Dear Dr. Sina Sedaghatnezhad,

in a study regarding the evaluation of the protection performance of three different organic coatings (Nylon and PVF based coating, obtained from different processes, applied after a proper surface treatment to steel components for the automotive sector, I used the electrode impedance spectroscopy (EIS) on sample with intact coating.

In order to make the evaluation as quicker as possible and easy to use, I suggested to analyse the Bode’s graphic representation where log Z is reported as a function of the frequence (log ω), we can consider the maximum asymptotic value of Z, reached through the low frequence values, as an index of the protective strength of a coating.

A conventional three electrodes type cell was used.

The corrosion solution was a 5% NaCl aqueous solution obtained with demineralized water and GPR purity grade laboratory reagent) leading to a pH = 4 with diluted HCl.

The main factors taken into account are hereafter reported:

1. Selection of the best cell geometry (area to be tested, position and shape of the electrode, external interferences, etc);

2. Selection of the measuring parameters (signal width, frequence range, etc);

3. Selection of the frequence used to carry out the measures and the amount of samples to be tested.

The impedance measures have been carried out on the samples which were immersed for a lapse of time of 2-240 hours (on the same samples in the solution because the test is not desctructive), in order to follow the development of the system over time.

Before and after the impedance measures the micrographs were performed on some transversal sections to obtain pieces of information about the thickness, the homogeneity and the adhesion of the organic coating, using a metallographic microscope (Leica – mod. DM/RME) combined with an image scanning system.

In conclusion, the EIS technique has shown to be a fast and proper method in order to evaluate the protection strength of the organic coatings, which can be used not only in the basic research field, but also in application contexts such as the problems concerning the production sector.

Best regards,

Pierluigi Traverso

Dear Pierluigi Traverso,

First of all I am so appreciated for your answer and sharing your experience with me.

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It is awesome that you did the tests in less than 10 days.

According to literature which I found, the time period for epoxy binder sample. was much longer.

Do you remember the dry film thickness (DFT) of your samples?

Dear Dr. Sina Sedaghatnezhad, I remember that, considering the different types of samples and type of process, the average coating thickness ranged from about 20 to 140 μm. I have often used EIS also for much larger thicknesses and very high protective coatings (eg perfluorinated coatings in the nautical sector). Sometimes, to speed up the test, a calibrated micro-defect can be made using a suitable microdrill with a stable support and then, before and after the measure, checking / documenting the defect by optical microscopy. Best regards, Pierluigi Traverso

Dear Pierluigi Traverso , Thanks one more time for your hints and help.

BR

Sina