When iron and steel are subject to conditions of
aqueous corrosion the incidence and rate at which
the corrosion takes place will alter if the steel is
coupled with other metals or alloys that are also
exposed to the electrolyte. Copper, brass, bronze,
lead and nickel are more "noble" and act as
auxiliary cathodes to the steel and accelerate its
anodic dissolution, that is, its corrosion.
Magnesium, zinc and zinc-base alloy are nearly
always less noble and tend to divert the attack
from the steel to themselves. The galvanic
relationship of various metals is an important factor
affecting corrosion.