Typically silver tarnish is silver sulfide, not what you stated which has me confused. Where did you learn about silicon being found in silver tarnish?
The dip can dissolve the silver sulfide from the surface, which will then be fresh silver and very susceptible to fast re-toning if not protected. As it stands, the native silver oxides and sulfide on the surface have passivated it, protecting the coin's surface to an extent. You take that away and more silver will react with the atmosphere. Repeating this process further removes silver from the coin's surface.