Corrosion is a naturally occurring phenomenon commonly defined as the deterioration of a substance (usually a metal) or its properties because of a reaction with its environment. The tendency of a metal to corrode depends on the grain structure of the metal, its composition as formed during alloying, and the temperature or deformation of a single metal surface developed during fabrication.
Corrosion occurs as a result of an electrochemical reaction driven by a potential difference between two electrodes, an anode and a cathode, connected by an electronic path and immersed in the same electrolyte. In the case of uniform corrosion, a multitude of microscopic anodic and cathodic sites exist on the surface of the metal structure.
There are various time-proven methods for preventing and controlling corrosion. One of these is cathodic protection (CP). Here we’ll take a look at cathodic protection, its types, and how it's used to protect buried pipelines against corrosion.
What is Cathodic Protection (CP)?
The metallic surfaces exposed to an electrolyte have a multitude of microscopic anodic and cathodic sites. Where anodes are more electronegative than cathodes, a potential difference is created between them, allowing for corrosion to occur. (To view a quick visual introduction to CP, watch Cathodic Protection in 2 Minutes Flat.)
The function of cathodic protection is to reduce the potential difference between anodes and cathodes to a neglected value. This reduction is due to the polarization of cathodes to the potential of most active anodes. In this way, corrosion current is mitigated according to Ohm’s law.
Cathodic protection can be accomplished by sending a current into the structure from an external electrode and polarizing the cathodic sites in an electronegative direction.
Cathodic Protection Criteria
In order to achieve adequate CP, the protected structure must be polarized to a certain value. The polarized potential is measured with respect to a certain reference electrode. A copper/copper sulfate reference electrode (CSE) is the most common electrode used in soil and freshwater.
There are two types of criteria for assessing cathodic protection. Either one may be used depending on the circumstances, although the first is considered superior in many cases.
- The Potential Criterion
The polarized potential of the protected structure is to be equal to or more negative than -850 millivolts (mV) with respect to CSE.
- The Polarization Shift Criterion
The protected structure is to be polarized by 100 mV with respect to CSE from its corrosion potential.
Note: These criteria are for carbon steel; criteria may differ for various metal types.
Types of Cathodic Protection
There are two types of cathodic protection:
- Galvanic Anode Cathodic Protection
- In this type of CP, protection is achieved by connecting the protected structure to a sacrificial anode, which is placed close to the protected structure.
- Sacrificial anodes are made from active metals such as zinc, aluminum, or magnesium, which are considered the most active metals according to the galvanic series.
- CP current is created by the potential difference between sacrificial anodes and the protected structure.
- The type of anode used depends on electrolyte resistivity and the chemical compositions of the electrolyte to which the substrate is exposed.
- Impressed Current Cathodic Protection
- In this type of CP, protection is achieved by connecting the protected structure to an anode bed through a transformer rectifier (TR). The anode bed is a series of buried anodes that are electrically connected and surrounded by certain backfill to reduce their resistance to the earth. The anode bed should be placed remotely from the protected structure. (For more information, see Cathodic Protection and Anode Backfills.)
- Three types of anodes are used: Soluble anodes (aluminum and steel), semi-soluble anodes (graphite and high silicon cast iron (HSCI)), and non-soluble anodes (platinum, mixed metal oxide, and polymer)
- The main component of this type of CP is the TR, which forces the current to flow from the anodes to the protected structure (cathode).
- The type of anodes used depends on the chemical composition of the electrolyte, to which the substrate is exposed and the area to be protected.)
Application of Cathodic Protection Systems on Buried Pipelines
Pipelines are used for transporting water, petroleum products, natural gas, and other utilities. There’s a huge network of piping systems used in every country all over the world. Pipelines may be onshore or offshore, and are subject to corrosion in both cases. If corrosion isn't mitigated, dangerous and expensive damage can be the result.
There are several corrosion control techniques used on pipelines; cathodic protection is one of them. It can be applied either to coated pipelines to mitigate the corrosion attack on areas where coating quality may be poor. It is also used on bare pipelines. Both types of CP can be applied to buried pipelines. The application of either of these types depends on several factors, such as the required current, soil resistivity, and the area to be protected.
CP aims to polarize a pipeline to a minimum potential of -850 mv, for carbon steel and for adequate CP. (Read An Overview of Cathodic Protection Potential Measurement for more information.) The polarized potential is to be measured through test stations, which are to be installed at the following locations along the route of pipeline:
- At frequent intervals (e.g. < 2 km / 1.24 miles)
- At crossings with foreign structures
- At points of electrical isolation
- At some galvanic anode locations
- At casings
- Near sources of electrical interference
- At the location of stray current discharge to earth
Problems Created by Cathodic Protection
In large pipeline networks, there are a lot of crossings, parallelism, and approaches, wherein the pipeline has its applied CP system. DC interference may occur between pipelines, accelerating corrosion. In order to overcome this problem, pipelines can be electrically coupled, either directly or through resistance.
For coated pipelines, where the applied coating quality is poor, cathodic disbondment may occur due to high CP levels. Higher temperatures may also promote cathodic disbondment. High pH environments are also a concern in terms of stress-corrosion cracking. In such cases, the polarized potential of the pipeline must be kept at a minimum value of -850 mV.
Remember, cathodic protection is just one method used to prevent corrosion, not just in pipelines, but in ships, offshore oil platforms, and other steel structures. Whether it's the best application for the job, or the only one to be used, is often specific to the structure being protected.