Feb 26 2002
P A / Shutterstock
Steel products can be given a high-grade architectural finish by using a polyester powder coating over hot-dip galvanized steel. This coating is considered as one of the most long-lasting protective coatings for steel.
What are Powder Coatings?
Polyester powders are essentially thermosetting resins that are electrostatically coated on the surface of steel and stoved at about 180 °C. This kind of technology creates highly uniform coatings that have an appealing architectural finish, with excellent atmospheric weathering properties.
The powder-coated product, integrated with hot-dip galvanized coatings, ensures maximum durability for steel components. Such components will stay rust-free for over 50 years in most architectural applications.
What are the Problems with Powder Coating over Galvanizing?
Hot-dip galvanized surfaces are not conducive to powder coating. This is a well-known fact because the technology was originally put together in the 1960s. The three leading issues associated with the powder coating of hot-dip galvanized steel products are discussed below.
During the stoving/curing cycle, minute gas bubbles are developed in the polyester coating, resulting in the pin-holing effect. These bubbles form tiny craters on the surface of the coating, and are unappealing.
Furthermore, they form perforations in the coating that reduce its long-term sturdiness, particularly in marine and other aggressive settings. It is said that the key reason for pin-holing is that the distinct polyester resin particles that make contact with the surface of the galvanized steel do not fuse concurrently with those on the surface of the polyester powder film.
This is credited to the galvanized steel mass*1 and the time required for it to reach the fusion temperature. With the aim of reducing this issue, exclusively formulated resins with “degassing” agents have been developed by postponing the onset of powder fusion. When the work is pre-heated in a pre-heat oven prior to the application of powder, heavier hot-dip galvanized sections are powder-coated and the problem of pin-holing is also solved when used along with “degassing” grades of polyester powder.
*1Note: Hot-dip galvanized items are inclined to have heavier section thickness than other steel items (typically sheet steel) coated with powder. Consequently, these items take a longer time to attain the oven temperature because of their greater mass.
In the final stage of the hot-dip galvanizing process, the work is quenched with water, typically in a weak solution of sodium dichromate. Consequently, the work is cooled, rendering it easy to handle, and enabling it to passivate the surface of the galvanized coating to stop premature oxidation of the surface.
The presence of a passivating film on the galvanized coating surface will obstruct the pre-treatment of zinc phosphate or iron phosphate and, in most cases, render these pre-treatments futile. Care should be taken that hot-dip galvanized items are not quenched*2 post galvanizing. This maintains the surface of zinc in a highly reactive state to receive the pre-treatment used in the powder-coating process.
*2Note: The unquenched hot-dip galvanized surface should also be maintained clean and dry before powder coating. If the surface is wet with rain or dew, it will rapidly oxidize and again lead to quality issues and coating adhesion.
As stated before, polyester powders are thermosetting resins, which when kept at a particular temperature (typically 180 °C) for about 10 minutes, will cross-link to their end organic structure.
This time-to-temperature combination is achieved by specially designed curing ovens. In the case of hot-dip galvanized items, with their heavier section thickness, sufficient stoving time should be given to meet the curing specifications. When the heavier work is pre-heated, it will help to accelerate the curing process in the curing oven.
Specification for Powder Coating over Hot-Dip Galvanizing
The following specifications are proposed:
- Only hot-dip galvanizing should be carried out, and the work should not be quenched with water or chromate.
- All surface imperfections and drainage spikes should be eliminated.
- Powder coating must be completed within 12 hours of galvanizing. Care must be taken to guarantee wet-free surfaces and they must not be left in the open.
- The surface must be clean. Uncovered loads should not be moved, if not the surface will get soiled by diesel fumes.
- If surface contamination is suspected or has happened, the surface must be cleaned with detergent or solvent exclusively made for pre-cleaning, prior to powder coating.
- Zinc phosphate pre-treatment must be used when the highest level of adhesion is required. The surface must be kept absolutely clean. Since zinc phosphate does not have any detergent effect, it will not get rid of any oil or soil.
- Iron phosphate must be used if regular performance is desirable. Since iron phosphate is said to have a mild detergent effect, it will eliminate small amounts of contamination from the surface. Iron phosphate is best used for pre-galvanized products.
- Only “degassing” grade polyester powder must be used.
- The work must be pre-heated prior to coating the powder.
- Solvent testing should be performed to check for correct curing. Pre-heat and line speed must be modified to guarantee a thorough cure.
Outstanding performance can be achieved through the precise application of polyester powder coatings over hot-dip galvanizing. In 1988, several architectural projects were conducted with this process and even today, these projects continue to be in exceptional condition.