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U. S. Steel Galvannealed Sheet is carbon steel sheet coated with
zinc on both sides by the continuous hot-dip
process described in Production
Methods. Immediately as the strip exits the coating bath, the
molten zinc coating is subjected to an in-line heat treatment that
converts the entire coating to a zinc-iron alloy. (The iron
diffuses from the steel into the coating.) The galvannealed surface
has a non-spangled matte finish, a finish very different than the
highly metallic appearance of the galvanized coating. At times, the
galvannealed coating may appear similar to uncoated cold rolled
steel.
 One of the primary attributes of the
galvannealed coating is that the surface accepts paint very
readily. It can be painted without the application of a
pretreatment, although a pretreatment will enhance the performance
after painting.
Other attributes of the galvannealed coating versus a galvanized
coating include: (1) the zinc-iron alloy coating can be welded more
easily than galvanized, and (2) the coating is harder than a
galvanized coating and is thus more resistant to scratching and
manufacturing damage.
The zinc-iron alloy coating on galvannealed product behaves very
differently than the soft galvanized coating. During fabrication,
there is almost always some degree of powdering of the coating. Heavier coatings will exhibit more powdering. For this reason,
typical galvannealed coatings are thinner than most galvanized
coatings. Since galvannealed product is intended to be painted,
a thinner coating can be successfully applied to provide long
product life. Additionally, the coating is less reactive when
exposed to the atmosphere, and as a result, the dissolution that
occurs during corrosion occurs more slowly than for a galvanized
coating. Thus, the coating does not have to be as thick as a
galvanized coating to provide the same level of protection. The
synergy between the lower overall corrosion rate of the
galvannealed coating and the paint can provide a long life
product.
 Galvannealed steel sheet can be readily postpainted, i.e.,
painted after fabrication of a part. It is not intended to be
prepainted, i.e., painted in coil form and then formed into a
shape. The brittle nature of the coating makes forming after
painting a difficult task to avoid degradation of the corrosion
performance in subsequent service.
As stated
previously, galvannealed sheet is specifically designed to be used
in the painted condition. The presence of iron aids to reduce the
rate of corrosion of the coating. However, because the coating
contains iron, typically from 8 to 11%, it tends to exhibit a
slight reddish-orange appearance when exposed to moisture in the
unpainted condition. After painting, only very slight staining is
found at sheared edges and other paint-film discontinuities. As a
result, the product performs excellently, and is used for such
aesthetically demanding applications as automobile exposed-body
panels.
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