I received this email:
I would like to know when titanium and gold accessories are worn together, will there be any chemical reaction affecting both metals. I am wearing a Gold Chain now and I wish to purchase a Titanium Chain but I do not know whether there will be any adverse reaction. As I know Gold and Silver accessories can’t be worn together, so how about titanium and gold?
I hadn’t previously considered that gold and silver might react on the skin. But, yes they do. It isn’t much of a reaction, but it is to the detriment of the silver.
Titanium is safe to wear with other metals.
Titanium always has a protective oxide coating that prevents any electrolytic reaction with other metals. Basically, moisture cannot reach the metal itself in order to complete a reaction. If it is scratched, the protective coating immediately re-forms (unless in a perfectly inert atmosphere (argon, krypton, etc) or a vacuum). That’s part of why titanium is such a good candidate for medical implants.
Go ahead and try it: http://mrtitanium.com/TitaniumChains.html
In detail, any two metals in a conductive solution (impure water) will exchange ions. If the metals touch, then an electric current forms, eating away at one of the metals. That’s how a battery works. Body moisture acts as an electrolyte between silver and gold, and hydrogen is produced at the gold side, and oxygen on the silver. Silver oxide is black and soluble; it tarnishes and eventually eats into the silver.
But for the reaction to continue, neither metal can be allowed to grow a continuous, non-soluble insulating layer. Titanium and niobium grow very good insulating layers when just exposed to most electrolytes. In fact, I force this insulating layer with enough voltage to produce the colors. That’s called “anodizing.”
How the two metals react is the interesting part. It depends on the relative electronegativity of the two metals or alloys.
When two metals (or alloys) with differing electronegativity are in electrical contact within solution (or sweat) the metal with the lower electronegativity will oxidize. In the case of Gold (2.54) touching Silver (1.93), the silver will oxidize (tarnish).
In the case of a Silver (1.93) spoon sitting in an Aluminum (1.61) pan in water with a bit of lemon juice and/or vinegar and a pinch of salt, the aluminum (being 0.32 lower) will tarnish and the silver will actually un-tarnish! And if you are careful to make sure the only electrical contact between the spoon and pan is through a volt meter, you can even measure the 0.32 volts. Although not perfect (as it still needs to be buffed) this is a funky way to polish silver.
I bring all this up because Titanium with an relatively low electronegativity of 1.54, when in contact with Gold, Silver, or even Aluminum it is the Titanium that would oxidize. And since it it this same oxide coating that protects the Titanium, this is a win-win situation.
To belabor the point, Magnesium (1.31) with Titanium (1.54) would have the reverse effect. Where the Magnesium would oxidize and the Titanium would actually un-oxidize and you could loose or change the coloring effect.
Titanium dioxide is an insulator, so no current can start to strip the oxygen off of it unless the relative electronegative voltage is greater than the breakdown potential. That is, if the titanium is anodized blue, the relative electronegativity would have to be at least 30 volts. No two elements are more than a couple of volts apart. (Carbon=2.55, Lithium=0.98, max voltage = 1.57)
Electronegativity Table of the Elements