The potential difference between the two dissimilar metals generates adequate electricity to form an electrochemical cell and drive galvanic or bimetallic corrosion. In passive cathodic protection systems, the sacrificial anode is connected directly or indirectly to the metal to be protected. This can be achieved by employing two distinct types of cathodic protection: passive cathodic protection and impressed current cathodic protection. (To learn more, read Galvanization and its Efficacy in Corrosion Prevention.) Types of Cathodic Protection (CP)Īs mentioned previously, cathodic protection works by intentionally forming a galvanic cell with another sacrificial metal. Another common type of cathodic protection, known as galvanizing, is commonly used to protect steel members and structures. In the marine industry, this protection method is also used on steel piles, piers, jetties and ship hulls. The oil and gas industry, in particular, uses cathodic protection systems to prevent corrosion in fuel pipelines, steel storage tanks, offshore platforms, and oil well casings. In some cases, external power sources can be used to supply additional electrons to the electrochemical process, which can increase the effectiveness of cathodic protection.Ĭathodic protection systems are employed in numerous industries to protect a broad range of structures in challenging or aggressive environments. By electrically connecting the metal to be protected to a more anodic (electronegative) metal, we can ensure that the anode sacrifices itself by corroding preferentially over its cathodic counterpart. In other words, if we want to protect a particular metallic structure, we can create conditions where this metal becomes the cathode of an electrochemical cell. By understanding the principles of this type of corrosion, we can purposely pair metals together to ensure that one cathodically protects the other. While the design of cathodic protection systems can be sophisticated, their operation is based on the concept of bimetallic or galvanic corrosion described earlier. Cathodic Protection (CP) and Its Method of Operation The further apart the contacting metals are in the galvanic series, the greater the potential difference between the metals, thus the more severe the corrosion at the anode. (See An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion for more information.) The metals positioned higher on the table are considered to be anodic (more electronegative), while the metals placed lower on the table are more cathodic (more electropositive). The electrode potential of various metals is displayed on a list known as the galvanic series. When a metal is immersed in an electrolyte, it adopts an electrode potential that represents the metal’s ability to be oxidized or reduced. In bimetallic corrosion, this potential difference is a direct result of the difference in electrode potential between the two dissimilar metals. Meanwhile, as electrons flow to the cathode, reduction occurs, further protecting the cathodic metal. As electrons flow out of the anode, oxidation occurs, causing the anodic metal to degrade or corrode. This potential difference causes electrons to flow from one metal in the cell (the anode) to the other metal (the cathode) while generating a small amount of electricity in the process. (This process is examined more fully in the article Why Do Two Dissimilar Metals Cause Corrosion?)Ĭorrosion in an electrochemical cell is driven mainly by a property known as potential difference. In other words, one metal sacrifices itself while protecting the other. Bimetallic corrosion is usually characterized by accelerated corrosion in one metal while the other remains unaffected. This corrosion is observed in several situations where dissimilar metals are in direct or indirect contact with each other. Bimetallic corrosion, as its name implies, is a unique type of corrosion that occurs between the pairing of two metals. To understand how cathodic protection works, we must first appreciate the basics of bimetallic corrosion, also known as galvanic corrosion. Understanding Galvanic or Bimetallic Corrosion
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