Argentium next to Sterling

One of my smiths recently made himself a ring out of Argentium
silver, looks nice, no tarnishing or skin reactions as he has been
wearing it. Over the last couple of days he started wearing it on a
different finger, next to a sterling silver ring he has; now he is
getting some skin reaction, the famous black line. Anyone got any
theories as to why the sterling and argentium would react this way?

Thank you,
Eric McCafferty

Without looking up specific reactions, I would say, dissimilar
metals in contact will create an electrical current. The human body
provides a capacitance to increase the flow of current and corrosion
is the result. This type of corrosion is often called galvanic
corrosion because it is like a galvanic cell; I believe the power
source for the first metal planting. This can happen between any two
dissimilar metals or the alloys of the same parent metal. The
corrosion can be any type of surface reaction with a gas, liquid or
solid. Because of the uneven nature of the current flow, pitting can
result. If corrosion can be avoided in the contact by use of a noble
cover gas or a vacuum than galvanic welding can occur. Pure silver
and pure gold in contact under these conditions will diffuse into one
another to completion. This of course will take a long time.

Daniel Culver