are the Problems with Powder Coating over Galvanizing?
Hot dip galvanized surfaces have been acknowledged as difficult to powder
coat since the technology was first developed in the 1960’s. Industrial
Galvanizers commenced research in this area in 1986.
The three main problem areas associated with the powder coating of hot
dip galvanized steel products are:
1. Pin holing of the coating
Pinholing is caused by the formation of small gas bubbles in the polyester
coating during the stoving/curing cycle. These bubbles form small craters
on the surface and are unsightly. They also produce holidays in the coating
that reduce its long term durability, particularly in aggressive (marine)
main reason for pin holing appears to be that the discrete polyester resin
particles in contact with the galvanized steel surface do not fuse at
the same time as those on the surface of the polyester powder film, because
of the mass of the galvanized steel *, and the time taken for it to come
up to fusion temperature. Specially formulated resins with 'degassing'
agents have been developed to alleviate this problem by delaying the onset
of fusion of the powder. Pre-heating the work in a pre-heat oven prior
to powder application allows heavier hot dip galvanized sections to be
powdercoated and deal with the problem of pin holing when used in conjunction
with 'degassing' grades of polyester powder.
*: Hot dip galvanized items tend to be of heavier section thickness than
other steel items, typically sheet steel, that are powder coated. These
items thus take longer to reach oven temperature because of their greater
2. Poor adhesion
The final stage in the hot dip galvanizing process involves water quenching
of the work, frequently in a weak sodium dichromate solution. This process
cools the work so that it can be handled and passivates the surface of
the galvanized coating to prevent early oxidation of the surface.
presence of a passivating film on the surface of the galvanized coating
will interfere with the zinc phosphate or iron phosphate pretreatment,
and in many cases, render these pre-treatments ineffective.
is essential that hot dip galvanized items are not quenched* after galvanizing.
This ensures that the zinc surface is in a highly reactive state to accept
the pre-treatment applied in the powdercoating process.
*: It is equally important that the unquenched hot dip galvanized surface
is kept clean and dry prior to powder coating. If wet with rain or dew,
it will rapidly oxidise and again cause coating adhesion and quality problems.
3. Incomplete curing of the polyester resin
Polyester powders are thermosetting resins that cross-link to their final
organic form by being maintained at a temperature (typically 356oF), for
about 10 minutes. Curing ovens are designed to provide this time at temperature
With hot dip galvanized items, with their heavier section thickness, it
is necessary to ensure that sufficient stoving time is allowed to meet
the curing specifications. Pre-heating of the heavier work will assist
in accelerating the curing process in the curing oven.
Galvanizers' investigations into these problems, in conjunction with
major polyester powder suppliers, resulted in the cause of these problems
being better understood, and polyester powder chemistry and plant procedures
and design to be modified to allow a consistent, quality assured polyester
powder coated hot dip galvanized product to be supplied.
The problems associated with powder coating over hot dip galvanizing have
not changed and inquiries are regularly received asking advice following
the failure of powder coatings over hot dip galvanized products.
for Powder Coating over Hot Dip Galvanizing:
Coater's Manual 1/98
the entire Powder Coaters Manual
to Powder-Coater Home Page