I am working on the corrosion of titanium alloys especially "Potentiodynamic test"- in Ringer's Solution.
My alloy composition is Ti-6Al-4V, and I want to determine the number of released ions -in the solution -individually for each of Ti,Al, and V.
The standard unit for the amount of substance is moles. So, I assume that you would like to estimate the moles of Ti, Al, and V dissolved for some known conditions. There are essentially three different approaches: (1) mass loss - gravimetrically determined change in mass is related to moles through the atomic masses assuming the mass fraction of each does not deviate from that of the alloy content. (2) Faraday's law - the current integrated over time is used through Faraday's law to determine the colomb of charge from which the moles of each species is estimated from the ions expected to form and the mole fraction of each in the alloy. This works for potentiostatic holds well away from the open circuit potential where one can assume the reduction reactions on the surface of the sample are negligible compared to the applied current. At open circuit conditions, one needs to estimate the steady state rate of reduction and oxidation by perturbing the system and measuring the response. There are a variety of techniques for producing perturbations and analyzing the results such as ac impedance (aka electrochemical impedance spectroscopy), potentiodynamic polarization, linear polarization resistance, electrochemical noise etc., and (3) Ionic concentration measurements (such as with the techniques mentioned above). The big advantage of this technique is that one does not need to assume a dissolution ratio of the different species in the alloy (typically assumed to follow the alloy composition n the other approaches), the disadvantage is that hydrolysis and precipitation of species can result in ionic concentrations in the solution that deviate from the amounts dissolved. For example, aluminic ions, Al(III), will reaction with water to form Al(OH)3 precipitate resulting in a solubility limit for this ion of less than 1 ppm in the pH range from about 4 to 9.
Probably the release and its distribution among the three metals will be very different if you apply potential or not, and for how long. If you have OCP (no applied potential), the release in Ringer's solution of TiAlV is probably very low, but could possibly be determined by GF-AAS or ICP-OES or ICP-MS. Chlorides in the electrolyte can interfere with the analysis, especially for ICP techniques. At the relatively neutral pH of Ringer's solution, you might also get precipitation of metal complexes and therefore be unable to detect all released metals. What you measure is generally Measured amount = Background contamination / matrix effects (interference of background) + released concentration - precipitated amount. To know the released amount, you need to run a reference sample (background without contact with TiAlV), and to test if you get precipitation (add known amounts of metal ions into Ringer's solution, expose it at similar conditions (time, termperature etc.), and measure then the amount. Prior to analysis, all solution samples should also be acidified to avoid adsorption on the walls or precipitation. If you are interested in the metal release at applied potential, you can take samples from the electrolyte after the polarization (but remember that the time of contact is important and should be the same for samples/conditions which you want to compare) and analyze them. You can also approximate the release from Faraday's law assuming that all increase in current is determined by electrochemical metal dissolution, and compare that with the measured release.
Thank you very much Prof. Richard E. Ricker and Dr. Yolanda Hedberg for your distinguish illustrations.
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