"Galvanized Yankees" was a term used to identify Confederate soldiers who
switched sides and served in Union regiments.....mostly, those who had been captured and imprisoned, who decided to denounce their allegiance to the Confederate cause and swear allegiance to the Union, and who then enlisted to serve the Union. The term may have also been used by Confederate soldiers to describe Union prisoners of war who switched sides and served in Confederate regiments.
Here's an example from 3d Battalion GA Sharpshooters: Robert F Wimpee Son of David & Emily Wimpee, Age 18 in 1860 Polk County GA census. According to the roster - "Enlisted 3/1/1862 in Co D Phillip's Legion, WIA at Fredericksburg 12/13/1862 (wounded in the head), Transferred to 3rd Ga. Sharpshooter Battalion May 1863, Captured at Gettysburg 7/3/1863, Sent to Fort Delaware, Deserted and joined Federal artillery unit in August 1863."
Robt. F Wimpey, buried in Winchester Indiana
http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=82221608 listed as a member of "Ahl's Del. H.A." Ahl's Independent Company, Heavy Artillery (officially known as the 1st Delaware Heavy Artillery.) The company was mainly composed of former Confederate prisoners of war who had sworn allegiance to the Union (over 200 galvanized Yankees) and was Delaware's only heavy artillery company during the war. It was organized at Fort Delaware July 27, 1863 soon after the Battle of Gettysburg. The company was assigned to garrison and guard duty at Fort Delaware for their entire period of service. Its commander was Capt. George W. Ahl, and nearly all the officers had come to the fort with Independent Battery G, Pittsburgh Heavy Artillery. The company mustered out on July 25, 1865.