INTRODUCTION
Unlike many manufacturing processes, the hot dip
galvanizing of fabricated steelwork involves many
variables that can impact on the appearance and
characteristics of the finished product.
Hot dip galvanizing is primarily an industrial protective
coating process designed to provide a tough and
durable protective coating for steel that will generally
outlast the design life of the element to which
it is applied.
As the benefits of hot dip galvanizing as a long-life,
totally reliable coating have been recognised, there
has been an increasing demand for ‘architectural’
standards for hot dip galvanized coatings which
can satisfy the aesthetic requirements of this segment
of the specification.
This special CORROSION MANAGEMENT section
illustrates many of the defects and variations that
can occur with hot dip galvanized coatings. Given
an understanding of the causes and effects of these
phenomena, it is possible for both specifiers and
operators to optimise the quality and appearance
of a hot dip galvanizing project.
FACTORS AFFECTING HOT
DIP GALVANIZED COATINGS
The factors affecting hot dip the appearance and
characteristics of hot dip galvanized coatings include:
1. The size and shape of the item.
2. The steel chemistry.
3. The steel surface condition.
4. The design of the item with respect to galvanizing.
5. The metallurgy of the galvanizing process.
The hot dip galvanizing process involves immersing
steel items in molten zinc at 450oC after
pre-treatment to remove organic materials, rust
and mill scale. This hot dipping process gives the
galvanized coating its unique characteristics.
SIZE AND SHAPE OF THE ITEM
Zinc freezes at 420oC, so there is very
little superheat in the molten zinc as the item
is withdrawn from the bath of molten zinc. Thicker
sections will retain their heat longer and promote
better drainage of the molten zinc from their surfaces
than thinner sections.
THE STEEL CHEMISTRY
The rate at which molten zinc reacts with steel
to form the galvanized coating is dependent on the
steel chemistry. Pure iron has a very low reaction
rate and for this reason, galvanizing kettles are
manufactured from steel of this type.
Structural grades of steel always contains alloying
elements, the most common of which are carbon, manganese
and silicon. Sulfur and phosphorous are residual
elements arising from the raw material used to make
the steel, although some special steels have these
elements added deliberately.
is an issue where the highest standards of surface
appearance are required.
Of these elements, silicon and phosphorous have
the most significant effect on the galvanized coating's
characteristics, with silicon being the most common
steel alloying additive affecting coating appearance.
