Hello
I monitored OCP on a bare alloy before and after Ar-purging (> 1500 s) the non-aggressive buffer solution as shown in the figure. The specimen was prior cathodically polarized in the same specimen. It can be seen that Ar-purging did not affect the OCP, which rather seems unusual. Does it meanthe specimen has superior corrosion resistance, which is not susceptible to the content of dissolved oxygen? Or could be the Ar flow rate was not sufficient enough to make the difference, though I suppose there was enough bubbling throughout the cell? Also I would be grateful to know the reason for rise in electrode potential initially before reaching the state?
OCP is not predictive of anything except position in the galvanic series (and even then, it must be in the same electrolyte as the other materials being compared to). The OCP is a mixed potential that is determined by a whole mess of half reactions (though usually dominated by a couple), and unless oxygen is one of the dominant reactions, its presence or absence should not be expected to affect the OCP. Even if it is dominant when present, if another reaction replaces it that has a similar E0 (or that the combined effect is similar) then the OCP may not move in that case either. If you want to know anything about the actual corrosion performance you need to do additional testing like LPR, Tafel, EIS, Potentiodynamic, and the old standby mass loss tests.
With regard to the rise: that's some form of settling into steady state behavior.
Your OCP vs time plot (P-t) results from the corrosion (aging, of your alloy) induced variation of the surface coverage. Ti and Ti-alloys[1] show similar plot (P-t), and the content of dissolved oxygen[2] is not, always, the main factor for the corrosion.
If you are interested to :
1) replace, temporarily, your buffer solution by distilled water, then you might see the effect of Ar-purging in the monitored OCP; also, if you are interested to
2) blade scratch, briefly, your electrode[3], then you might see your (same) OCP vs time plot, but in shorter scale of time (as an accelerated aging test).
1. Effect of Fluoride Ions on Ti6Al4V Alloy Passivation in Lactated Ringer’s Serum http://www.scielo.br/pdf/mr/v6n2/16027.pdf
2. Corrosion resistance of Ti and Ti–Pd alloy in ... H2O2 addition http://www.sciencedirect.com/science/article/pii/S1003632613625418
3. Initiates a partial un-coverage of the (corroded alloy surface) electrode. Also, some "big sparks" in P-t might appear due to a sudden (pinhole) un-coverage of the corroded electrode, mainly, at very long time (t >>3 hours).
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