Galvanizing Steel Posts

24 Sep 2012

In an effort to deliver the best shade structures Abacus Shade Structures uses galvanized steel poles. Time and time again galvanizing has proven to outlast and outperform other steel finishes. This article answers some questions about the galvanizing process.

Abacus Shade Structures prefers to use galvanized steel poles. Painting is offered as an option on galvanized poles. The following document references research from the Galvanizing Association of Australia and galvanizing companies to elaborate on the galvanizing process, the benefits of galvanizing, standards for galvanising and painting methods suitable for a galvanized pole.  

Why Galvanize

No other protective coating for steel provides the long life, durability and predictable performance of hot dip galvanizing. Galvanizing is a once only process, committed to the concept of the maintenance-free use of steel, ensuring long service life and virtually eliminating disruptive maintenance.

This long-term protection is well documented world-wide in terms of being ahead of any other protective coating, and galvanizing continues to find new applications in almost every field of engineering.

Galvanizers Association of Australia

The Galvanizers Association of Australia (GAA) comprises many of the leading galvanizing companies throughout Australia, New Zealand and Asia. GAA is an industry association established in 1963 to represent galvanizing companies and to provide technical consulting services on a not for profit basis. 

The Galvanizing process

 

 

Degreasing

Scale, rust, oil, paint and other surface contaminants are carefully removed from the steel by suitable preliminary treatment and subsequent acid cleaning or pickling in sulphuric or hydrochloric acids, followed by rinsing.

Fluxing

The acid-cleaned steel article is immersed in a flux solution, usually 30% zinc ammonium chloride with wetting agents, maintained above 65°C. The flux solution removes the oxide film which forms on the highly reactive steel surface after acid cleaning, and prevents further oxidation before galvanizing. The work is then dried ready for galvanizing.

Galvanizing

Prepared items are galvanized by immersion in molten zinc. On immersion in the galvanizing bath the steel surface is completely covered by the molten zinc, which reacts with the steel to form a series of zinc-iron alloy layers, producing a uniform coating. The thickness of these layers is determined principally by the mass of the steel being galvanized.  This is an important advantage of the galvanizing process – a standard minimum coating thickness is applied automatically regardless of the operator. [1]

Metallurgy of Galvanizing

When the cleaned and fluxed steel surface contacts the molten zinc of the galvanizing bath the protective flux layer is removed leaving a clean steel surface which is immediately wetted by the zinc.  This results in a reaction between zinc and steel with the formation of zinc-iron alloy layers.

The photomicrograph section of typical galvanized coating consists of a progression of zinc-iron alloy layers bonded metallurgically to the base steel, with the relatively pure outer zinc layer. [2]

10 Benefits of Galvanized Steel

The use of galvanizing for structural steel protection gives you ten major measurable benefits.

1. Lowest first cost.Galvanizing is lower in first cost than many other commonly specified protective coatings for steel. (The application cost of labour intensive coatings such as painting has risen far more than the cost of factory operations such as galvanizing.)

2. Less maintenance/Lowest long term cost.Even in cases where the initial cost of galvanizing is higher than alternative coatings, galvanizing is almost invariably cheapest in the long term (because it lasts longer and needs less maintenance).  And, maintenance causes problems and adds to costs when structures are located in remote areas, and when plant shutdown or disruption to production is involved.

3. Long life.The life expectancy of galvanized coatings on typical structural members is far in excess of 50 years in most rural environments, and 20 to 25 years plus, even in severe urban and coastal exposure.

4. Reliability.Galvanizing is carried out to Australian / New Zealand Standard 4680, and standard, minimum coating thicknesses are applied. Coating life and performance are reliable and predictable.

5. Toughest coating.A galvanized coating has a unique metallurgical structure which gives outstanding resistance to mechanical damage in transport, erection and service.

6. Automatic protection for damaged areas.Galvanized coatings corrode preferentially to steel, providing cathodic or sacrificial protection to small areas of steel exposed through damage. Unlike organic coatings, small damaged areas need no touch up.

7. Complete protection.Every part of a galvanized article is protected even recesses, sharp corners and inaccessible areas. No coating applied to a structure or fabrication after completion can provide the same protection.

8. Ease of inspection. Galvanized coatings are assessed readily by eye, and simple non-destructive thickness testing methods can be used. If coatings appear sound and continuous, they are sound and continuous.

9. Faster erection time.As galvanized steel members are received they are ready for use. No time is lost on-site in surface preparation, painting and inspection. When assembly of the structure is complete, it is immediately ready for use, or for the next construction stage.

10. A full protective coating can be applied in minutes.The galvanizing process is not dependent on weather conditions[3]

Galvanizing to Australian Standards

Australian Standard AS/NZS 4680:2006 – Hot dip galvanized (zinc) coatings on fabricated ferrous articles is the defining standard for hot dip galvanized coating specifications in Australia. It is closely aligned with the International Standard ISO 1461:1999 – Hot dip galvanized coatings on fabricated ferrous products, in keeping with the Australian Government’s policy of aligning all Australian standards with appropriate international standards.

Abacus Shade Structures uses only galvanizing companies that conform to Australian Standards.

AS/NZS 4680 defines the minimum requirements for coating mass (thickness) for various steel sections, methods of test and repairs to galvanized coatings, as well as containing informative information on design for galvanizing, surface preparation for painting and metallurgical information.

Some of the key parts of the Standard have been reproduced here in condensed form. Copies of all Australian and International standards can be downloaded from the Standards Australia web site at www.standards.com.au.

Associated Standards
The original galvanizing standard, AS 1650:1989, has been replaced by a series of standards dealing with each type of galvanized product; sheet, wire, tube and after fabrication galvanized products. This hasresolved a deal of confusion as each product and process produced galvanized coatings of differing durability and metallurgical characteristics. The additional standards are:

AS
1397 Steel sheet and strip—Hot-dipped zinc-coated or aluminium/zinc coated
AS/NZS
4534 Zinc and zinc/aluminium-alloy coatings on steel wire
4791 Hot-dip galvanized (zinc) coatings on ferrous open sections, applied by an in-line process
4792 Hot-dip galvanized (zinc) coatings on ferrous hollow sections, applied by a continuous or a specialised process.

Coating thickness specification
AS/NZS 4680:2006 specifies minimum coating thickness requirements for steel of various section thicknesses. While most galvanized coatings are classified by the mass of the galvanized coating in g/m2, the practical requirements of testing and measuring galvanized coatings non-destructively require the coating thickness to be a measure of conformance.[4]

The finished galvanized product

Unlike many manufacturing processes, the hot dip galvanizing of fabricated steelwork involves many variables that can impact on the appearance and characteristics of the finished product.

Hot dip galvanizing is primarily an industrial protective coating process designed to provide a tough and durable protective coating for steel that will generally outlast the design life of the structure to which it is applied.

The variables inherent in the process can each affect the appearance of the galvanized product. Many of these variables are classified as ‘coating defects’ but most are aesthetic and do not affect the performance of the galvanizing as an anti-corrosion coating.

Factors affecting galvanizing coatings

The factors affecting the appearance and characteristics of hot dip galvanized coatings include:

• The size and shape of the item
• The steel chemistry
• The steel surface condition
• The design of the item and
• The metallurgy of the galvanizing process.

The hot dip galvanizing process involves immersing steel items in molten zinc at 450oC after pre-treatment to remove organic materials, rust and mill scale. This hot dipping process, where the steel is immersed in the molten zinc for several minutes, gives the galvanized coating its unique characteristics.

Painting galvanized poles

Abacus Shade Structures offers a painted finish option on every shade structure. Painting is done on a galvanised steel surface. Painting can add visual aesthetics and colour matching beyond the industrial finish of galvanizing. Abacus is currently trialing electrostatic painting on galvanised posts and getting excellent results.

Where steel is exposed to highly corrosive environments or where access is difficult and freedom from maintenance is required, duplex systems of galvanising-plus-paint provide very long service life, resulting in outstanding economics.

By combining heavy duty industrial paint coatings which are typically at least 100 microns in thickness, with hot dip galvanized coatings which are also around 100 microns in thickness, true zero maintenance coating systems can be produced for steel with corrosion free life expectancy in excess of 50 years.[5]

In preparing galvanizing for painting, anything that prevents the paint wetting out or adhering to the surface needs to be removed.  Oils, dirt, dust, salts, corrosion products and other friable material and soluble salts are removed prior to subsequent treatment. 

For painting unweathered galvanizing in normal low corrosive conditions (C1 & C2)  cleaning and degreasing is adequate. Light scuffing with sandpaper will enhance paint adhesion. Brushing and abrasive blasting is not required unless under medium or higher corrosive conditions (C3+) such as in extreme industrial areas, tropical and ocean environs.


Back
  • Abacus Shade Structures
  • Level 14, Lumley House,
  • 309 Kent Street,
  • Sydney NSW 2000
  • Australia: 02 9831 1218
  • Int: 61 2 9831 1218
  • [email protected]
  • © Copyright 1999-2016, Abacus Shade Structures. All rights reserved. Trading TermsShade Sails - Sydney based supplier