Electro-surgical instrument and attachments for converting a mechanical surgical instrument into an electro-surgical instrument are disclosed for applying electrical current to tissue, such as a blood vessel, to seal or cauterize the tissue. A cover element is slidable over a distal end of a component of a mechanical surgical instrument, such as a forceps. Each cover element defines a single pocket or cavity and a single aperture or opening at one end thereof for receiving and surrounding the distal end of the component and an electrically conductive element attached to an outer surface of the cover element. RF energy can be applied to tissue through the electrically conductive elements. After the procedure, the cover elements can be slidably removed from the forceps member and are disposable.