Science

Crumbling Away: How Climate Change Could Accelerate Corrosion

Critical pieces of infrastructure may fail sooner in the coming decades, warns study

ivil infrastructure is increasingly vulnerable to the effects of climate change. The floods of 2021 saw cities including London, Zhengzhou, and New York temporarily under water. The extreme temperatures of northern hemisphere summer 2022 have led to wildfires, power cuts, buckled rail lines, melting roads, and broken water mains. While here in the southern hemisphere, weeks of stormy winter weather have caused significant damage to roads, beaches, and homes.

But our changing climate is also having a slower, less visible impact on infrastructure. It seems that increases in temperature and humidity are beginning to accelerate corrosion – the process by which strong, shiny metals turn into weak, crumbly metal oxides. Over time, water and oxygen work together to rip electrons away from the atoms of an exposed metal surface, forming other more chemically-stable compounds. The presence of a familiar orange-red color on a surface tells you that there is a ferrous, or iron-containing, metal (like steel) nearby, and that it is starting to rust.

When it comes to materials used in the construction of today’s urban landscapes, steel is always nearby. Sometimes it’s out in the open, in the form of beams, sheets, and rails. In those instances, weathering steel – a material specifically designed to rust in a very slow, controlled manner – tends to be the go-to option. In the right conditions, and with only nominal maintenance, weathering steel can retain its strength for 100 years. A different approach is needed for structures in coastal regions or those that experience deep winters. There, the main enemy is salt, in the form of seawater and de-icing compounds. The chloride ions present in salt speed up the corrosion process, so the main goal is keeping them away from the steel. This is usually done using specialist paints that are regularly reapplied.

Chloride ions are the biggest threat to steel that’s out of sight, too. Embedded within layers of concrete, steel bars provide additional support and strength to a structure. But chloride ions suspended in water can intrude into even fully intact concrete. If there also happens to be oxygen present, corrosion can begin, causing the slow, steady deterioration of the steel. The process speeds up when temperatures and humidity are elevated. Infrastructure in coastal regions is particularly susceptible to this type of degradation.

Another cause of concrete deterioration is carbonation; the slow chemical reaction that can occur between CO2 in the air and compounds commonly used in cement, e.g. calcium hydroxide. This reaction forms carbonates which can actually improve the strength and durability of the concrete. However, it also reduces the concrete’s pH, which makes it easier for chloride to start corroding the steel. Carbonation is highly dependent on the relative humidity of the concrete – below 25%, it’s of almost no concern, but between 50-75%, it can be very problematic. The rate of carbonation is also higher at higher concentrations of atmospheric CO2.

Reinforced concrete and steel are ubiquitous in modern infrastructure – buildings, tunnels, sewers, airport runways, railways, roads, and ports all rely on one or both of these materials. But as a new, open-access paper suggests, they’re increasingly under threat. Writing in the journal Resilient Cities and Structures, the trio of US-based researchers looked at the potential impact of climate change on the deterioration of coastal infrastructure.

The US coastline is home to over 128 million people; that’s 40% of the population. And wherever there are lots of people, there’s a need for infrastructure to support them. In this study, the authors focused on bridges in 223 coastal counties; specifically, the 8,736 concrete and steel bridges that were constructed between 2000 and 2020.

They then considered how much it would cost to replace a bridge in that specific region, based on data from the US Federal Highway Administration. This replacement cost can vary widely between states – the authors write that in 2020, it ranged from $806/m2 in Texas to $13,226/m2 in Hawai’i – so rather than take an average, they used these state-specific figures.

Data for annual air temperature, relative humidity, and wind speed came from NA-CORDEX, the North American arm of an international project called the Coordinated Regional Climate Downscaling Experiment. Its goal is to provide scientifically credible data and “climate scenarios for use by impacts and adaptation researchers and decision-makers.” The authors of this paper referenced two (of four) internationally-recognized future climate scenarios:

RCP 8.5: a high emissions pathway, sometimes referred to ‘business as usual’

RCP 4.5: a pathway that involves taking ‘moderate action’ is taken to mitigate greenhouse gas emissions.**

These scenarios show that in coastal US counties, temperatures will rise by 2100. For RCP 8.5, temperature may increase between 3.1 and 5.6 °C, with higher-latitude regions experiencing a greater increase than lower-latitude regions. For RCP 4.5, the range is between 1.4 and 2.9 °C by 2100. Under both scenarios, humidity is likely to decrease in some southern regions, but increase elsewhere, with wind speed set to increase in lower-latitude regions but decrease in higher-latitude regions.

With all of that data, the authors calculated the rate of deterioration of reinforced concrete under these two climate scenarios. They concluded that the corrosion rate may increase by 0-24 μm/year across the coastal counties by 2100. In terms of what that means for the life of the bridge, they found that under RCP 8.5, high-grade concrete structures designed to last 100 years might now last 97. It might not sound like much, but if taken across all regions, the total loss for concrete bridges could be as much as $251.8 under RCP 8.5.

Structures made from weathering steel will also deteriorate more quickly too; particularly those in regions experiencing increases in temperature and humidity. In some cases, their useful lifetime will shorten by almost eight years. By 2100, carbon steel structures designed to stand for 75 years might only last for 63. The total cost for this loss may reach $628 million under RCP 8.5.

And that’s just bridges in coastal counties in the US. Scale that up to all of the critical infrastructure that relies on these materials, in regions all across the globe…..What you’re left with is a very worrying picture of the future. One that lays bare the lack of resiliency in our infrastructure, and our over-reliance on unsustainable construction materials.

Because don’t forget, the production of these materials comes at an enormous environmental cost – the manufacture of concrete accounts for at least 8% of the world’s CO2 emissions, as well as requiring vast quantities of sand. And as for steel, Mark Peplow said it best in c&en magazine, “If the steel industry were a country, its carbon dioxide emissions would rank third in the world, below the US and above India.”

It’s almost bleakly poetic. The very climate ushered in by these materials is slowly but surely ripping them apart.

**: the numbers 8.5 and 4.5 refer to the degree of ‘radiative forcing’ (in W/m2). This is “a measure of the combined effect of greenhouse gases, aerosols, and other factors that can influence climate to trap additional heat.” The higher the number, the more significant the outcomes for warming.


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