By Wesley Crump
[Note that this article is a transcript of the video embedded above.]
This is the old Howard Frankland Bridge that carries roughly 180,000 vehicles per day across Old Tampa Bay between St. Petersburg and Tampa, Florida. A replacement for the bridge is currently under construction, but the Florida Department of Transportation almost had to replace it decades earlier. The bridge first opened for traffic in 1960, but by the mid-1980s it was already experiencing severe corrosion to the steel reinforcement within the concrete members. After less than 30 years of service, FDOT was preparing to replace the bridge, an extremely expensive and disruptive endeavor. But, before embarking on a replacement project, they decided to spend a little bit of money on a test, a provisional retrofit to try and slow down the corrosion of steel reinforcement within the bridge’s substructure. Over the next two decades, FDOT embarked on around 15 separate corrosion protection projects on the bridge. And it worked! The Howard Frankland Bridge lasted more than 60 years in the harsh coastal environment before needing to be replaced, kept in working condition for a tiny fraction of the cost of replacing it in the 1980s.
The way that bridge in Tampa was protected involves a curiously simple technique, and I’ve built a ridiculous machine in my garage so we can have a corrosion protection shootout and see how it measures up. I’m Grady and this is Practical Engineering. In today’s episode, we’re talking about cathodic protection for corrosion control.
Of all the structural metals in use today, most applications in infrastructure consist of mild steel (just plain old iron and carbon). There are so many applications where steel infrastructure comes into contact with moisture, including bridges, spillway gates, water tanks, and underground pipelines. That means there are so many opportunities for rust to deteriorate the constructed environment. We’re in the middle of a deep dive series on rust, and in the previous video about corrosion, I talked about its astronomical cost, which equates to roughly $1,400 per person per year, just in the United States alone. Of course, we could build everything out of stainless steel, but it’s about 5 times as expensive for the raw materials, and much more difficult to weld and fabricate than mild steel. Instead, it’s usually more cost effective to protect that mild steel against corrosion, and there are a number of ways to do it. Paint is an excellent way to create a barrier so that moisture can’t reach the metal, and I’ll cover coatings in a future video. But, there are some limitations to paint, including that it’s susceptible to damage and it’s not always possible to apply (like for rebar inside concrete). That’s where cathodic protection comes in handy.
Let me introduce you to what I am calling the Rustomatic 3000, a machine you’re unlikely to ever need or want. It consists of a tank full of salt water, and a shaft on a geared servo. These plastic arms lower steel samples down into the saline water and then lift them back up so the fan can dry them off, hopefully creating some rust in the process. Corrosion is an electrochemical process. That just means that it’s a chemical reaction that works like an electrical circuit. The two individual steps required for corrosion (called reduction and oxidation) happen at separate locations. This is possible because electrons can flow through the conductive metal from areas of low electric potential (called anodes) to those of high potential (called cathodes). As the anode loses electrons, it corrodes. This reaction is even possible on the same piece of metal because different parts of the material may have slightly different charges that drive the corrosion cell.
However, you can create a much larger difference in electric potential by combining different metals. This table is called the galvanic series, and it shows the relative inertness or nobility (in other words, resistance to corrosion) of a wide variety of metals. When any two of these materials are joined together and immersed in an electrolyte, the metal with lesser nobility will act as the anode and undergo corrosion. The more noble metal becomes the cathode and is protected from corrosion.
You can see that steel sits near the bottom of the galvanic table, meaning it is less noble and more prone to corrosion. But, there are a few metals below it, including some commonly available ones like Aluminum, Zinc, and Magnesium. And wouldn’t you know it, I have some pieces of Aluminum, Zinc, and Magnesium here in my garage that I attached to samples of mild steel in this demo. We can test out the effects of cathodic protection in the rustomatic 3000. Each time the samples are lifted to dry, the arduino controlling the whole operation triggers a couple of cameras to take a photo. One of the samples is a control with no anode, then the other three have anodes attached consisting of magnesium, aluminum, and zinc from left to right. I’ll set this going and come back to it in a few minutes your time, three weeks my time.
One application of cathodic protection you might be familiar with is galvanizing, which involves coating steel in a protective layer of zinc. The coating acts kind of like a paint to physically separate the steel from moisture, but it also acts as a sacrificial anode because it is electrically coupled to the metal. Galvanizing steel is relatively inexpensive and extremely effective at protecting against corrosion, so nearly all steel structures exposed to the environment have some kind of zinc coating, including framing for buildings, handrails, stairs, cables, sign support structures, and more. Most outdoor-rated nails and screws are galvanized. You can even get galvanized rebar for concrete structures, and there are applications where it is worth the premium to extend the lifespan of the project.
But because it’s normally a factory process that involves dipping assemblies into gigantic baths of molten zinc, you can’t really re-galvanize parts after the zinc has corroded to the point where it’s no longer protecting the steel. Also, in aggressive environments like the coast or cold places that use deicing salts, a thin zinc coating might not last very long. In many cases, it makes more sense to use an anode that can be removed and replaced, like I’ve done in my demonstration here. Cathodic protection anodes like this are used on all kinds of infrastructure projects, especially those that are underground or underwater.
I let this demonstration run for 3 weeks in my garage. Each cycle lasted about 5 minutes, meaning these samples were dipped in salt water just about 6,000 times. And here’s a timelapse of those entire three weeks. Correct me if you find something better, but I think this might be the highest quality time lapse video of corrosion that exists on the internet.
It’s actually really pretty, but if you’re the owner of a bridge or pipeline that looks like this sample on the left, you’re going to be feeling pretty nervous. You can see that the unprotected steel rusts far faster than the other three and the rust attacks the sample much more deeply. The sample with the magnesium looks like it was most protected from corrosion, but watch the anode. It’s nearly gone after just those three weeks, and that makes sense. It’s the least noble metal on the galvanic series by a long shot. The samples with aluminum and zinc anodes do experience some surface corrosion, but it’s significantly less than the control.
In fact, this is exactly how the lifespan of the Howard Frankland bridge in Tampa was extended for so long. Zinc was applied around the outside of concrete girders and in jackets around the foundation piles, then coupled to the reinforcing steel within the concrete so it would act as a sacrificial anode, significantly slowing down the corrosion of the vital structural components.
Here’s a closeup of each sample after I took them down from the Rustomatic 3000, and you can really see how dramatic the difference is. The pockets of rust on the unprotected steel are so thick compared to the minor surface corrosion experienced by the samples with magnesium, aluminum, and zinc anodes. The anodes went through some pretty drastic changes themselves. After scraping off the oxides, the zinc anode is nearly intact, and you can even see some of the original text cast into the metal. The aluminum anode corroded pretty significantly, but there is still a lot of metal left. On the other hand, there’s hardly anything left of the magnesium anode after only three weeks. And here’s a look at the metal after I wire brushed all the rust off each sample. The difference in roughness is hard to show on camera, but it was very dramatic to the touch. There’s no question that the samples with cathodic protection lost much less material to corrosion over the duration of the experiment.
There’s actually one more trick to cathodic protection used on infrastructure projects. Rather than rely on the natural difference in potential between different materials, we can introduce our own electric current to force electrons to flow in the correct direction and ensure that the vulnerable steel acts as the cathode in the corrosion cell. This process is called impressed current cathodic protection. In many places, pipelines are legally required to be equipped with impressed current cathodic protection systems to reduce the chance of leaks which can create huge environmental costs. The potential between the pipe and soil is usually only a few volts, around that of a typical AA battery, but the current flow can be in the tens or hundreds of amps. If you look along the right-of-way for a buried pipeline, especially at road crossings, you can often see the equipment panels that hold rectifiers and test stations for the underground cathodic protection system. The Howard Frankland bridge also had some impressed current systems in addition to the passive protection to further extend its life, proving a valuable lesson we learn over and over again.
The maintenance and rehabilitation of existing facilities is almost always less costly, uses fewer resources, and is less environmentally disruptive than replacing them. You don’t need a civil engineer to tell you that an ounce of prevention is worth a pound of cure (or the whatever the metric equivalent of that is). It’s true for human health, and it’s true for infrastructure. Making a structure last as long as possible before it needs to be replaced isn’t just good stewardship of resources. It’s a way to keep the public safe and prevent environmental disasters too. Corrosion is one of the number one ways that infrastructure deteriorates over time, so cathodic protection systems are an essential tool for keeping the constructed environment safe and sound.
Watch Video At: Practical Engineering.