By Wesley Crump
[Note that this article is a transcript of the video embedded above.]
In July of 1995, Folsom Lake, a reservoir created by Folsom Dam in Northern California, reached its full capacity as snow continued to melt in the upstream Sierra. With the power plant shut down for maintenance, the dam’s operator needed to open one of the spillway gates to maintain appropriate flow in the river below. As the gate began to rise, one side suddenly collapsed and swung open, allowing an uncontrolled torrent of water to flow past the gate down the spillway. With no way to control the flow, the water level of Folsom Lake began to drop… and drop and drop. By the time the deluge had slowed enough that operators could block the opening, nearly half the water stored in Folsom Lake had been lost.
Forensic investigation of the failure revealed that the gate malfunctioned because of corrosion of its pivot mechanism, called the trunnion, creating excessive friction. Essentially, the gate was stuck at its hinges. When the hoist tried to raise it, instead of pivoting upwards, the struts buckled, causing the gate to collapse. This gate operated flawlessly for 40 years before the failure in 1995. However, corrosion is an insidious issue. Because it occurs gradually, it’s hard to know when to sound the alarms. But, there are alarms to sound!
It’s been estimated that we lose roughly two-and-a-half trillion dollars per year globally because of the collective corrosion of the things we make and build. That is a colossal cost for a simple chemical reaction, and there’s an entire field of engineering dedicated to grappling with the problem. So, this is the first in a series of videos on corrosion engineering. Make sure you subscribe to catch them all. You probably don’t have a line item in your household budget for rust, but you might add one after this video. I’m Grady, and this is Practical Engineering. In today’s episode we’re talking about corrosion engineering for infrastructure.
It will come as no surprise to you that we build a lot of stuff out of metal. Entire periods of human civilization are named after the kinds of metals we learned to use, like the bronze age and the following iron age. These days nearly every humanmade object is made at least partly of metal or in a metallic machine, from devices and vehicles to the infrastructure we use everyday, including bridges, pipelines, sewers, pumps, tanks, gates, and transmission towers. Metals are particularly useful for so many applications, and we humans have invented a plethora of processes (like smelting, refining, and alloying) to assemble metallic molecules in various ways according to our needs. But, mother nature is resolved to dismantle (in due course) the materials we create through a process called corrosion. It seems so self-evident that structures deteriorate over time that it might not seem worth the fuss to worry about why. But, infrastructure is expensive and we all pay for it in some way or another, so we need it to last as long as possible. Not only that, but the failure of infrastructure has consequences to life safety and the environment as well, so keeping corrosion in check is big business. But what is corrosion anyway?
You’re here for engineering, not chemistry, so I’ll keep this brief. Corrosion is an electrochemical descent into entropy: a way for mother nature to convert a refined metal into a more stable form (usually an oxide). Corrosion requires four things to occur: an anode (that’s the corroding metal), a cathode (the metal that doesn’t corrode), a path for the electrical current between the two, and an electrolyte (typically water or soil) to complete the circuit. And the anode and cathode can even be different areas of the same piece of metal with slightly different electrical charges. The combination of these elements is a corrosion cell, and the process that corrode metals in nature are nearly identical to those used in batteries to store electricity. In short, corrosion is a redox (that is, reduction-oxidation) reaction, which means electrons are transferred, in this case from the metal in question to a more stable (and usually much less useful) material called an oxide. For corroded iron or steel, we call the resulting oxide, rust.
Here’s a little model bridge I made from steel wires in a bath of aerated salt water. I added a little bit of hydrogen peroxide to speed up the process so you could see it clearer on camera. This timelapse ran for a few days, and the corrosion is hard to miss. Of course, we don’t keep our bridges in aquariums full of salt water and hydrogen peroxide, but we do expose our infrastructure to a huge variety of conditions and configurations that create many forms of corrosion.
You’re probably familiar with uniform corrosion that happens on the surface of metal, like the beautiful green patina of copper oxides and other corrosion compounds covering the Statue of Liberty. But corrosion takes many forms, and corrosion engineers have to be familiar with all of them. These engineers know the common design pitfalls that exacerbate corrosion like not including drainage holes, leaving small gaps in steel structures, and mixing different types of metals. Corrosion can occur from the atmosphere or simply by allowing dissimilar metals to contact one another, called galvanic corrosion. Even using an ordinary steel bolt on a stainless steel object can lead to degradation over time. Corrosion can happen in crevices, pits, or between individual grains of the metal’s crystalline structure. Even concrete structures are vulnerable to corrosion of the steel reinforcement embedded within. When rebar rusts, it expands in volume, creating internal stresses that lead to spalling or worse.
Just as there are lots of kinds of corrosion, there are also many, many professionals with careers dedicated to the problem. After all, the study of corrosion and its prevention is a topic that combines various fields of chemistry, material science, and structural engineering. There’s even a major professional organization: the AMPP or Association for Materials Protection and Performance, that offers training and certifications, develops standards, and holds annual conferences for professionals involved in the fight against corrosion. Those professionals employ a myriad of ways to protect structures against this insidious force, that I’ll cover in this series.
One of the simplest tools in the toolbox is just material selection. Not all metals corrode at the same rate or in the same conditions, and some barely corrode at all. Gold, silver, and platinum aren’t just used in jewelry because they’re pretty. These so-called noble metals are also prized because they aren’t very reactive to atmospheric conditions like moisture and oxygen. But, you won’t see many bridges built from gold, both because it’s too expensive and too soft.
Steel is the most common metal used in structures because of its strength and cost. It simply consists of iron and carbon. Steel is easy to make, easy to machine, easy to weld, and quite strong, but it’s also one of the materials most susceptible to corrosion. I’ve got another demonstration set up here in my garage. This is a tank full of salt water, a bubbler to keep the water oxygenated, and a few bolts made from different materials. I’ll let the time lapse run, and let you guess which bolt is made from steel. It doesn’t take long at all for that characteristic burnt orange iron oxide to show up. Even the steel bolt to the left that has a protective coating of zinc is starting to rust after a day or two of this harsh treatment. That humanmade protective layer on the galvanized bolt gives a hint about why the other ones shown are able to avoid corrosion in the saltwater. Unlike iron oxide that mostly flakes and falls off, there are some oxides that form a durable and protective film that keeps the metal from corroding further. This process is called passivation. Metals that passivate are corrosion resistant precisely because they’re so reactive to water and air.
In my demo I included several metals that undergo passivation, including an aluminum bolt (or aluminium for the non-north-americans), which is typically quite corrosion resistant in air, but struggled against the saltwater. I also included a bronze bolt which is an alloy of copper and (in this case) silicon. Finally, I included two types of stainless steel, created by adding large amounts, sometimes as much as 10%, of chromium and nickel to steel. There are two major types of stainless steel, called 304 and 316 in the US. 316 is more resistant to saltwater environments, but I didn’t really notice a difference between the two over the duration of my test.
I should also note that there are even steel alloys whose rust is protective! Weathering steel (sometimes known by its trade name of Corten Steel) is a group of alloys that are naturally resilient against rust because of passivation. A special blend of elements, including manganese, nickel, silicon, and chromium don’t keep the steel from rusting, but they allow the layer of rust to stay attached, forming a protective layer that significantly slows corrosion. If you keep an eye out, you’ll see weathering steel used in many structural applications. One of my favorite examples is the Pennybacker bridge outside of Austin. The U.S. Steel Tower, the tallest building in Pittsburgh, Pennsylvania, was famously designed to incorporate corten steel in the building’s facade and structural columns. Rather than fireproof the columns with a concrete coating, the engineers elected to make them hollow and fill them with fluid so the corten steel could remain exposed as an exemplification of the material. Corten steel is in wide use today. Architects love the oxidized look, engineers love that it’s just as strong as mild steel and almost as cheap, and owners love not having to paint it on a regular schedule. That saves a lot of cost. In fact, the cost of corrosion is the main point I want to express in this video.
In 1998, the Federal Highway Administration conducted a 2-year study on the monetary impacts of corrosion across nearly every industry sector, from infrastructure and transportation to production and manufacturing. They found that the annual direct costs of corrosion in the U.S. made up an astronomical $276 billion dollars, over three percent of the entire GDP. Assuming we still spend roughly as much today, that amounts to over 1,400 dollars per person per year, more than the average American spends on gasoline! Of course, you don’t get a monthly rust bill. Corrosion costs show up in increased taxes to pay for infrastructure; increased rates for water, sewer, electricity, and natural gas; increased costs of goods; and shorter lifespans for the metal things you buy (especially vehicles). But corrosion has costs that go even beyond money.
In 2014, the City of Flint Michigan began using water from the Flint River as their main source of drinking water to save money. The river water had a higher chloride concentration than the previous supply sourced from Lake Huron, making it more corrosive. Many cities add corrosion inhibitors to their water supply to prevent decay of pipe walls over time, but the City of Flint decided against it, again to save on costs. The result was that water in the city’s distribution system began leaching lead from aging pipes, exposing residents to this extremely dangerous heavy metal and sparking a water crisis that lasted for 5 years. A public health emergency, nearly 80 lawsuits (many of which are still ongoing), government officials fired and in some cases criminally charged, and upwards of 12,000 kids exposed to elevated levels of lead all resulted because of poor management of corrosion. Sadly, it’s just a single example in a long line of infrastructure problems caused by corrosion. Metals are so necessary and important to modern society that we’ll never escape the problem, but the field of corrosion engineering continues to advance so that we can learn more about how to manage it and mitigate its incredible cost.
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