The last six months have been the driest on record for Perth, leading to a rise in the number of bushfires and putting vast areas of vegetation under high levels of stress.
Meteorologists predict the drying trend will continue throughout South East Queensland for years to come, likely due to climate change.
Dam levels are well below seasonal averages, but, for the thousands of businesses and residents with properties in coastal areas, there’s another serious consequence of the lack of rainfall: damaging salt corrosion.
“If there’s no rain to wash off the salt that’s blown from the ocean, areas of exposed steel are at an elevated risk of corrosion and rust,” said Dr David Harrison, Market and Specifications Engineer for the Galvanizers Association of Australia (GAA). Lower humidity can decrease corrosion rates, but conversely, it can also allow salt aerosols to travel further inland, thus increasing corrosion.
In coastal areas, galvanized steel that gets regularly washed by rainwater is less at risk from corrosion than steel that’s protected from the elements.
The two posts pictured above, manufactured from pre-galvanized strip, are within a few metres of each other, but the one on the left is exposed to rainfall while the other is sheltered from rainfall but still exposed to the prevailing winds.
“You can see the effect of unprotected cut edges and how much more corrosion occurs when salt isn’t regularly washed off,” Harrison said. “So prolonged droughts or changing climates can have a significant impact.
“It’s a great illustration of how environmental and structural factors affect how quickly the steel breaks down, and how important it is for engineers to understand how the metal will perform in different locations over long periods of time.”
Dramatic differences in corrosion
GAA has a dedicated corrosion testing site at Belmont, a suburb of Newcastle, New South Wales, where the gradual degradation of galvanized products is monitored over several years.
The dramatic effects can even be seen on two parts of the same object.
The bracket pictured above is coated in a zinc aluminium magnesium (ZM) alloy that has effectively protected the part regularly washed by rain (top photo), while the surfaces underneath have experienced corrosion. As well as looking at the effects of sheltering, these tests are designed to look at the effects of exposed cut edges and bolted connections on durability.
It’s therefore critical for engineers to factor in local corrosion risks when choosing the type and thickness of steel coating for every construction project.
That process is set to become a lot easier thanks to the Interactive Corrosion Map of Australia, developed by GAA in conjunction with CSIRO and RMIT University. It not only calculates salt deposition from the nearest coast but considers other factors such as weather patterns, rain washing and the degree to which it’s shielded by tall buildings.
The GAA testing site has revealed that developers and design engineers must also assess the accuracy of claims made by steel manufacturers.
“It’s shown that some of those claims simply don’t stand up to scrutiny,” Harrison said. “We’ve found that some brands carry out accelerated performance testing on light gauge products and then claim similar results for thicker steel sections, ignoring the effect of the greater uncoated area on the thicker steel section.
“So it might be that a ZM coating that works well on a 0.8mm gauge steel sheet fails prematurely on a 5mm gauge structural section. We’ve found several products that didn’t live up to the claims made in their advertising.”
Danger of exposed edges
A common problem with all pre-galvanized coatings is exposed, uncoated edges. Manufacturers of ZM coatings claim a “self-healing” effect, where a zinc-based film containing magnesium migrates over the cut-end, thus protecting it from corrosion.
If the sheet of metal is only one or two mm thick, then self-healing usually occurs, although problems arise when it’s thicker, as is the case with those pictured below, where the area is too great for the migrating species to cover so the adjacent coating corrodes as a faster rate. Batch hot dip galvanizing to AS/NZS 4680 avoids this problem by providing complete coating coverage of the article.
Another reason why the GAA app will be so critical to the industry is that it advises on sheltering — something not covered by building codes.
“The Interactive Corrosion Map of Australia builds in additional sophistication that will allow engineers to make informed decisions. As our testing facility has demonstrated over the last nine years, it’s crucial to choose the optimal galvanized steel for every project, based on comprehensive, long-term testing and up-to-date climate data.”
As other parts of Australia experience the changes in climate that Perth is currently enduring, the new map and the findings from Belmont will become even more critical for infrastructure and design engineers in choosing the most appropriate galvanized steel for every construction project.
Find out more about how GAA helps engineers assess corrosion rates and galvanizing solutions.
