I have read some papers that Ti is mostly used as a cathode substrate during electrodeposition because the deposits easily peel off. What make this easy peeling off on Ti substrates in comparison to Ni or SS substrates where we have very adherent coating?
I have to guess a bit, but I think the following is reasonable:
- Like Al, Ti oxidizes at the surface. Meaning you get a stable layer of TiO2 at the electrode's surface.
- A nice "feature" of TiO2 (other than Al2O3) is the fact that it is conductive. Rendering it suitable for electrodeposition.
- While metal-metal bonds are quite strong, with TiO2 you'll get an oxide-metal interface which I expect to be not too stable due to differing lattice constants.
Hence while allowing for electroplating, Ti(O2) would be great to get the deposited layer separated. Even if some of the TiO2 might be lost during separation, it will easily regenerate (aka "oxidize") unter normal ambient conditions.
Dear Thomas and Abdelkader Thank you for your inputs.
Dear Dreher, thank you for your inputs. However going by your inputs the same logic must be applicable for the stainless steel (SS) substrates where a thin layer of Cr2O3 is existing and it easily regenerates if damaged. The lattice parameters of rutile TiO2 is vaguely 4.5 angstroms (can change depending on structure) and that for Cr2O3 is 4.9 angstroms (a quick google search) which is a difference of almost 9%. In-fact during electrodeposition, I have observed that the coatings are very adherent on the SS substrate whereas they easily peel off for the Ti substrate. So I think lattice parameter may not be playing a decisive role here. Should I be wrong or incorrect, please do not hesitate to correct me. Thank you for your time.
The comparison with stainless steel is not bad. And "stainless steel" is a wide field - just starting with Chrome steels.
The "funny" thing: Cr2O3 is reported to be a "conductor" if the same kind as Al2O3. So electroplating either requires a breakdown of the Cr2O3 barrier or happens in the "gaps" between Cr2O3 locations. Either way you'll get some metal-metal bond. Eventually "disturbed" by Cr2O3 "islands", but obviously not too much.
Other stainless steels (e.g. with Nickel) implement differing anti-corrosion schemes - not exclusively relying on some oxide layer. In these cases, the adherence of deposited layer is even easier explained by metal-metal bonding.
An afterthought: rutile does not build-up a smooth layer. It's more like some "forest of needles" eventually preventing some too-tight bonding. Which might allow for even easier peel-off. Just an afterthought ...
Electroplating of titanium is commonly considered as an, almost, “impossible” process (in aqueous solutions), mainly, due to its large negative redox potential. The standard electrode potentials of titanium is much more negative than that of hydrogen, among other shortcoming factors (pH, Temperature, surface modification and morphology, etc.) results, intermittently, in reduction of (gaseous) hydrogen[1].
1. An Alternative Approach to Plating of Titanium... https://www.materialstoday.com/metal-finishing/features/an-alternative-approach-to-plating-of-titanium/
Dear Samaras, thank you for the comments. But I would like to point out that my question was pertaining to the electrodeposition of alloys (say Ni-Cu, Zn-Cu etc.) on Ti substrates. When we use Ti substrates the electrodeposited layer easily peels off but not necessarily when Stainless steel is used. Why ?
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