Crevice[1] (and pitting) corrosion[2] come out as the main issue[3] in your investigation. Could you change the substrate with copper, "same material" as the Cu-foil, e.g. try to form a "Cu-foil//Cu-substrate" electrode[4] ?

1. Crevice Corrosion https://www.corrosionpedia.com/definition/347/crevice-corrosion

2. Corrosion Resistance https://www.copper.org/applications/marine/cuni/properties/corrosion/

3. Effect of connection type on galvanic corrosion between lead and copper pipes https://www.awwa.org/publications/journal-awwa/table-of-contents/articleid/38771369/issueid/38771370.aspx?getfile=%5C%5Cpers75apppcr%5Cpersonify%5Cserverfiles%5Cdcdfiles%5C38771369%5Cjaw201310clarkb_pr.pdf

4. In case of minor changes, you might be interested, also, to introduce a "3rd (sacrificial protection) metal", that can be another Cu-foil-2 to suffer more from the corrosion, not your main Cu-foil(-1), between them.

Dear Prof Ioannis Samaras,

Thanks for your kind answer! However, I am not sure it is about crevice corrosion. Because I also tried using something like nail polish to seal the gap between copper and waterproof tape. Under this condition, failure occurs at the boundary of nail polish and copper.

Regards,

GAO Yang

Dear Dr Christian Lupó,

Thanks for you kind answer! I will try to add some stirring procedures in the etching process.

Regards,

GAO Yang

Thanks for your kind answer. However, avoid, please, the extraneous step of the "nail polish to seal...", because it outsets[1], exactly, a mechanically[2] assisted crevice[3] corrosion pre-step; in other words, it forms a topological (mechanical-electrical-electrochemical) catalyst, acting, altogether, precisely, on this boundary[2,3], just inside[3,4] the waterproof tape.

1. Different Types of Corrosion http://www.corrosionclinic.com/types_of_corrosion/stress_corrosion_cracking.htm

2. The Mechanical and Electrochemical Processes Associated with Taper Fretting Crevice Corrosion: A Review https://www.astm.org/DIGITAL_LIBRARY/STP/PAGES/STP12020S.htm

3. Mechanically assisted crevice corrosion of the head-neck taper in a large head metal-on-metal total hip arthroplasty http://www.arthroplastytoday.org/article/S2352-3441(15)00024-2/pdf

4. Electrochemical and mechanical behavior of implantable materials https://www.tandfonline.com/doi/full/10.1080/10667857.2016.1239673