In our previous posts, we discussed the basics of galvanized coatings, their performance in the Australian atmosphere, and the impact of various environmental factors. In this final post, we will focus on the performance of galvanized coatings under specific conditions, including high temperatures, underwater environments, and soil.
Effect of Temperature
Hot dip galvanized coatings can withstand continuous exposure to temperatures of up to approximately 200ºC and occasional spikes up to 275ºC without significant effects. At higher temperatures, the outer zinc layer may start to separate, although the alloy layer typically remains intact. This resilience makes galvanized coatings suitable for a variety of high-temperature applications, though exceeding the melting point of the alloy layer (around 650ºC) may compromise protection.
Underwater Performance
General Considerations
The corrosion rate of zinc under submerged conditions varies with water composition. In acidic solutions below pH 6 and alkaline solutions above pH 12.5, the corrosion of zinc is rapid. However, within the pH range of 6 to 12.5, the rate is quite low, providing effective protection in many natural water systems and when exposed to various chemical solutions.
Impact of Water Composition
In mains supply water (typically pH 6-8), calcium carbonate often precipitates onto the galvanized coating, forming an adherent and protective scale. This layer, combined with zinc corrosion products and virtually halts further corrosion. However, the presence of uncombined carbon dioxide can prevent the formation of this scale, compromising protection. Additionally, even small quantities of dissolved copper can cause rapid pitting corrosion.
It is also important to recognise that the “Plumbing Code” does not permit the use of galvanized pipes for drinking water but does allow them for firefighting systems. Some states of Australia have differing rules in relation to the galvanized steels to collect rainwater (e.g., storage tanks) and the subsequent use.
Sea Water
Galvanized coatings may perform well in submerged sea water conditions due to the formation of a protective layer by dissolved salts, although submerged corrosion rates can be much higher than general atmospheric corrosion rates. In areas of severe sea water exposure, such as the tidal area or splash zone, a suitable paint or wrap system over the galvanized coating should be applied for optimal protection as the development of a patina in such environments is unlikely.
Performance in Soil
The corrosion behaviour of galvanized steel buried in soil is highly variable and depends on local conditions. Generally, galvanized steel performs better in alkaline and oxidizing soils, where a 600g/m² coating may add approximately 10 years of life to steel pipes. In highly reducing soils, zinc may be consumed at rates exceeding 13μm per year.
To enhance durability, buried galvanized steel can be protected further using paint coatings, bituminous compounds, tape wraps, or concrete encasement. AS/NZS 2041.1 provides detailed design advice for buried structures, which can be useful for all buried galvanized steel applications.
In this post, we examined the performance of galvanized coatings in specific conditions such as high temperatures, underwater environments, and soil. These insights underscore the versatility and durability of galvanized coatings across a wide range of challenging conditions.
This concludes our series on the performance of galvanized coatings. For more detailed information on this topic, you can download the Guide to the Service Life of Galvanizing from the guide section of our Technical Publications page. Thank you for following along, and we hope you now have a comprehensive understanding of the importance and performance of galvanized coatings in various environments.