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CM Nov 2001 - Page 3

DEVELOPMENT OF A DURABILITY BRANDING SYSTEM FOR STEEL CONSTRUCTION PRODUCTS

By John Robinson - Editor

Zinc Coating Parameters Affecting Durability ; Coating Life & Coating Thickness ;
Existing Building Codes & Standards ; Standards Related to Various Zinc Coated Products ;
Durability Branding of Zinc Coated (Galvanized) Steel Products ; Coating Certification ;
Corrosion Mapping Developments.

INTRODUCTION

Galvanized or zinc and zinc alloy coated products represent one of the largest construction material segments used in Australia, with more than one million tonnes of sheet, wire, tube and structural sections being used annually.

While the galvanized and zinc based coatings on these products are defined in various Australian, Australian/New Zealand, and ISO standards, these standards are largely prescriptive and define minimum requirements for coating mass without reference to durability.

There is a significant difference between the various galvanized and other zinc based coatings in their physical and metallurgical characteristics, and in particular, their durability.

The differences in these coatings in terms of durability are poorly understood at specifier level, and as a result, many products are selected for critical construction applications that do not deliver the expected maintenance free life.

It is not possible to determine the characteristics of many zinc-based coatings simply by their appearance, as coating durability is a function of coating mass (coating thickness), which can only be determined with specialised equipment.

The Galvanizers Association of Australia (GAA) has initiated a program to identify its members’ hot dip galvanized products with a Durability Certification label that will allow specifiers and consumers to obtain durability information on the particular galvanized product.

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ZINC COATING PARAMETERS AFFECTING DURABILITY

The most significant factor affecting zinc-based coating durability is coating mass. This is usually specified in grams per square metre (g/m2). In practice, coating mass is usually converted to a relative coating thickness to allow non-destructive testing of the coating to be done with an appropriate thickness gauge, where the coating thickness is defined in microns (um).

Other factors affecting coating durability include the method of zinc coating application, the presence of other alloying elements in the coating and the existence of surface treatments applied as an integral step in the coating process. A brief description of the main non-proprietary zinc based coatings and their method of application follows.

Zinc electroplating
Zinc electroplating involved immersing pre-treated components in a zinc plating solution and applying a DC current. Zinc electroplating is widely used as a coating on builders’ hardware, fasteners and appliance components.

The coating consists of pure zinc with the coating thickness rarely exceeding 15um and generally measuring less than 10um.

The coating produced is bright and smooth. Chromate post treatment on some electroplated products is done to improve durability performance and is characterised by a yellow-brown colour.

Continuous or in-line galvanizing
Sheet, wire and some hollow and open sections are galvanized in a continuous process, where the pre-treated steel passes through a galvanizing bath at speeds up to 180 metres per minute. Control systems in the process closely control the coating thickness within the limits of the process. Coatings exceeding 300 g/m2 are rarely applied in these processes and coatings are more commonly in the 15-25um range.

The coating consists of largely pure zinc with a thin (less than 5um) zinc-iron alloy layer at the steel/coating interface.

The coating produced is smooth and uniform and may have a ‘spangled’ appearance that arises from alloying elements in the zinc – specifically lead.

Continuously galvanized coatings are applied to semi-finished products, which are always subject to further processing. This results in areas of the steel in the finished product being uncoated because of cutting and punching.

Hot dip galvanizing
Batches of fabricated items are immersed in a bath of molten zinc after pre-treatment for periods ranging from 3-10 minutes, depending of the mass and shape of the articles. This produces a relatively thick coating that will vary in appearance depending on the surface condition of the steel, its chemistry, its shape and other design elements.

The coatings consist largely of zinc-iron alloys (75%+) with a surface coating of zinc. The zinc-iron alloys are hard and relatively brittle. This metallurgical characteristic results in the significant differences in abrasion resistance and flexibility when compared to continuously galvanized coatings.

Hot dip galvanized coatings are applied to steel items after fabrication and thus all surfaces are coated. Coating thickness variation will occur as a result of the orientation of the work to the galvanizing bath that affects drainage characteristics. The hot dip galvanized coating can be variable in thickness due to local variations in the surface chemistry of the steel.

Hot dip galvanized coatings range from around 40m in thickness on thin (less than 3 mm) steel sections to over 200um on heavy structural sections.

Figure 1
Zinc (galvanized) coating thickness applied by different methods.

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COATING LIFE AND COATING THICKNESS

Galvanized (zinc) coatings behave differently to applied organic coatings in that their life is determined by the rate at which they oxidise from the surface. In atmospheric exposures, the rate of corrosion is approximately linear and is determined by the classification of the environment.

Thus, the coating thickness is the most important element in determining the maintenance free life of a zinc-based coating. Other secondary, but still important factors determine corrosion rates of zinc (galvanized) coatings. These include orientation of the surface and degree of sheltering. Recent CSIRO test programs have indicated that the presence of significant zinc-iron alloy layers, absent in continuously galvanized products, and always present in hot dip galvanized products, may have significantly lower (2X-3X) rates of corrosion that zinc coatings.

This phenomenon provides a reliable method of determining the expected life of a zinc-based coating in any nominated environment, as long as the coating thickness can be determined.

There is a large body of corrosion data collected over 100 years on the corrosion rate of zinc. In more recent times, computer modelling of surface oxidation reactions has been added to the empirical data to further refine corrosion rate data for a wide range of actual and theoretical environments.

The International Standards Organisation (ISO) has developed a suite of standards to quantify zinc corrosion rates and classify environments to which they are exposed.
The key ISO standards are:

• ISO 9223 – Corrosion of metals and alloys – Corrosivity of atmospheres – Classification
• ISO 9223 – Corrosion of metals and alloys - Corrosivity of atmospheres – Guiding values for the corrosivity categories
• ISO 9225 – Corrosion of metals and alloys – Corrosivity of atmospheres - Measurement of Pollution
• ISO 9226 - Corrosion of metals and alloys - Corrosivity of atmospheres- Determination of corrosion rate of standard specimens.

These standards take a structured approach to classifying atmospheres using time of wetness, temperature, chloride and SO2 levels.

These ISO standards use a C1-C5 classification to define atmospheric corrosion classification. These categories are defined as follows:

Table 1.
Classification of atmospheric corrosivity

Table 2.
Categories of corrosivity of the atmosphere

These ISO categories are broad and non-descriptive and need to be related to practical environments to which they are applied. AS/NZS 2312 has used ISO 9223 to produce a set of atmospheric classifications more appropriately descriptive of regional environmental exposures.

These are:

1. Interior exposure (ISO Category C1) - Steel corrosion rates less than 1.3um/year.
2. Mild (ISO Category C2) - Steel corrosion rates up to 10um/year. Most rural environments are in this classification.
3. Moderate (ISO Category C2) - Steel corrosion rate from 10-20um/year. Most capital city and suburban areas are in this classification.
4. Tropical (ISO Category C2) – In Australia, this includes coastal areas north of the Tropic of Capricorn. This is a category that cannot be readily delineated by ISO 9223 parameters. Measurements put this into an ISO C2 category for metals but it is classed as more aggressive for organic coatings.
5. Industrial (ISO Category 3-4) – First year steel corrosion rates greater than 25um/year, but may extend into ISO Category 4 (50um/year). Significant reductions in industrial pollutant levels through regulation has resulted in Australian heavy industrial centres being in the lower range of corrosivity classification.
6. Marine (ISO Category 3) – First year corrosion rates of 25-50um/year, and within 1 km of the ocean. Topography and climatic conditions will influence the transport of chlorides, which are the major determining factor.
7. Severe marine (ISO Category 4-5) – First year steel corrosion rates exceeding 50um/year includes off-shore and coastal areas subject to ocean surf and prevailing on-shore winds. Most of the Australian oceanfront with exception of those areas sheltered by reefs and islands are represented in this category.

While there is relativity in the corrosion rate of zinc and steel in a given corrosivity classification, it is not constant across all corrosivity classifications.

Table 3 lists typical corrosion rate ranges in the above AS/NZS 2312 atmospheric classifications.

Table 3
Typical corrosion rate range – Zinc and steel

EXISTING BUILDING CODES AND STANDARDS

There is a large number of standards related to the application and constitution of steel coatings of all kinds, along with their pre-treatments. AS/NZS 2312:1994 Guide to the protection of iron and steel against atmospheric corrosion, references over 50 standards for various types of galvanizing, priming, paint, surface preparation and testing.

These standards are then referenced in other documents such as the Building Code of Australia (BCA) produced by the Building Code of Australia Board (BCA), which is a federal government body convened to standardise state, territory, local government and building industry codes and practices.

Other organisations such as (CIS) Construction Information Systems (previously NATSPEC) provide specification services by pre-packaging standards and specifications into project related documents to facilitate the documentation and administration of project specifications.

The main focus of the BCA is to document engineering and safety standards for dwellings. It largely references other standards for durability information, and in some areas, such as shelf angles and lintels, contradictions exist within the referenced standards.

The main weakness with the BCA is that there are no clearly defined durability standards (e.g. that an element in a dwelling must have a design life of X years.)

The onus of performance thus frequently resides with the builder, and without enforcement, inadequately protected steel elements are used in many jurisdictions.

This is not necessarily because individuals wish to take short cuts, but because of the poor understanding of the differences in performance between metallic coatings on steel building products.
It is difficult, without the necessary measuring equipment or experience, to tell the difference between many zinc (galvanized) coatings and a the durability of a similar looking steel section may be reduced by 5X due to the different technology that has applied the zinc (galvanized) coating.

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STANDARDS RELATED TO VARIOUS ZINC-COATED PRODUCTS

A suite of Australian (and NZ) standards covers the range of zinc and galvanized coatings that are applied to most building and construction products. Additional standards also cover coatings on specific products such as fasteners, which are not referenced here. These Standards are:

• AS/NZS 4680 - Hot dip galvanized (zinc) coatings on fabricated ferrous articles
• AS/NZS 4534 - Zinc and zinc/aluminium coatings on steel wire
• AS/NZS 4791 - Hot dip galvanized (zinc) coatings on ferrous open sections applied by a continuous or specialised process.
• AS/NZS 4792 - Hot dip galvanized (zinc) coatings on ferrous hollow sections applied by a continuous or specialised process.
• AS 1397 - Steel sheet and strip – Hot dipped zinc coated and aluminium/zinc coated coated
• AS1789 - Electroplated coatings – Zinc on iron and steel
• AS 4750 (Int.) - Electro-galvanized (zinc) coatings on ferrous hollow sections.

Each of these standards defines coating mass/coating thickness requirements and the method of designation, using a coating descriptor and a number representing the mass per m2 of the coating.

For example, Z is used for zinc (galvanized), ZA for zinc-aluminium alloy, E for electroplated and ILG for in-line galvanised tube and open sections. The numeral (150, 300, etc) is the coating mass in g/m2.

All standards with the exception of AS 1397 specify the single-side coating mass. AS 1397 specifies the total coating mass on both sides of the galvanized sheet. Thus a Z350 class coating on galvanized sheet is equivalent to a Z175 class on other galvanized products. This, in itself, is a constant source of confusion to specifiers attempting to define durability.

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DURABILITY BRANDING OF ZINC COATED (GALVANIZED) STEEL PRODUCTS

The need to classify the performance of buildings and building components is well established. Local, state and federal authorities have implemented performance based ‘branding’ protocols to determine energy efficiency of buildings and the engineering and energy ratings of building elements such as windows.

‘Star’ rating systems are commonly used on items and services as disparate as appliances and accommodation to certify the performance of the ‘product’ in a way that is easily recognised by the consumer.

The Galvanizers Association of Australia (GAA) has long been faced with the problem of differentiating the high performance, heavy-duty hot dip galvanized coatings applied by its members and the many other coatings that claim the same credentials as a ‘galvanized’ coating.

While the characteristics of many of these many coatings are clearly defined in the Standards relating to these coatings, these differences are sometimes reluctantly revealed to the marketplace and at specifier level. There is little knowledge of the specific Australian standards related to these types of products or their relative performance.

The first stage of the national Durability Branding Program being implemented by the GAA involves the certification of its members’ hot dip galvanized products with a labelling system that will allow specifiers to easily identify coating durability with an identifier attached to the product (sticker, label, tag).

Because the corrosion rate of galvanized coatings is essentially linear, and the coating thickness is clearly defined by the process and is relative to steel section thickness and process metallurgy, a clear definition of galvanized coating durability can be provided on products hot dip galvanized after fabrication.

GAA member galvanizers have accredited coating measuring equipment to accurately determine coating thickness on any steel section processed.

By allocating one ‘Star’ for each 25 microns of galvanized coating thickness, a rating system covering the full range of hot dip galvanized coatings can be provided. The ‘Stars’ are then related to an atmosphere corrosivity classification table to give an immediate determination of coating life expectancy in any environmental classification.

A sample of a GAA Durability Branding Certificate is illustrated in Figure 2.

This labelling system provides a simple and comprehensive identifier for specifiers seeking to have galvanized coatings supplied to a performance standard.

Figure 2.
Sample GAA Durability Branding Certificate

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COATING CERTIFICATION
Where independent certification of the coating is required, this is available through organisations such as CSIRO or BRANZ, who are already actively involved in product and coating evaluation and also support the development of performance-based standards as a policy.

BRANZ (Building Research Association of New Zealand) has considerable regulatory authority to enforce durability-based standards in New Zealand. A minimum building design life of 50 years for structural elements and lower specified life for more easily replaced components.

The protocols established by BRANZ in New Zealand could well be duplicated by Australia’s BCA in providing a regulatory framework with more enforceable elements that the present BCA.

The long-term aim of the Durability Branding initiative is to expand the program to include a full range of coated steel products used for building and construction, including fasteners, coated cladding and roofing, and process controlled paint coatings such as powder coating.

Because of the nature of these various coatings, independent methods of durability certification will need to be developed to allow them to be durability certified. Both CSIRO, BRANZ and other testing organisations have developed accelerated durability testing procedures that may satisfy the requirements for defining long-term durability performance.

Certification of zinc coated (galvanized) of products outside the jurisdiction of the GAA can also be done through one the above certifying agencies using the same performance parameters as are proposed by the GAA.

This will allow a large range of existing steel building products to be ‘Durability Certified’, by having them submitted for certification to the certifying agency by their supplier/manufacturer. This may not be a welcome initiative with many existing steel building products, in particular builders’ hardware and some in-line galvanized light structural and hollow sections, as the relatively poor performance of the zinc coatings on these products in outdoor exposures will be immediately evident to the consumer.
Many of these ‘galvanized’ products would not rate one using the GAA Durability Certification and this will ensure that the higher durability products get due consideration in the selection process, where they are currently disadvantaged because of the low cost of those coatings with less durable zinc (galvanized) coatings.

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CORROSION MAPPING DEVELOPMENTS FOR DURABILITY MANAGEMENT

In 2001, Industrial Galvanizers Corporation and the CSIRO entered into a research agreement to develop an Internet based Corrosion Mapping System (CMS) to provide on-line environmental corrosion data for a range of materials, but specifically steel and galvanized coatings.

The CMS will allow specifiers to obtain corrosion rate data in any location in Australia and relate this to durability, given section or coating thickness of the material being evaluated.

While this has been developed independently by Industrial Galvanizers, the synergy with the Durability Branding program is obvious, as it not only allows the Durability Certification provided through GAA members and other participants to be verified independently (all the corrosion data comes from the CSIRO’s database) but also allows durability assessments to be made on existing galvanized steel products.

The CMS is now fully operational and is available online via membership. (Member login | New Member Registration)

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SUMMARY

It is not possible to accurately determine the durability of structures unless the components used in their construction are able to satisfy the durability requirements of the design. Without a method of certifying the durability of coated steel products, the selection of materials becomes subjective and is often dictated by expediency and cost rather than performance.

Premature corrosion of steel building components is one of the major causes of degradation and impacts on the life-cycle costs of the building, its future value and its safety.

Support of this GAA Durability Branding initiative by the specifying community, and by the administrators of building codes at national, state and local government level will ensure:

• that optimum material performance is incorporated into buildings;
• that environmental impacts caused by corrosion are eliminated;
• that maintenance costs are minimised;
• that a technically sound basis for determining durability is available at the ‘point of sale’ using Durability Certification labelling.

The development of broader-based Durability Branding protocols for other coated steel building products will ensure that better control of building durability can be implemented from the design stage through to construction.

The long-term strategy is to facilitate the development of Australian standards and building codes that incorporate performance-based durability requirements, and ultimately extend this to ISO standards so that international practices are consistent in ensuring the durability of coated steel construction materials.

REFERENCES

Porteous Dr.W, Harmonising What? – The Difference Between Codes and Standards, Proc. Second Asia/Pacific Conference on Durability of Building Systems Harmonised Standards and Evaluation, Vol. 1, July 10-12, 2000, Bandung, Indonesia. pp 3/1 – 3/9
Standards Australia AS/NZS 2312:1994, Guide to the Protection of Iron and Steel Against Exterior Atmospheric Corrosion, Section 2 Atmospheric Environments, Standards Australia Homebush pp 11-12.
Porter, F.C. Corrosion Resistance of Zinc and Zinc Alloys. Chapter 2 – Resistance to Atmospheric Corrosion. Marcel Dekker, New York, pp 101-105.
Cole I.S, Neufeld A.K., Kao P., Ganther W.D., Chotimongkol L., Bhamornsut C., Hue N.V., Bernado S., and Purwadia S. Factors Affecting Atmospheric Corrosion in Five Tropical Countries. Proc. Second Asia/Pacific Conference on Durability of Building Systems Harmonised Standards and Evaluation, Vol. 1, July 10-12, 2000, Bandung, Indonesia.pp 18/1-18/11.
Van Gaal P, Its Coating Thickness that Counts. Corrosion Management, Vol. 6 No.1 March 1997. IGC, Brisbane. pp 5-7.
Roberts D. A New Standard of Durability for Self-Drilling Building Fasteners. Corrosion Management Vol. 8 No 1 March 1999. IGC Brisbane pp 17-20
Haberecht P, Bennett A.F, Experience with Performance Based Building Codes. Corrosion Management Vol. 8 No. 3 November 1999. IGC Brisbane pp 11-19.

Return to Corrosion Management - November 2001 Index

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