Zinc has been used in marine boilers for many years, but it was not
until the publication in 1880 of the report of the Admiralty committee
that the use of zinc became general. It has been used in various ways:
1.—Virgin spelter, as imported in oblong slabs of various sizes.
2.—Cast, or remelted zinc. 3.—Cast zinc buttons, generally made from
virgin spelter or new clean zinc trimmings. 4.—Zinc spheres. 5.—Rolled
zinc blocks, generally 12 inches by 6 inches, and thicknesses varying
from 1/4 inch to 1-1/2 inch, generally with a 13/16-inch hole in the
centre.
It is desirable that close-grained zinc of uniform structure and free
from impurities should be used, and rolled zinc appears to meet this
want. The wear is entirely confined to the surface. It does not appear
to become distorted or broken up. On the contrary, it gradually wastes
away till only a slight shred, a sort of skeleton frame work, remains
to indicate what it has been.
The primary object in the use of zinc in boilers is the prevention
of corrosion, but it has also some effect in reducing the amount of
incrustation, and rendering it softer and less adherent.