Understanding Cathodic Protection and its Measurement
Cathodic protection may be referred to as a commonly used technique for controlling the corrosion on metal surfaces. This methodology is used mostly for protecting submerged and underground metallic structures from corrosion. The technique is based around transforming active metal surface areas to passive. Technically speaking, this involves converting the surface to be protected into an electromagnetic cell’s cathode.
In order to create cathodic protection, the potential of the metal is reduced by supply of current, which ceases corrosion attack. Sacrificial anode cathodic protection and impressed current cathodic protection are the two methods used for achieving this type of corrosion resistance.
Though the primary application of cathodic protection is in protecting steel structures immersed in water or buried in soil, this technique can be applied for corrosion control of any metal in contact with a bulk electrolyte. Some of the most common applications of these systems include water and fuel pipelines, storage tanks, offshore oil platforms, ships and boats, and oil well casings.
Cathodic Protection Principles
An electrochemical process, corrosion reverts ore extracted metals to their primary state when they are exposed to water and oxygen. It occurs normally at the anode, but never at the cathode. The principle of cathodic protection involves connecting external anode to surface of the metal that needs to be protected. Also, a DC current is run between them, which makes the metal cathodic and remains protected from corrosion.
Galvanic systems have minimal maintenance requirements, low operating costs, and can be installed easily. Also, they don’t interfere with foreign structures and need no external power supply. They are suitable for quite localised CP applications, but can only provide limited protection to large structures.
If you are looking to protect pipelines and underground storage tanks, impressed current systems are better suited for you. As a result of their high current output, these systems can protect large underground metal structures. They are less susceptible to soil resistivity and have the flexibility to deal with varying conditions. However, they often interfere with other buried structures located nearby and rely entirely on the availability of an AC power source.
Please remember regular maintenance is a must for impressed current systems used for cathodic protection. Also, the level of current applied by the system plays an important role. While too little current may lead to corrosion damage, hydrogen embrittlement and disbanding of the coating may be caused by excessive current.
Cathodic Protection Measurements
Measurement of cathodic protection is carried out by the following standard methodologies.
Pipe-to-Soil Potential: pipe-to-soil potential may be referred to as a pipeline’s potential at a given location. It is the result of corrosive electrolytic reaction that takes place between the electrolyte (the soil) and the buried pipe.
Instant OFF Potential: As a result of IR drop errors, the pipeline potential for pipe-to-soil measurement tends to appear more negative compared to its true potential. These errors are corrected by instant OFF measurement, which provides a truer measure of corrosion protection.
Coupon Current: Effectiveness of a CP system can also be monitored by connecting cathodically-protected structures to corrosion coupons. Representative samples of the material, coupons are buried nearby so that it encounters the same environment.
If you have more questions about corrosion protection, measurement and inspection, please contact our experts at Coating Management Solutions.