Cathodic protection is an effective method to mitigate corrosion. To illustrate, I’ll go over a solar farm cathodic protection calculation.
We’ll calculate the number of required anodes per stanchion, on which the solar panels will mount.
Before we begin though, we’ll discuss the details of cathodic protection. This will give you a better understanding of the calculation.
What is cathodic protection?
It’s a technique used to control the corrosion of a metal surface. The goal is to turn the metal you want to protect into a cathode.
When the metal becomes a cathode, you prevent metal oxidization, as oxidation only happens to an anode.
It’s also important to know, rust is the result of corroding metal. More specifically, when you expose the metal to any one of the following:
- Moist air
- Moist soil and even moist concrete
Important Note: The deterioration of metals is caused by electron transfer, known as corrosion. There are two types of corrosion processes: oxidation and reduction.
Oxidation occurs when a metal atom loses electrons in a chemical reaction with oxygen, resulting in the formation of a metal oxide. On the other hand, reduction is the transfer of electrons from a metal to another material.
We define a cathode and anode as follows:
- Anode (active site): the metal, which loses electrons during a chemical reaction
- Cathode (less active site): the metal, liquid, or gas, which gains electrons during a chemical reaction
Next, for corrosion to occur, the following three things must be present:
- Two different types of metal, such as steel and aluminum
- A medium (electrolyte) such as seawater and the Earth, which allows ions to flow and transport electric charges
- An electrical connection between the cathode and anode, enabling current flow between the metals
Important Note: the two different metals can be separate metals. But also, a single piece of metal with metallurgical differences on the surface will work. These differences can be macroscopic, resulting in non-uniformity on the metal surface. For example, a single pipe can have an anode part and a cathode part.
Imagine a pipe half dipped in water with low oxygen concentration. The dipped half becomes the anode, while the other half in high oxygen concentration becomes the cathode.
How does cathodic protection prevent corrosion?
Cathodic protection prevents corrosion by converting all anodic (active) sites on metal to cathodic (passive) sites. This is done by pumping electrons onto the metal, which needs protection. It can be a large steel pipe or the hull of a ship, it doesn’t matter.
Additionally, there are two different cathodic protection methods. Each has its own advantages and disadvantages in real-world applications.
Method #1: sacrificial anode
The anode is a metal, which is more reactive than the protected metal. When a metal is more reactive, it loses electrons more easily and forms ions.
For example, when protecting iron, you’ll use a more reactive metal than iron such as zinc or magnesium.
Important Note: the additional metal source is also called a sacrificial anode. Because the galvanic anode sacrifices itself to protect another metal from corrosion.
What happens next is the sacrificial anode oxidizes in the electrolyte, which can be soil or the ocean. The oxidation generates electrons.
These electrons then flow to the metal you want to protect, forcing it to become a cathode. Additionally, the incoming electrons heal the protected metal by causing any oxidized parts of the cathodic metal to reduce and return to their original state.
The presence of the electrons will cause the ferrous ions to turn back into iron solid.
Important Note: the sacrificial material needs to oxidize before the protected material. Otherwise, cathodic protection won’t work.
Method #2: impressed current protection
This method is similar to the sacrificial anode method. Except, we use an external power source to generate the electric current.
We pump electrons onto the protected metal through an alternate power source. For example, using a DC power supply, connect the negative end to the protected metal. This makes the protected metal a cathode, thus protecting it from oxidization.
The cool thing is, the anode material can be anything as long as it’s electrically conductive. For example, iron would work, while plastic wouldn’t.
An important benefit of this approach is you don’t need to replace the anode. Whereas with the sacrificial anode method, you need to replace the anode when it corrodes.
Important Note: a big difference between the two methods is oxidation. In the anode of a sacrificial system, the metal oxidizes. Whereas in an impressed current system, the water oxidizes.
Example of cathodic protection with a ship
Imagine a large ship traveling through the ocean. The ship sits in an electrolyte solution of saltwater, the ocean. The goal is to protect the ship’s steel from corrosion.
To do this, a zinc anode is dropped into the water and connected electrically to the hull of the ship. The reactive metal zinc oxidizes and produces electrons, which are then pumped onto the hull of the ship, making it a cathode.
Now assume the ship’s hull’s material is iron (Fe) and it has already started to oxidize. The anode’s electrons will reduce the Fe ions, by forming Fe atoms again. This prevents corrosion, as only Fe bonds can bond to other atoms and cause corrosion.
Important Note: Zinc anode oxidation:
Assume some iron has oxidized on the ship’s hull. The below chemical reaction shows the Fe ions capturing the two electrons from zinc. In return, we again have solid iron, which is what we want!
Now, assume iron hasn’t oxidized on the ship’s hull. Instead of reducing Fe ions back to Fe solid, a different reaction occurs. We reduce water and oxygen into with the addition of the electrons. The electrons need to go somewhere.
Your water heater and submarines
Your water heater works using cathodic protection. If you replace your water heater’s sacrificial anode every couple of years, your unit will last longer.
What’s more, submarines use cathodic protection. It’s how these large engineering marvels can safely stay underwater for so long.
In short, you use cathodic protection when the environment around a metal acts as an electrical conductor.
Cathodic protection calculation for a solar farm
We want to install 378 steel stanchions for our solar farm, and they need corrosion protection.
First, we calculate how much current we need to output from each anode. Or as we learned, the number of electrons, which will flow out from an anode.
The required current output for the protection of embedded steel structures we define as:
: factor for uncoated steel is 150,000
f: anode factor is 1.90 for a 42lb long shape 3″ x 3″ x 72″ magnesium anode
Y: structure to soil potential correction factor for standard -0.85 volt differential is 1.0
: soil resistivity in ohm-centimeters = 1951 ohm-centimeters for our location per test report
: current output in milliamperes
= 146.08 milliamps output per anode
Our install location has low-resistivity (high corrosion) soil. And in our application, we’ll require a current of 15 milliamperes/ of surface.
The total embedded surface area for 378 stanchions is 1,669,920 or 11,597
So, (11,597 ) x (15 x Amps/) = 173.95 Amps
I = 173.95 Amps (total current requirement) = 173,950 milliamps
Based on a 10-year life, the total weight of anodes is given by the following equation:
W: weight of anodes
: projected life in years
I: total current required in milliamps
= 40,633 pounds of magnesium anodes
Next, we calculate the number of anodes required:
= 967.5 anodes
The number of anodes per stanchion calculates to:
With 3 anodes per stanchion = 3 x 378 = 1134 total anodes
= 11.72 years anticipated life
Important Note: anodes are 3-inch x 3-inch x 72-inches, installed in 6-inch diameter holes, which are by 72-inches in length. Additionally, you install the anodes approximately 10-feet from the stanchion they’re protecting.
General rules of thumb with sacrificial anode system designs
drives the amount of electric current an anode can discharge. This is Ohms’s Law!
I: the flow of current in amps
R: circuit resistance
V: voltage difference between the anode and cathode
In application, the current is high at first due to the large voltage difference between the anode and cathode. Over time, however, the potential difference drops as more current travels from the anode to the cathode. Eventually, the current decreases with the polarization of the cathode.
Finally, regarding circuit resistance, we’ll go back to our ship example. The path through the saltwater and metal, and any connected cables, make up the resistance.
Important Note: the following are general rules of thumb with anodes:
- The length of an anode determines the amount of generated current. This in turn determines the area of metal, which you can protect.
- The cross-sectional area of an anode, determined by its weight, determines how long you can protect a metal.
Cathodic protection calculation wrap up
Without cathodic protection, the world we live in today would be much different. Because we constantly install metals in corrosive environments, such as ships traveling across the oceans.
It’s always surprising how much goes into every installation, which isn’t visible to the naked eye.
What are your thoughts on the cathodic protection calculation? Which type of cathodic protection do you most commonly see installed?
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Koosha started Engineer Calcs in 2020 to help people better understand the engineering and construction industry, and to discuss various science and engineering-related topics to make people think. He has been working in the engineering and tech industry in California for over 15 years now and is a licensed professional electrical engineer, and also has various entrepreneurial pursuits.
Koosha has an extensive background in the design and specification of electrical systems with areas of expertise including power generation, transmission, distribution, instrumentation and controls, and water distribution and pumping as well as alternative energy (wind, solar, geothermal, and storage).
Koosha is most interested in engineering innovations, the cosmos, our history and future, sports, and fitness.