Background

Galvanised coatings have an unusual characteristic compared to other protective coatings in that they fail by weathering and oxidation from the surface. Paint coatings, once breached, deteriorate through under-film corrosion and can suffer rapid failure as a result. Because of the electrochemical protection provided to steel by zinc (galvanised) coatings, no corrosion of the steel will occur while there is any zinc present, regardless of the thickness or condition of the galvanised coating. Galvanised coatings, in atmospheric exposure conditions, corrode at an approximately linear rate. Once this rate has been established for a particular environment, the expected life of the coating can be defined by relating the rate of corrosion to the thickness of the coating.

Factors Affecting Galvanised Coating Life

The durability of galvanised coatings depends on a number of environmental factors. These include: · Time of wetness · Ambient temperature · pH of moisture · Chloride levels in atmosphere · Sulfate levels in atmosphere · Contact with other chemicals · Contact with dissimilar metals · Orientation of exposure (vertical, horizontal) · Nature of exposure(sheltered, open) · Ventilation conditions Corrosion engineers take these factors into account when assessing the life-cycle performance of galvanised coatings. Organisations such as the CSIRO have developed environmental assessment techniques based on atmospheric computer models that facilitate the accurate assessment of metallic corrosion rates. A number of international (ISO) standards have also been developed that use combinations of the parameters listed above to tabulate corrosion rate data for zinc (galvanizing) and other metals. Classification Of Environments Most standards and documents associated with coating performance use exposure classifications to define corrosivity of the atmosphere. For metallic coatings such as galvanizing, factors such as UV exposure do not impact on coating life, where with paint coatings, UV levels are an important factor in their durability. For galvanised coatings, common Australia exposure classifications are arid/rural, mild/urban, industrial, marine and tropical. Much exposure testing has been done to obtain corrosion rate data in these environments, and this work is ongoing. Testing done in a number of long-term case studies has indicated that hot dip galvanised coatings in service may have lower corrosion rates than those of zinc coupon samples exposed in test facilities. Reasons for this apparent lower rate of in-service corrosion have not been quantified, but are thought to be related to the quite different characteristics of a hot dip galvanised coating compared to pure zinc, typical of the samples used in exposure testing. The hot dip galvanised coating contains alloys of iron, aluminium and sometimes nickel, each of which may modify the way the coating reacts with the environment. The following table shows typical corrosion rates of hot dip galvanised coatings in the various environmental classifications. Table 1. Corrosion Rates of Hot Dip Galvanised Coatings Environment Corrosion Rate – microns/year Arid/Rural <1 Mild/Urban* 1-3 Industrial 3-5 Marine** 5-15 Tropical 1-3 * Metropolitan and urban areas within 25 km of the Australian coastline outside the ocean surf spray zone. ** Within the ocean surf spray zone from 0 – 1000 metres from ocean surf, depending on topography. Coating Thickness Versus Coating Life All continuously galvanised and after-fabrication galvanised steel products have coating thickness specified in various Australian, New Zealand and international standards. By relating this coating thickness to the corrosion rates in the table, an accurate estimate of galvanised coating life can be obtained. Hot dip galvanised coatings that comply with AS/NZS 4680 – 1999 are those that will give the longest life, as they are typically 3 –5X the thickness of zinc coatings applied to continuously galvanised products. On structural steel sections, 50 years life before first rust in other than marine or heavy industrial environments is a reasonable expectation. Case history studies of existing installations in tropical and industrial environments indicate that 100 year life is achievable with galvanised coatings applied after fabrication.