INTRODUCTION
One of the most important characteristics of zinc (galvanized) coatings on steel is the ability of the zinc to protect adjacent steel from corrosion. The electrical potential between zinc and steel ensures that zinc will act as an anode to adjacent exposed steel and prevent its corrosion.

All continuously galvanized products are dependent on the cathodic protection of their galvanized coatings to protect them from localised corrosion, as the galvanized coatings on all of these products are ‘damaged’ during the manufacturing process. They are cut, formed or punched after being manufactured from sheet, wire, or hollow and open sections to feed the next stage of the manufacturing process.

The degree of protection provided by the zinc coating to the exposed cut edges will be determined by:

1. The thickness of the material.
2. The mass of zinc available in the coating adjacent to the exposed steel.
3. The nature of the environment to which the product is exposed.

The cut edge corrosion phenomenon is almost always ignored in Standards and specifications. Australian Standard AS/NZS 4680:2006 Hot dip galvanized coatings on fabricated ferrous items, is a good example of this. The requirement to repair damaged areas on items galvanized after fabrication is clearly defined in the Standard, where the maximum permissible un-coated areas are nominated in terms of area.

In other standards associated with continuously galvanized products, there is no such requirement for repair of cut edges on sheet, tube, or wire products that are galvanized prior to fabrication. This is particularly relevant with cold-formed structural sections (purlins, channels) used for commercial construction, where the exposed steel per unit of surface exceeds the maximum allowable repairable area in the hot dip galvanizing standard.

HOW FAR DOES GALVANIC PROTECTION EXTEND?
For many years, the only type of product reliant on galvanic protection of its cut edges was hot dip galvanized sheet used for roofing and cladding. The base steel used for these applications is very thin, typically 0.5-0.7 mm in thickness and thus the surface area of bare steel exposed is negligible.

Development in continuous manufacturing and coating of pipe and open structural sections, as well as heavier (up to 3.2 mm) sheet has resulted in a wide range of products being developed that have significant areas of exposed steel between the coated surfaces.

The other factor common to all galvanized products is the coating’s ability to protect any exposed steel in the event of damage to the coated surface. While the ability of galvanized coatings to cathodically protect adjacent exposed steel has been evaluated empirically for over 100 years, few attempts have been made to quantify this phenomenon.

Work done in by X.G.(Gregory) Zhang of the Cominco Product Technology Centre, Mississauga, Canada, has allowed a more scientific approach to be taken.

EXPERIMENTAL RESULTS
The Cominco research by Gregory Zhang was aimed at determining the consumption rate of zinc as a barrier coating along with its galvanic protection of the exposed steel. Samples were prepared to simulate various zinc/steel combinations and included a number of degrees of separation, using zinc and steel strips of various thicknesses embedded in epoxy resin mounts. The samples were subjected to testing in the following environments:

• ASTM standard salt spray test
• full immersion test
• cyclic wet-dry humidity tests
• actual atmosphere exposure test.

After testing in each corrosion environment, the samples were evaluated with an optical microscope. Each sample was inspected for areas of rusted and non-rusted steel surface based on both colour and depth of corrosion. Brown to dark brown discoloration on the steel surfaces was used to define the range of galvanic protection provided by the zinc.

CONCLUSION
In atmospheric exposure conditions, the Cominco report concludes that cathodic protection afforded to exposed steel by a zinc (galvanized) coating will be about 2 mm. This will vary somewhat depending on exposure conditions, and while rusting may occur outside this area, partial galvanic protection will cover a much larger area and significantly moderate the rate of steel corrosion in this area. The thin (0.2 mm) lateral surface of the zinc coating will be subject to galvanic corrosion in protecting the adjacent exposed steel while the rest of the zinc coating will corrode normally.