Secure Your Metal Infrastructure With Cathodic Protection

Steel infrastructure is all around us—everywhere we look, we see pipelines, buildings, bridges, and infrastructure (to name a few) designed from iron-based materials in order to make our lives safe and secure. But what’s keeping your infrastructure from corroding?

Anything that is exposed to the elements is at risk of being broken down. To protect your infrastructure, you need cathodic protection.

What is cathodic protection?

When steel rusts, it gives up electrons to oxygen and forms an iron oxide. This process is accelerated by the presence of an electrolyte (in nature, the electrolyte is typically water with minerals and/or salt.)

One way to protecting steel from rusting is to provide a source of electrons to the steel structure. This type of protection is called cathodic protection, because a cathode is by definition a part of an electric system which receives negatively charged electrons. The steel structure to be protected is the cathode.

When steel structures are in contact with an electrolyte, typically underground or underwater structures, they need to be protected from corrosion by cathodic protection. There are two types of cathodic protection: passive cathodic protection and impressed current cathodic protection (ICCP).

Passive cathodic protections work by attaching a metal anode, which corrodes easier than the steel in the structure to be protected. Zinc is a metal commonly used as an anode—it corrodes and gives up electrons to the steel structure until the zinc metal is gone, at which point the steel starts to corrode. In this case, the zinc anode is a sacrificial electrode.

Passive cathodic protection is less expensive and less complicated. The disadvantages are that the sacrificial anode does eventually disappear and must be replaced. In stronger electrolytes, like sea water, the zinc will disappear faster and must be replaced more often.

The ICCP method is used to protect the steel structure by providing electrons to the structure by a direct current (DC) power supply powered by alternating current (AC) source. The DC power supply is connected on one side to the cathodic steel structure and on the other side to the buried anode, also called the groundbed.

The power supply acts as a pump, moving electrons from the anode to the cathode through the electrical wires. The surplus of electrons at the cathode prevents the steel from giving up its electrons to oxygen—thereby preventing it from forming iron oxide (rust)—and protecting the steel structure from corrosion.   

Because the lack of an electrolyte, steel structures in air do not normally require any other protection from oxygen and water besides being painted or galvanized. Galvanizing is a process where the steel is coated with zinc (which is actually a type of passive cathodic protection, too.)

Having a cathodic protection system, which transports corrosion to a sacrificial piece of metal buried away from a pipeline, for example, protects your infrastructure.

How can you ensure your infrastructure is secured by cathodic protection?

Cathodic protection design requires that you perform a ground resistivity soil test in order to determine what kind of cathodic protection is needed and how to design and where to locate the groundbed.

The soil resistivity determines the degree of corrosivity the soil can hold. For example, soil can be classified as follows, where ρ is the soil resistivity:

• ρ > 100 Ωm: slightly corrosive
• 50 < ρ < 100 Ωm: moderately corrosive
• 10 < ρ < 50 Ωm: corrosive
• ρ < 10 Ωm: severe

With AGI equipment, you can seamlessly measure soil resistivity—our SuperSting tools and EarthImager modeling software will produce a map of the resistivity distribution under the site. This map makes it easy to locate the ideal location for the groundbed.

Tools Designed For So Much More

Additionally, you may use resistivity to map the ground to find depth to bedrock, for example, or to locate cavities in the ground in order to know where to put up a building or a bridge foundation, etc. There are two ways to do this:

1. The typical, old-school method uses the ASTM G57 soil test or the vertical electrical sounding (VES) method: Do a sounding around a central point, and try to get an idea of the ground with the limited data you get.
2. At AGI, we offer a better, more efficient way to do it: Simply set out whatever specific number of electrodes you need for the given survey— 20, 50, 100, or more—and perform automatic soundings right next to each other. With this method, you can use the EarthImager processing package to create a cross-section of the earth under the survey line, showing the underground resistivity distribution instead of getting only one point of data—with your 2D or 3D scan, you’ll be able to choose the best location of many instead of going with the first one you find.

Quite simply, AGI’s tools are value-added equipment. For the same cost and time you’d expend in the field, our automatic system will give you a long 2D transit that allows you to determine where to build or in the case of ICCP where to find the best location for the anode ground bed.

Other Use Cases

If you’re interested in cathodic protection systems and design, you may also be interested in these parallel uses for conductive surfaces:

• Grounding for IEEE standard 81 Fall-Of-Potential (FOP) method.
• ASTM G57 soil test as an application of the IEEE FOP method.