Galvanic Attack is another name for corrosion. Any two substances with different galvanic potentials with the presence of oxygen and moisture can corrode. Zinc and Brass (or steel and brass) have different galvanic potentials and their contact can result in corrosion.
Iput, Galvanic attack is just field name for a type of corrosion know as Galvanic corrosion. What happens is two electro-chemically dissimiliar materials are brought into contact through an electrolyte creating a galvanic cell. The metal ions from one of the metals is transferred to the other through the electrolyte,creating a weak electrical current and the appearance of corrosion on the metal giving up the ions.