSIVTM 4900), we can expect a very good long-term corrosion protection performance.
Systems, which will fail, are loosing their capacitance during 1 week or even quicker. It can be seen, that the stability of the EIS spectra or loss of the pure capacitance behaviour is parallel to the results found in 4.5, a good or bad performance in VDA cycling test.
It should again be noted, that systems (samples #5 and #6) composed by a pure epoxy primer or a Zn-rich epoxy primer top coated with the same EP top coat as in CORRPASSIV™ /2-C EP (sample #1) again perfom worse (i.e., they loose impedance very quickly, Fig. 12).
Intermediate conclusion
Scratch test, OCP, RKS and EIS are probing each different aspects of the integral named „corrosion protection performance“. Ennobling and passivation, primer adhesion (delamination/underrusting) or coating barrier properties, being tested somewhat in a separated manner, belong and work together. Both underrusting (delamination) and barrier property (impedance) are not only dependant on the primer or the topcoat, but the combination of both.
