Building projects demand durable long-lasting products and no protective coating for steel can match the strength of hot-dip galvanizing.
High-traffic areas like multi-storey car parks are exposed to more than the average level of wear and tear. Along with the risk of abrasion, localised pollutants from car exhausts can increase the rate of corrosion. So, when the car park of Melbourne’s Austin Hospital was extended to provide an additional 752 spaces, hot-dip galvanized steel was the only product tough enough to withstand the pressure.
No other protective coating for steel can match the durability, longevity and predictable performance of hot-dip galvanizing. A process of coating iron or steel with a layer of zinc to enable greater resistance to corrosion, it can be used in building projects across a range of demanding environments.
Made to last
Dr David Harrison, Market and Specifications Engineer at Galvanizers Association of Australia (GAA), explained the durability of galvanized coating is a function of its thickness and the environment to which it’s exposed.
“We have an Australian Standard AS 4312, which defines the corrosivity Australian environments, and AS/NZS 2312.2 which provides guidance on the durability of zinc coatings in different environments,” he said, adding that the GAA has developed a free online estimator based on these Standards that provides guidance on the durability of galvanizing.
“These Standards allow you to determine the expected life to first maintenance for galvanized coatings based on the coating thickness and the environment to which the coating is exposed. However, they may be a little conservative, because zinc corrosion rates actually slow down over time.”
The shiny lustre of newly galvanized steel develops a patina over time, which is essentially a thin film of protective zinc carbonate.
“This protects the zinc by slowing down its corrosion rate,” Harrison said. “This means galvanized coatings work in a very different way to paint coatings, for example.”
Ann Sheehan, Corrosion and Sustainability Officer at GAA, said the durability of galvanized products makes them ideal for a range of building project applications. She cited the example of Austin Hospital’s car par extension, which used approximately 1500 tonnes of hot dip galvanized steel.
“For that sort of environment, galvanized products are going to have at least 50 years before requiring maintenance,” she said.
“Their ability to withstand impact and their abrasion and scratch-resistance in high traffic areas is really valuable. Paint just can’t withstand as much impact and abrasion.”
Responding to corrosion
Galvanized building products are used in a wide range of applications and in many different environments, including in contact with materials such as concrete, soil or timber.
“Corrosion mechanisms vary in different environments,” Harrison said.
“One really good application, which hasn’t received lot of recognition in the past, is galvanized steel reinforcement.”
Harrison explained that in a steel reinforced concrete structure, black non-galvanized steel is passivated. Overtime, however, carbon dioxide works its way through the concrete.
“This is the process of carbonation,” he said.
“Once the concrete is carbonated at the bar surface, the pH in concrete has dropped into a region where the black steel isn’t passive anymore and can actively corrode.
“In contrast, when a galvanized reinforcement is initially set into concrete, it develops a calcium hydroxyzincate film on the surface, which is stable over a very broad pH range. This means the concrete can be fully carbonated and the galvanized steel bar will remain immune to that effect.”
Importantly, added Harrison, the calcium hydroxyzincate layer also has a much greater tolerance to chloride ion concentration, increasing the critical chloride threshold of HDG reinforcement two to six times above that of black steel reinforcement.
The durability and longevity of galvanized products also present sustainability benefits for the construction industry.
“With a reinforced concrete structure, for example, durability modelling can determine when back steel will start to corrode, due to carbonation or chloride ingress, and so builders may choose to make the concrete thicker to buy more time before is depassivates,” Harrison said.
“By using the more corrosion resistant reinforcement, you’re reducing the amount of raw materials and concrete that’s consumed in infrastructure.”
Sheehan noted that the zinc coating and the steel substrate is also 100 per cent recyclable.
“If it wasn’t going to be reused in the same context, it could be recycled and used for other things,” she said.
“Also, given the long life cycle of galvanized products, you don’t need to regularly replace materials.
“This means you’re not taking more materials out of the ground and using energy to create steel and melt the zinc. Galvanized products are strong enough to stand the test of time.”
More information on durability of galvanizing in the Australian environment is available at www.gaa.com.au