Application of chronocoulometry method for measuring Fick’s diffusion constant of thin coating layers

Abstract

An applied electrochemical method in the form of a chronocoulometry technique is proposed for measuring Fick’s diffusion constant of a thin protective coating layer over the conductive base materials. Using this method, the capacitance is monitored for a capacitor formed using the base material, an electrolyte such as aqueous NaCl, and an insulating coating layer in between. The capacitance increases when the distance between the base material and the electrolyte decreases in the diffusion of the electrolyte through the insulating coating layer. Therefore, the change in the capacitance provides a diffusion rate related to Fick’s diffusion constant. Conventional gravimetric methods require a long measurement time to reach saturation, and thin coating layers of minimal weight increment through water diffusion were demonstrated to be inadequate for the methods. However, the new electrochemical method is appropriate for thin coating layers and requires a limited duration after the onset of diffusion. Experimental for 1-mm thick PA6 films provided Fick’s diffusion constants of safer side with the new method of 15h by reflecting the heterogeneous properties of the surface, while the conventional gravimetric method in 360h provided smaller or danger-side values. 

Keywords:

• Fick’s diffusion constant
• thin coating
• electrochemical method
• gravimetric method
• chronocoulometry
• coating
• thin film
• corrosion

Acknowledgements

We would like to thank Editage (www.editage.jp) for English language editing.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by METI [International standardization of reliability and durability evaluation methods for metal-CFRP dissimilar material adhesive joints].