INTRODUCTION
Industrial Galvanizers Australian Galvanizing Division has introduced a range of custom coating packages to better serve the needs of specific projects.
Traditionally, when hot dip galvanized coatings are specified, AS/NZS 4680:2006 Hot-dip galvanized (zinc) coatings on fabricated ferrous articles is referenced. This standard defines minimum requirements for coating mass (thickness) and appearance.
This standard was developed on the basis of hot dip galvanizing being an industrial coating process intended to improve the durability of fabricated steel products, and recognizing that the coating is a result of a metallurgical reaction between steel and molten zinc.
The galvanizing process has defined these minimum thickness standards, based on a range of steel section thicknesses. Unlike applied coatings such as paint, where specific or additional thickness can be applied if required, the process rather than the supplier defines the thickness of hot dip galvanized coatings.
The surface finish is also process controlled to a large degree. A steel fabrication is submerged in a molten zinc bath and then withdrawn at a rate and in an orientation designed to allow the molten zinc to drain off the steel as uniformly as possible.
Variations in the steel’s surface condition, the steel’s chemistry and the design of the fabrication will determine the appearance of the galvanized coating.
The development of the Industrial Galvanizers’ Custom Coating Packages has been in response to requests for hot dip galvanized coatings that exceed the basic requirements of AS/NZS 4680:2006.
GALVANIZED COATING CHARACTERISTICS
Unlike other zinc-based protective coatings, hot-dip coatings predominantly consist of zinc-iron alloys that are crystalline in appearance. The surface of the coating is covered by a layer of zinc that freezes on the surface as the steel is withdrawn from the bath.
The thickness of the zinc alloy layers determines the final thickness of the galvanized coating. The alloy layer thickness is determined by a number of interrelated factors. These are:
• The steel’s sections thickness.
• The steel chemistry
• The galvanizing bath temperature
• The time that the steel is immersed in the molten zinc
• The alloy additions to the zinc bath.
Other processes associated with hot-dip galvanizing operations can affect the appearance of the final product and its suitability for additional processing – e.g. painting or powder coating.
It is possible for the galvanizer it take these issues into account when processing specific items, to produce a galvanized coating that may be thicker, shinier or duller than a standard coating.
DURABILITY OF GALVANIZED COATINGS
The anti-corrosion performance of zinc-based coatings is largely a function of the thickness of the coating in any given environment. The rate at which zinc corrodes is approximately linear so by doubling the coating thickness, the coating life will also be doubled.
The following table has been reproduced from AS/NZS 4680:199 and indicates the minimum spot and average galvanized coating thicknesses required for various steel sections to comply with the standard
TABLE 1
REQUIREMENTS FOR COATING THICKNESS AND MASS FOR ARTICLES THAT ARE NOT CENTRIFUGED
Article
thickness mm |
Local coating
thickness minimum µm |
Average coating thickness minimum µm |
Average coating mass minimum g/m2 |
51.5 |
35 |
45 |
320 |
>1.5 <3 |
45 |
55 |
390 |
>3 <6 |
55 |
70 |
500 |
>6 |
70 |
85 |
600 |
NOTE: 1 g/m2 coating mass = 0.14 pm coating thickness.
In practice, hot dip galvanized coatings will almost always exceed the minimum specified thickness, particularly on hot-rolled structural sections.
CUSTOM COATING PACKAGES
There are five custom coating packages being offered by the Industrial Galvanizers. Not all plants offer all options, as plant size and material handling characteristics will determine the individual plant’s capabilities.
1. INGAL HDG – Australian Standard AS/NZS 4680 compliant coating
2. INGAL HDG Plus – Over-standard coating. 2X over for hot rolled sections and 1.5X over for cold rolled sections. Provides extra-long life in heavy-duty applications.
3. INGAL SSQ – Australian Standard AS/NZS 4680-compliant coating. Special surface quality to meet specific customer requirements for overcoating or appearance.
4. INGAL HDG Matt - Australian Standard AS/NZS 4680 compliant coating. Treated to provide a non-shiny galvanized finish for environmental aesthetics.
5. INGAL HDG Colorcote - Australian Standard AS/NZS 4680 - compliant coating. Top coated with a paint system to client specifications, for architectural or high durability applications.
Each of these Custom Coating Packages can be supplied with a performance guarantee on the coating, which is typically in the range of 10-25 years, depending on the exposure conditions. This guarantee provides the client with an assurance that the hot dip galvanized coating will protect the steel from corrosion for the term of the guarantee, and any liability for maintenance during the guarantee period lies with Industrial Galvanizers.
Because of the logistics involved in providing these packages, Industrial Galvanizers can only make them available on specific projects and not on general galvanized products.
More details on each of these Custom Coating Packages are as follows:
INGAL HDG
This specification complies with current Australian and international (ISO) hot dip galvanizing standards for items galvanized after fabrication. Negotiated long-term coating performance guarantees are available with INGAL HDG specifications.
INGAL HDG Plus
INGAL HDG Plus exceeds Australian and International (ISO) standard minimum coating thickness requirements by a minimum of 50% for cold-formed sections and 100% for hot rolled sections. This specification guarantees a minimum average coating mass on cold-formed sections of 900 g/m2 (130 microns thickness) and 1200 g/m2 (170 microns thickness) on hot rolled sections.
The differing steel characteristics produced by the steel section’s manufacturing processes account for the different minimum extra coating mass.
Galvanized coating life is proportional to its thickness, and the additional thickness provided by INGAL HDG Plus significantly increases coating life proportionately.
INGAL HDG SSQ
INGAL SSQ (Special Surface Quality) offers specifiers the option of nominating surface quality requirements above and beyond the obligations listed in AS/NZS 4680. Depending on the specifiers requirements, details such as steel chemistry, design of the product and handling of the products through the galvanizing process are negotiated to ensure a managed outcome. Where items are to be painted after galvanizing, the INGAL HDG SSQ process insures a smoother surface better suited to the application of architectural paint finishes.
INGAL HDG SSQ has been specified on a number of major projects, including the award-winning Olympic Tennis Centre at Homebush Bay in Sydney.
INGAL HDG Matt
INGAL HDG Matt has been developed to provide an environmentally acceptable coating where shiny traditional galvanized coatings are not appropriate. INGAL HDG Matt has been specified in the telecommunications and power distribution industries as a custom treatment on large poles used in these industries. The INGAL HDG Matt process produces a non-reflective galvanized coating that blends well with the natural environment and reduced the visual impact of products such as pole structures.
INGAL HDG Colorcote
INGAL HDG Colorcote offers a one-stop duplex coating service to specifiers requiring painted galvanized structures for heavy-duty or architectural projects. Industrial Galvanizers’ has many years’ experience in specifying and applying paint coatings (including powder coatings) over hot dip galvanized surfaces. Comprehensive test programs have been undertaken by Industrial Galvanizers for more than 20 years to determine the performance of appropriate surface preparation, priming and top coating systems used on hot dip galvanized surfaces.
Strong relationships with selected paint suppliers and certified applicators allow Industrial Galvanizers’ to manage the application of the complete coating system on specific projects.
COATING GUARANTEES
Each of the above Custom Coating Packages comes with the option of a negotiated performance guarantee. More information on how these coating guarantees are developed is included elsewhere in the Specifiers Manual. Guarantees are currently in place with clients such as the NSW Roads and Traffic Authority for major projects such as the M5 East Motorway and the Western Ring Road for periods from 25 to 40 years, and on other landmark projects in Melbourne such as the MCG grandstands and the Eastlink Motorway.
SUMMARY
Industrial Galvanizers is unique in the hot-dip galvanizing industry, worldwide, in offering this range of additional coating options to the specifiers. These options add to the already well-established reputation of hot-dip galvanized coatings as the most durable and predictable method of protecting steel from corrosion.
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