What do water towers, storage tanks, and oil pipelines have in common? They’re all giant objects made with a lot of metal.
Normally, if you leave a piece of metal outside, within a couple years rust and corrosion have eaten away at it. Yet, after decades of use, those massive metal containers, pipelines, and structures show minimal signs of damage.
Is it magic? Not really. What you’re seeing is an electro-chemical process called cathodic protection. This preservation method makes it possible to extend the lifespan of metals far beyond what they would experience without it.
Cathodic protection uses a sacrificial metal to protect steel or other alloys by giving up its electrons. As it sheds electrons, the anode prevents the metal from corroding.
You’ll typically see cathodic protection used with steel products. This is because as soon as steel products are produced, they want to degrade into their original components. Steel and other alloys corrode by shedding electrons, creating rust pockets on the surface of the metal. Over time, the rust flakes off, and the metal returns to its natural form.
Anodes are placed directly on the structure or connected using a cathodic protection cable. Over time, instead of the metal shedding its electrons, the anode gives up its electrons to replenish what’s lost by the structure.
Although the process does a good job of slowing down corrosion, it doesn’t stop it entirely. Everything breaks down eventually, but cathodic protection gives the structures a much longer lifespan than what they would have without it.
There are two types of cathodic protection used to protect metals: galvanic or impressed current. Both accomplish the same goal but take slightly different approaches to do it.
Galvanic Cathodic Protection – This method requires attaching metal pieces directly to the structure that needs protecting. For the electron flow to occur, the sacrificial metal has to be more electro-negative than the steel.
Magnesium, zinc, and aluminum are commonly used as protective metals because they can shed electrons easily.
Impressed Current Cathodic Protection (ICCP) – An anode is connected to the metal it needs to protect via a cable. Electricity is run through the anode to the steel or other metal, creating a complete circuit and protecting the metal from corrosion.
Unlike galvanic protection, ICCP doesn’t require a more electro-negative metal to be used. It’s also more cost-effective than galvanic protection, though you’ll typically find the two methods used together for optimal protection.
For cathodic protection to work, several pieces are needed to care for metals that have been buried or submerged in water.
Anode – In cathodic protection, this material gives up its electrons to protect the metal from corrosion.
Cathode – This is the metal that will receive the electrons from the anode.
Electrolyte – This could be soil, water, or other materials between the anode and cathode. For the best results, the electrolyte should be low resistivity.
Path to complete the circuit – This is the wire used to connect the anode to the cathode, completing the circuit and allowing electrons to flow.
Although the methods may differ between galvanic and IC cathodic protection, each allows for electrons to go from one metal to another. The result, if done correctly, is a well-protected pipeline designed to last for decades.
Cars in the Northeast rust faster than those in the Southwest. Though the makes and models may be the same, their environments are drastically different.
Soil in the Northeast has low resistivity and high moisture content, which corrodes metals faster. Using our car analogy, roads in New York are covered with snow for several months out of the year. Salt is used to clear the snow and ice, but the combination leads to much faster corrosion.
The story is the exact opposite in Arizona, where dry air and soil offer higher resistivity and lower moisture content. Electrons have a harder time flowing through the soil, leading to slower corrosion rates.
Acidity and temperature also impact how quickly corrosion takes place. Acids speed up the process by drawing ions from the metal. High temperatures also increase corrosion.
Remember how we mentioned salt being a problem for cars? Salt is one of many natural electrolytes found in soil and water that interact with metals to pull ions from them.
Lastly, the type of metal we use for the structures can determine how corrosion works. Iron and steel corrode faster than aluminum and copper. Other metals, like gold and silver, don’t corrode at all. But good luck finding a solid gold storage tank!
The goal of cathodic protection is to slow corrosion as much as possible, so we need to ensure every metal surface is protected.
The first step is to use a coating to protect the outside of the metal. Although the coating typically does an admirable job, it isn’t perfect. Scrapes, abrasions, and dings are almost guaranteed to occur while the structure is being installed, which could potentially remove some of the coatings. When bare metal is exposed, it eventually creates holes in the pipe or metal structure called “holidays.” Over time, the holidays create issues for exposed metal as corrosion occurs.
The second step is to create a cathodic protection system using cables to connect the sacrificial anode to the cathode. When combined with the coating applied in step one, crews establish a cost-effective method of corrosion protection for decades to come.
Companies spend a lot of money on cathodic protection methods, but how do they know they work?
To check the status of buried metals like pipelines, workers use test stations or coupons to simulate what’s happening to the metal underground. For underground utilities, test stations are often plastic tubes with multi-colored heads.
Inside those tubes, a piece of metal is subjected to the same conditions found underground, generating real-time data to determine how fast corrosion is happening. Depending on how corroded the coupon is, it’s possible to get a general idea of how the pipe is doing underground.
Replacing underground utilities is expensive. Current pipeline construction hovers around $1 million per mile… just for the pipe.
On top of the cost of the pipeline, there could be additional costs if there are issues caused by a leak or poor maintenance. Fines can stretch into the million-dollar range, and clean ups tend to be expensive and arduous.
So, not only is it expensive to replace corroded pipelines and other structures, but if there’s a leak or other problem, it quickly compounds costs. Additionally, there are hundreds of thousands of miles of underground pipelines across the U.S. – some of which have been buried for decades. In that time, cities have been built on top of them, making it an incredible hassle to replace them.
Thankfully, companies can retrofit utilities with cathodic protection to get as much use as possible before replacement.
With more than 3.3 million miles of regulated pipelines and nearly 17,000 natural gas storage wells, it makes sense to have an agency controlling how those utilities are maintained. This is where the Pipeline and Hazardous Materials Safety Administration (PHMSA) comes in.
PHMSA ensures underground utilities are in working order and levies fines for unsafe situations. The organization is also in charge of developing new rules to maintain environmental and human health standards.
No matter what type of active metal structure you want to save, cathodic protection is a cost-effective way to get it done.
Despite some limitations for assets that have already been buried or submerged, retrofitting them with ICCP cathodic protection systems can go a long way toward protecting what is still there.