Fairly high resistances measured. Perhaps external effects from electromagnetic radiation are the reason in combination with too high an impedance in the reference electrode circuit. It is best to check the impedance of the reference electrode first and to shield the measurement components.

You can check on ocp first.....if its stable for longer time then the we can consider certain number of reactions are at constant rate ..then carry out eis ...depends upon the nature of coating and state their capacitive and resistive behaviour...hence repeatation is way to confirm...if same thing happens to other samples to then need to explore reaction chemistry...and mechanism.

the scattering at higher frequencies could come from net frequency and its overtones the scattering at low frequencies could come from too high impedances for the potentiostat to resolve or also from a not steady state of the sample itself. you can also try to enhance the amplitude of exitation. What amplitude do you use? You can also do a voltage/current resonse analysis with gamry (i think gamry does this automatically when switching to the next frequency) this so called lissajous plots give you an indication if there are overtones in current response or if the current response is not linear. I recommend plotting coating EIS spectra in Bode plot due to the high impedances. If you get a second time constant (onset of corrosion) you will not see it in Nyquist because impedance magnitude is much smaller of the second time constant. Another advantage is that you have frequency information in the Bode plot. What was the lowest frequency applied in these plots?

probably, you have a problem with the resistance of the electrolyte, and on the other hand you have to check the conductivity of the working electrode because it seems that it has a very high electron transfer resistance.