I want to measure the corrosion rate of different TixNb alloys (x=wt%) from Ti20Nb to Ti50Nb obtained by laser cladding. I don`t know which valences for Ti and Nb can be used.
Interesting, there are actually three assumptions that are required: (1) change in charge for the Ti product, (2) change in charge for the Nb product, and (3) the stoichiometry of dissolution. It sounds like you intend to use the atomic ratios of the alloys for #3. Looking at the Electrochemical Equilibrium Diagrams (Pourbaix, 1974) one sees that Ti will dissolve in low pH solutions with the primary product at the metal surface being Ti(+2) and passivates on the metal surface to form the suboxide of TiO at higher pHs. Niobium on the other hand, should not dissolve in any pH unless the solution contains a chelating agent (such as F-). However, it should also form a suboxide, NbO, first on the surface of the metal. So, you should probably use +2 for both. Notice that I used the phase, or ion, that can be at equilibrium with the metal surface for these estimates. This is because this phase is that which is in contact with the solution, and attempting to reach equilibrium with the solution by dissolving. Of course, during dissolution you will be quite far from equilibrium so equilibrium thermodynamics are only an indicator of behavior. You should state your assumptions clearly in any report including the stoichiometry assumption because Ti is considerably more active than Nb, and will form soluble ions in low pH solutions, while Nb will not. That is, Ti will be dissolved preferentially, and the surface will tend to become enriched with Nb and insoluble NbO. The tendency of the surface for retention of Nb and the formation of NbO will interfere with mass loss measurements.
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