Engineers are being offered greater accuracy and location-specific insights around galvanizing-related corrosion and coating life from a new data-driven tool.
Established standards have long ruled over the durability of hot dip galvanized steel, providing a consistent baseline for the estimation of corrosion trends and life expectancy of materials.
However, those standards come with limitations, including the fact that they are often conservative, they rely on broad environmental assumptions and they provide relatively limited guidance around atmospheric corrosion conditions and their often real-world complexities.
A new online calculator has been developed by the Galvanizers Association of Australia (GAA) in partnership with RMIT University. It brings multiple data sources, including highly localised weather conditions, together to offer a more precise way to measure durability of models containing galvanized materials.
What’s wrong with today’s durability measures?
Dr David Harrison, Market and Durability Engineer at the Galvanizers Association of Australia, said in durability analysis for galvanized steel the two key variables are coating thickness and environmental corrosivity.
“The durability is proportional to the coating thickness, so you need to know the coating thickness,” Harrison said. “The other thing you need to know is the corrosivity of your environment.”
Coating thickness, of course, can be determined with relative certainty. But accurately determining the environmental corrosivity is significantly more difficult.
The current method for determining the zone of atmospheric corrosivity described in two Australian standards. AS 4312 describes six different ISO 9223 zones of atmospheric corrosivity in terms of the corrosion rate of steel.
“The tricky part is knowing which zone of corrosivity you’re in,” he said. “The standard classifies different types of coastline in terms of sheltered bays and surf beaches, etc., because surf beaches are going to generate more salt aerosols that will then travel inland.”
“As those aerosols travel they drop out of the atmosphere, so concentration lessens. So, the standard essentially relies on a fairly linear relationship between distance from the coast and the type of coast.”
While this provides a useful, high-level framework, it creates a degree of approximation.
Adding to this challenge is the way durability is calculated. Current standards rely on measures developed from the first-year corrosion rate of zinc, which tends towards over-stating the long-term degradation.
“When zinc corrodes, initially you’ve got a nice, fresh zinc surface that will react with the atmosphere and develop what’s known as a patina – a thin film of corrosion products on the surface,” Harrison said. “Once that patina has developed, the corrosion rate slows down, so the long-term corrosion rate of zinc is less than the first-year corrosion rate.”
Despite this, the standard approach uses the initial, higher corrosion rate as the basis for predicting time to first maintenance.
“That’s very conservative, because that’s based on that first-year corrosion rate,” he said.
Another limitation comes from the treatment of local atmospheric corrosion conditions. Standards acknowledge factors like sheltering, moisture retention and salt accumulation, but they stop short of quantifying effects.
“It will say, for example, that if you are exposed to salt-laden winds but sheltered from rain, it will be more corrosive,” Harrison said. “But it doesn’t say how much more corrosive. You can’t quantify the effect.”
The outcome of these variables is a gap between standardised assumptions and actual performance.
What is the data-driven alternative?
The new GAA corrosion calculator enables location specific durability modelling through the use of meteorological data from local weather stations. Conditions over the most recent 10-year period, using data gathered every three hours, generate the model for an average year.
This data is combined with new modelling of salt aerosol generation, developed through the use of satellite observations of whitecaps on ocean surfaces around Australia. This is integrated with other datasets such as prevailing winds and local topography to develop new, more locally accurate conclusions around atmospheric corrosion effects.
“We’re able to look at the primary drivers,” Harrison said. “We can look at rain washing, we can look at the effects of sheltering. We can move around the structure for a much more granular understanding of corrosivity, not just at a site level, but across different parts of the structure.”
For engineers, the benefit will be in the provision of more accurate durability modelling data.
The free, web-based version on the GAA website, will allow engineers to input a location as an address, as a point on a map or even as a use-my-location option, and then to receive information after defining coating thickness. It is in pre-release testing ahead of an imminent release.
“That will allow you to work out how long your galvanizing is going to last,” Harrison said. “Also, it will tell you what zone of corrosivity you’re in, and it will spit out a list of Building Code of Australia-compliant galvanized coatings for the structural steel.”
A more functional version, for members of the GAA and their customers, will include extra calculations for parts of a structure that might be sheltered from wind or rain, etc.
“I think it will give engineers more confidence around what sort of environment they’re dealing with,” Harrison said.
“The current standards are conservative, but engineers want longer design lives. Now, they’ll be able to achieve that with a good safety margin.”
For more information on the science behind this technology, seeEngineers Australia, Industry Partners Series webinar: mapping corrosion for structural steel in Australia with Professor Ivan Cole, Dr Milan Patel, Dr Mahsa Esfahani and Dr David Harrison
Send an email to [email protected] and we’ll let you know when the Durability App is up and running.
