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Galvanized coatings have extraordinary properties than other protective coatings, in their resistance to oxidation and weathering. When paint coatings are breached, they become degraded by under-film corrosion and are prone to rapid failure.
Zinc (galvanized) coatings offer electrochemical protection to steel. Due to this reason, steel does not corrode in the presence of zinc, regardless of the condition or thickness of the galvanized coating.
In atmospheric exposure conditions, galvanized coatings corrode at a nearly linear rate. If the rate has been established in a particular environment, then the predicted life of the coating can be delineated by associating the rate of corrosion with the thickness of the coating.
Factors Affecting the Galvanized Coating Life
The durability of the galvanized coatings depends on the following environmental factors:
- pH of moisture
- Ambient temperature
- Nature of exposure (open, sheltered)
- Contact with other chemicals
- Contact with different metals
- Ventilation conditions
- Chloride levels in the atmosphere
- Sulfate levels in the atmosphere
- Time of wetness
- Orientation of exposure (vertical, horizontal)
Generally, corrosion engineers consider the aforementioned factors when assessing the life-cycle performance of galvanized coatings. Organizations such as the CSIRO have developed environmental evaluation methods based on atmospheric computer models. These models enable the accurate evaluation of metallic corrosion rates.
In addition, several international (ISO) standards have been developed that utilize the combinations of the above-mentioned parameters to tabulate corrosion rate information for various metals, including zinc.
Classification of Environments
Most of the documents and standards associated with the coating performance utilize exposure classifications to illustrate the corrosivity of the atmosphere. In the case of metallic coatings such as galvanizing coatings, factors like UV exposure do not affect the coating life. But in paint coatings, UV levels play a crucial role in their durability.
General Australia exposure classifications for galvanized coatings are arid/rural, mild/urban, industrial, marine, and tropical. A considerable amount of exposure testing has been performed to obtain corrosion rate information in these environments, and this work is still ongoing.
Tests performed in several case studies have established that hot-dip galvanized coatings in service can have lower corrosion rates in comparison to zinc coupon samples in test facilities.
Even though the reasons for this obvious lower rate of in-service corrosion have not been established, they are believed to be connected to the relatively different properties of a hot-dip galvanized coating when compared to pure zinc.
The hot-dip galvanized coating includes alloys of iron, aluminum and, at times, nickel. Each metal may alter the way in which the coating reacts with the environment.
The following table demonstrates the standard corrosion rates of hot-dip galvanized coatings in different environmental categories.
Table 1. Corrosion Rates of Hot-Dip Galvanized Coatings
||Corrosion Rate – microns/year
* 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 to 1000 m from the ocean surf, depending on topography.
Coating Thickness vs. Coating Life
The coating thickness for each continuously galvanized and after-fabrication galvanized steel product is indicated in different New Zealand, Australian, and international standards. An accurate estimation of the life of the galvanized coatings can be obtained by associating the thickness of these coatings with the corrosion rates given in the table above.
Hot-dip galvanized coatings that comply with AS/NZS 4680 – 1999 are the ones that provide the longest life. These coatings have a thickness that is usually three-to-five times that of zinc coatings applied to continuously galvanized products.
A reasonable expectation in the case of structural steel sections is 50 years before the first rust materializes. This does not apply to heavy industrial or marine environments. With respect to present installations in tropical and industrial environments, case history investigations have demonstrated that a 100-year life can be realized with galvanized coatings applied after fabrication.