Powder Coating over Galvanizing           Back to Powder-Coater Home Page




abrasive blasting
adhesion problems
caps and plugs
chemical cleaning
cure ovens
fluidized bed
hi-temp tapes
infrared oven

powder formulations


Paint Arrestors
powder selection
powder vs. liquid
spray booth
spray gun
types of powder


Specs for powder coating over hot dip galvanizing


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.

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) 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 *, 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 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* 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 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.

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 asking advice following the failure of powder coatings over hot dip galvanized products.

Specification for Powder Coating over Hot Dip Galvanizing:

More information

Web www.powder-coater.com

Powder Coater's Manual 1/98

Print the entire Powder Coaters Manual

Back to Powder-Coater Home Page

April 21, 2010