Powder
Coating over Galvanizing |
Introduction
One of the most
durable protective coatings for steel is provided by applying a
polyester powder coating over hot dip galvanized steel to provide
a high grade architectural finish to steel items.
What are
Powder Coatings?
Polyester powders
are thermosetting resins that are applied electrostatically to the
steel surface and stoved at temperatures around 180 o C. This technology
produces very uniform coatings that have an attractive architectural
finish with excellent atmospheric weathering characteristics. In
combination with hot dip galvanized coatings, the powder coated
product ensures maximum durability for steel components, which will
generally provide 50 year+ rust free life spans in most architectural
applications.
What 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 which culminated in the
construction of a purpose built powder coating plant at its Hexham
(NSW) site in 1988 in conjunction with its galvanizing operations
there. The three main problem areas associated with the powder coating
of hot dip galvanized steel products are:
- Pin holing of the coating
- Poor adhesion
- Incomplete curing of the polyester resin.
Industrial 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 from throughout Australia asking advice following
the failure of powder coatings over hot dip galvanized products.
1. Pinholing: 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) environments. The 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 * 1 , 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.
* 1 Note :
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 mass.
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. The 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. It is essential that hot
dip galvanized items are not quenched * 2 after
galvanizing. This ensures that the zinc surface is in a highly reactive
state to accept the pretreatment applied in the powdercoating process.
* 2 Note :
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:
Polyester powders are thermosetting resins that cross-link
to their final organic form by being maintained at a temperature
(typically 180 o C), for about 10 minutes. Curing ovens are designed
to provide this time at temperature combination. 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.
SPECIFICATION FOR POWDER
COATING OVER HOT DIP GALVANIZING
The following specification
is recommended:
- Hot dip galvanize and do not water or chromate quench
- Remove all drainage spikes and surface defects
- Powdercoat within 12 hours of galvanizing. Do not get surfaces
wet. Do not leave outside
- Keep the surface clean. Do not transport uncovered loads. Diesel
fumes will contaminate surface
- If surface contamination has occurred or is suspected, clean
surface with proprietary solvent/detergent designed for pre-cleaning
prior to powdercoating
- Use zinc phosphate pretreatment if highest adhesion is required.
Surface must be perfectly clean. Zinc phosphate has no detergent
action and will not remove oil or soil.
- Use iron phosphate if standard performance is required. Iron
phosphate has a slight detergent action and will remove small
amounts of surface contamination. Best used for pre-galvanized
products
- Pre-heat work prior to powder application
- Use 'degassing' grade polyester powder only
- Check for correct curing by solvent testing. Adjust pre-heat
and line speed to ensure full cure.
SUMMARY
Properly applied polyester
powdercoating over hot dip galvanizing will give exceptional performance.
Many architectural projects done with this process in 1988 remain
in excellent conditions today.
One of the first 'heavy' powder coating projects undertaken
in Australia - pedestrian bridge rail on the Janet Street
overpass on the Wallsend-Sandgate expressway link,
hot dip galvanized and polyester powder coated in 1989
still in excellent condition in 1997.
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