i’m fairly new to anodizing niobium and only use the techique as called for with a project. I don’t do this every day as you likely do in your shop. Thus, my TSP and etching solutions may be ”older” than yours at times. I can always make up fresh TSP solution in small quantities and ensure I’m always using fresh solution, but the etchant is more problematic. The way it is packaged I have to make up the whole 1/2 gallon of the 2X concentrate all at one time. (I would make smaller lots if I could divide the dry powder accurately, better yet buy it in smaller units.) I then make up only 1 cup of the 1:1 dilution for a hot etch. After use I always decant it into a heavy polystyrene (?) lidded container for reuse later. I toss this diluted solution if I notice any color tint.
Sorry for the long intro ... Because my use is so limited at times, these soultions tend to ”age in place”. Are there any tell-tales to look out for as an early indicator that it is time to use fresher reagents, or explain what I’m seeing when things go differently than expected?
Thanks,
Rick
From Multi Etch LLC:
First, have you tried anodizing without Multi-Etching? Niobium, unless it's really dirty, doesn't usually need etching in order to achieve good color. Your electrolyte solution of TSP will last years. Remember to not mix it up too strong as burning or pitting can occur on your anodized metal. Although Multi-Etch is available only in one size, the mixed solution has a shelf life of at least 11 months. Tints in the solution indicate contamination: blue for copper, pink for iron/steel.
Over time the unused solution will pick up hydrogen and trend towards a lower pH, which actually makes it work better, so "age in place" has merit. When heated, Multi-Etch will lose water through evaporation which makes the solution a little more concentrated, so etch time might be shortened a bit or you can add a little water. You can use small scraps of metal to check the anodized color--in general, if the color's good, the etch is good, but this really applies to titanium as we don't typically etch niobium.
We etch titanium for 7 seconds in heated Multi-Etch but as the solution is used, that time can go up and down. When the etch gets full of suspended titanium, this will plate back onto the metal and cause splotchy color.