How to keep etch times consistent with Multi-Etch
When is the Multi-Etch solution used up?
The limiting factor is that as you etch, molecules of titanium (or whatever metal you’re etching) collect in your bath. When you accumulate too much metal, it will start to plate back on to your workpiece and inhibit good color in titanium. And unfortunately, pH is not helpful in determining when the bath is spent. pH does gradually becomes more acidic as the etchant ages, whether or not it has been used.
So, how can you tell when the etchant is spent? The short answer is that the etchant is still working if you’re able to get good color. We have a rough guide that a gallon of regular-strength Multi-Etch will clean approximately 1,500 square inches of titanium when dipped for three seconds at 120—160°F. As the etchant gets used, you may find that longer etching times lengthen the lifespan of the etch bath.
If you are working on one-offs or something special it’s best to use fresh etchant. But if you need to etch with used etchant, first check for good color by etching and anodizing some titanium scrap of the same type/alloy as your workpiece.
If you work with gallons of etchant at a time and repeatedly anodize lots of similarly-sized parts, you want to spend the time to determine the quantity of parts you can regularly etch before the etchant is spent. This will require some “sacrificial” parts at least toward the end of your etchant bath lifespan.
Once you’ve determined how much you can etch before you no longer get good color, you might want to back off by 5% or so with all subsequent etching baths to make sure there’s never any bad anodizing. This is especially true for dental and medical implants, when protocol prohibits removing “bad” anodizing and redoing it.
Another thing you can do to keep your etch bath consistent is to make periodic cuts of the etch tank and replace the cut volume with fresh etch. As an example, were you to estimate an etch life of four weeks, instead of dropping the etch tank and making up a fresh batch every four weeks, it would be better to cut the etch tank by ¼ every week then make it up with fresh chemistry. Maintaining the etch this way, each week you would begin with roughly the same etch as any other week.