application+guideline+for+hot+dipped+galvanized+steel+v1 (1)

7/28/2019 Application+Guideline+for+Hot+Dipped+Galvanized+Steel+V1 (1)

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Application Guide

Powder Coatings for Hot-Dipped Galvanized Steel

Powder Coatings on Hot Dipped Galvanized Steel

Application GuidePowder Coatings on Hot-Dipped Galvanized

Introduction

This application guideline is a recommendation on effective application of powder coatings on hot-

dipped galvanized substrate.

General

There are two significant problems associated with the use of polyester powders on hot dipgalvanized products. These are:

- Adhesion of the coating to the galvanized surface:

- pin holing of the cured coating.

The critical points that must be controlled are:

1. Adhesion2. Pinholing3. Chemical pre-treament4. CuringA review of each step follows.

1. Adhesion

When steel is hot dip galvanized, it is degreased in a hot caustic bath, pickled in hydrochloric acid

to remove oxides, fluxed in a zinc ammonium chloride solution and then galvanized by immersing

the work in molten zinc at 455C.

The zinc reacts with the steel at that temperature to form the galvanized coating, which is

typically 80-100 microns in thickness and consists of a series of crystalline zinc-iron alloy layers

comprising typically 80% of the coating thickness, coated with a layer of zinc,making up the

balance of the coating.

Depending on the metallurgy of the steel and the galvanizing bath conditions, the galvanized

coating is sometimes made up entirely of alloy layer with no free zinc layer. These coatings are

typically dull gray and tend to be thicker than standard galvanized coatings.

After galvanizing, the work is generally quenched in water containing a low percentage, typically

less than 0.25%, of sodium dichromate. The dichromate solution applies a passivation film to the

surface of the zinc that inhibits the zincs reaction with atmospheric moisture prior to the

formation of the basic carbonate films that give zinc its outstanding corrosion resistance in

atmospheric exposure conditions.


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Application Guide



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-treatmentsineffective. 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 pretreatment applied in

the powder coating process.

2. Pinholing

The phenomenon of pinholing in polyester powders applied over hot dip galvanized surfaces has

been identified as the most serious problem associated with coating integrity. 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.

Causes of pin holing:

a. Hot dip galvanized sections are generally heavier than those typically powder coated.The greater section mass delays uniform fusion of the coating, entrapping air in the

lower level of the coating.

b. The thicker phosphate films formed, on the highly reactive hot dip galvanized zincsurface, may release water of crystallization as curing temperatures are approached

that give rise to water vapor becoming entrapped in the paint film.

c. The high reflectivity and lower emissivity of the galvanized surface, coupled with thegenerally heavier section thickness of the steel used delays heat absorption by the

steel.

Pin holing can be controlled by:

- pre-heating the work prior to applying powder;- use of `degassing grades of polyester powder.

Elements of each of these factors may contribute to the pin holing phenomenon, and it should be

noted that the pre-heating of the work to higher than normal temperatures prior to powder

application would contribute beneficially to reducing the effects of each of these factors.

It is very difficult for any applied coating less than 100 microns in thickness, applied in a single

coat, to be free of holidays in the coating. While galvanized substrates supply a high level of

corrosion resistance in their own right, and will generate stable corrosion products that willusually seal any pinholes in the coating, the presence of chlorides will result in excessive leaching

of the oxides.

For this reason, single-coat polyester coatings are not recommended in marine environments,

and two-coat systems are now available that offer exceptional performance where high levels of

airborne chlorides are present. These systems consist of an epoxy based primer coat over thehot dip galvanized surface, followed by a polyester top-coat.


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Application Guide



3. Chemical Pre-treatment

Zinc phosphate conversion coatings have been the preferred pre-treatment over hot dip

galvanized surfaces producing a fine grained crystalline zinc phosphate at coating weights of 1-2

g/m2. More recently, chrome phosphate has been found to provide superior performance in high-

durability applications.

Control of zinc phosphate coating mass is important in achieving good adhesion. With batch pre-

treatment processes, extended residence time in the zinc phosphate may result in coating

weights exceeding 4g/m2. At this coating mass, adhesion problems can arise because of the

inherent weakness of the thicker phosphate film.

The use of alkaline degreasing operations on galvanized product prior to phosphating must becarefully controlled because of the risk of attack on the zinc by the alkali.

In continuous process operations, the residence time in the phosphate pre-treatment can be

controlled by the line speed, and better control of the phosphate coating is possible. The

temperature and concentration of the pre-treatment chemicals can also be modified to match line

speed requirements.

Iron phosphate is commonly used to pre-treat pre-galvanized products and this is usually applied

at iron phosphate concentrations of 0.3-1.0 g/m2. A number of pre-galvanized products,

particularly hollow sections, have a clear polymer coating applied over the galvanized coating by

the manufacturer, and this will influence the type of pre-treatment best suited for this type of

product.

Thermal (Dry-off) Pre-treatment

Following phosphating, it is normal procedure to pass the work through a dry-off facility to

remove residual moisture from the work prior to powder application. Trials showed that

conventional powder coating plant dry-off facilities operated at too low a temperature, and a dry

off oven capable delivering enough heat is required, with an optimum achieved work piece

temperature of 140C. At surface temperatures over 140C, the phosphate coating may break

down.

Heavier sections with more mass require higher pre-heat oven temperatures to obtain optimum

surface temperatures.

4. Curing

The introduction of hot dip galvanized products, which in general are fabricated from heavier

sections, requires considerable attention to the curing phase of the process to ensure that the

powder is fully cured. lt is the surface temperature of the steel that determines the curing of the

polyester powder, not the air temperature inside the oven.

While the coating may appear normal on exiting the oven, it will not have achieved its design

properties if the time-at-temperature requirements have not been met. Simple solvent testing

procedures will quickly determine whether the coating has been fully cured.

With curing ovens using radiant heaters for curing, care must be taken to ensure that effectivecuring is being obtained, especially on white or pale coloured coatings which have high reflectivity

and may delay heat transfer to the steel substrate.

The following specification/process is highly recommended:


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Application Guide



1. Hot dip galvanize without water quench containing sodium dichromate (nopassivation film).

2. Remove all drainage spikes and surface defects3. Keep the surface clean. Do not transport uncovered loads. Diesel fumes will

contaminate surface.

4. If surface contamination has occurred or is suspected, clean surface withproprietary solvent/detergent designed for pre-cleaning prior to powder

coating.

5. Use chrome (preferred) or zinc phosphate pretreatment if highest adhesionis required. Surface must be perfectly clean. Zinc phosphate has no

detergent action and will not remove oil or soil.

6. Use iron phosphate if standard performance is required. Iron phosphate hasa slight detergent action and will remove small amounts of surfacecontamination. Best used for pre-galvanized products

7. Pre-heat work prior to powder application and use degassing polyesterpowder.

8. Powder coat with within 12 hours of galvanizing. Do not get surfaces wet.Do not leave outside.

9. Use 'degassing' grade polyester powder only10. Check for correct curing by solvent testing. Adjust pre-heat and line speed

to ensure full cure.

Note: The information on this Application Guide is given to the best of the manufacturers knowledge, based on laboratory

testing and practical experience. Jotun Powder Coatings reserves the right, without notice, to alter or change the content

of this Application Guide. This Application Guide supersedes all previously issued versions.

Jotun Powder Coatings 2012. All rights reserved.

application+guideline+for+hot+dipped+galvanized+steel+v1 (1)

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