In metalurgical terms and the source of the term was to coat one type of metal with another coat of metal. This did not change the structural part of the under coating , only the surface.

So in other words Galvanizing a Southerner was to put a coat of Union Blue on him, knowing he was truly at heart a Southerner.

The same for Galvanizing Yankees, it didn't matter how you galvanized a Yankee, he would always be a Yankee.

Why a Galvanized Yankee was either Reb or Yankee? I can't answer that question.