INTRODUCTION
The durability of metal structures is determined by the environment to which the structures are exposed, and the way that the metals or their protective coatings respond to that environment.

The way that metallic coatings such as zinc-based coatings protect the base steel is quite different to the mechanism of applied paint coatings.

Zinc coatings, specifically hot dip galvanized coatings are metallurgically alloyed to the base steel and form what is essentially a composite material. They deteriorate through surface oxidation when exposed to moisture and corrodents such as chlorides and sulfates.

Paint coatings, in the other hand, depend on the condition of the surface to which they are applied (surface preparation), the performance of the primer used to promote adhesion, and the resistance of the topcoat to the atmospheric or service conditions to which the coating is exposed.

Paint coatings can be formulated to deal with a wide range of service conditions, while zinc-based coatings have clearly defined performance boundaries that will set the appropriateness of their use.

Paint coating may fail due to loss of adhesion for various reasons, degradation due to ultra violet light exposure or undercutting corrosion of the steel adjacent to damaged areas. The paint film itself may still be in good condition except that it no provides an effective barrier for the steel.

Galvanized coatings fail by being consumed from the surface through some form of oxidation. In any given environment, this oxidation progresses at a steady rate for the life of the coating. In a stable environment, the rate of corrosion is also stable.

By defining the expected rate of corrosion in any given environment, the service life of galvanized (and any zinc-based) coating can be determined. This is the basis of the INGAL Corrosion Mapping System.

THE DEVELOPMENT OF THE CORROSION MAPPING SYSTEM
The CSIRO’s Manufacture and Infrastructure Technology Division has compiled a large body of research on environmental conditions affecting the corrosion of metals, and has used the data from this research to develop corrosivity models that can be applied on a geographically large-scale.

Previously coupon testing of corrosion parameters at specific sites was the standard method of measurement of corrosivity. This is time-consuming and costly and also defines performance over a limited time span. Detailed corrosion maps of Newcastle, NSW, Melbourne, VIC, and parts of South Australia have been developed using coupon exposures and in Newcastle, for example, the closure of two major metallurgical industries has changed the corrosion landscape significantly.

Industrial Galvanizers Corporation and CSIRO entered into a research agreement in 2000 to develop a web based system that would allow the corrosivity of specific locations throughout Australia to be determined, based on time of wetness, chloride deposition rates and orientation of the surface being assessed.

This joint venture resulted in the web-based INGAL Corrosion Mapping System (CMS) being set up on the Industrial Galvanizers web site (www.indgalv.com.au), with additional modules dealing with in-ground performance of galvanized steel and environmental sustainability factors associated with materials of construction.

Also included in the INGAL CMS is a module to assist in the determination of the corrosivity of soils with respect to buried metal structures, as well as a sustainability of materials calculator for environmental assessment of materials.

HOW THE CMS WORKS
It is not possible to get the degree of precision required from screen-based maps, which on the INGAL CMS allow only approximate locations to be defined.

For this reason, a co-ordinate system has been included in the INGAL CMS, permitting accurate latitude and longitude co-ordinates to be entered to deliver the corrosivity data for the location. This service is provided through a link with the Geoscience Australia web site. By entering the locality identity, the Geosciences Australia system will return a set of co-ordinates for the location.

These can then be entered onto the co-ordinate fields in the INGAL CMS, which will then deliver the corrosivity data for the site. For ocean-side locations, further refinement of the corrosivity data can be implemented by defining the exact distance from the coastline in metres, up to I km.

The data from the above actions, again on the INGAL CMS web site, can then be further refined to take into account the effect of position in its service environment, of the item being assessed. The position may be classified as exposed, partly sheltered or sheltered.

Where airborne chlorides are present, these qualifications are important, as sheltered (not washed by rainwater) areas may be subject to chloride accumulation that will significantly increase corrosion stress. The CMS accounts for these higher rates of corrosion.

If refinement of corrosivity data is required, further assessment can then be done in-house, by Industrial Galvanizers trained staff. This second-level assessment evaluates local topographical data (coastal forms (cliffs, etc) height above sea level, prevailing winds, vegetation and the built environment.

SUMMARY
The large-scale corrosivity modeling that has been built into the CMS by CSIRO allows assessment to be made of the durability of steel and galvanized coatings throughout Australia. Industrial Galvanizers’ experience in assessing the performance of galvanized steel in a wide range of applications and environments over the past 30 years, using carefully documented case-histories, combined with its CMS, allows the company to offer its industry leading Coating Guarantees to customers seeking assurance of the long term durability of their steel structures.

NAVIGATING THE INGAL CORROSION MAPPING SYSTEM

1. Go to the Industrial Galvanizers Web Site at www.indgalv.com.au
2. Scroll down to the CORROSION button and select CORROSION MAPPING
3. Select NEW USER and register.
4. The INGAL CMS will load.
5. Go to FIND COORDINATES . This will link to the Australian Geosciences web site. Follow the instructions to obtain locality coordinates.
6. Return to the INGAL CMS Map and enter coordinates. The corrosion data for the location will be displayed.