Cathodic Protection is a method of protection for iron and steel structures against electrochemical corrosion. It takes place through the application of an electrical DC current and stops the corrosion reaction from occurring. To prevent damage to costly structures like pipelines and tanks from corrosion, protective measures must be taken.
Corrosion and Cathodic Protection
First, it’s important to understand the process of corrosion. The processing of metals into usable materials expends a considerable amount of energy. Corrosion is the reaction where refined metals release that energy and return to their natural state.
The corrosion reaction is an oxidation-reduction reaction – a chemical reaction where the metal reacts with the environment to reduce it to an oxide form. This happens through a process of free electrons moving from a more active metal (anode) to a less active metal (cathode). Since before the free electrons from the anode reach a cathode they are exposed to oxygen in the air, the ions can then recombine to produce ferrous hydroxide or, in other words, rust.
Therefore, corrosion occurs where current leaves the structure and combines with oxygen in the air to form rust. Cathodic protection stops the energy from leaving the metal and prevents corrosion by applying current to the surface.
How does Cathodic Protection Work?
Cathodic Protection involves the introduction of a “sacrificial metal” that corrodes instead of the base metal. The base metal is preserved by providing a highly active metal that acts as an anode and provides free electrons. The active metal then sacrifices its ions, keeping the less active steel from corroding.
CP works by placing an anode or anodes (external devices) in the electrolyte to create a circuit where current flows from the anode, through the electrolyte to the surface of the structure. Current moves to the anode to stop further corrosion from occurring.
Advantages of Cathodic Protection
- Cathodic protection is a reliable solution to corrosion that has been used for many years to protect structures that suffer long-term exposure to corrosive environments.
- Corrosion control helps preserve expensive assets and reduce their maintenance (Or replacement) costs.
- Government regulations include corrosion protection requirements.
- Cathodic Protection is one of the few methods of corrosion control that can effectively protect a submerged metal surface from corrosion.
- CP can be used on various types of metal structures in various environments.
Ground Storage and Elevated Storage Tanks
Cathodic protection is necessary for steel storage tanks and piping according to government regulations. Just like other metal structures, corrosion is an ever-present concern for storage tanks. The effects thereof include premature failures and disruptions in service during repairs. By the use of Cathodic protection, the corrosion reaction can seize to take place thereby preserving the structure.
Impressed current cathodic protection involves a cathodic protection rectifier generating a protective current which is led into the ground via impressed current anodes. The cathode is situated at the pipeline so that the protective current flows from the anode via the ground to the imperfections. To monitor the effectiveness of the cathodic protection, the potential is measured through permanent reference electrodes at the measuring points.
Due to the large interception areas of the pipelines and their galvanic connection to the cathodic protection rectifier, lightning and surge protection measures are required to prevent failure of the cathodic protection rectifier.
Internal and external corrosion of storage tanks
Internal corrosion protection
To prevent tanks and pipelines from corroding, paint and coatings are often used as a primary defense. This is not effective enough and the internal wetted surfaces of tanks and piping are susceptible to degradation over time. Installing a cathodic protection system inside the storage tank is a common means of supplementing the coating system. Depending on the specific situation, there are various configurations to protect against internal corrosion including suspending, hanging, or attaching anodes to the tank.
External corrosion protection
Just like how internal wetted surfaces are susceptible to corrosion, so are external surfaces like tank bottoms. The most common configuration for external corrosion protection involves the use of linear anodes in concentric rings. In a linear system, everything is factory assembled and tested before installation and the installation is simple and not very time-consuming. Think of it as a low-effort, highly reliable system whereby installation costs are minimal.
Corrosion Protection for Tanks
As is evident, one of the best ways of preventing the electrochemical reaction from taking place in the case of corrosion is impressed current cathodic protection (cathodic protection rectifier). With the installation of a cathodic protection system comes the risk of fire or failure of the rectifier, for instance, due to large interception areas of the pipelines and their direct galvanic connection to the cathodic protection rectifier. To safeguard your tanks and pipelines, it’s important to make sure lightning and surge protection measures are in place.
DEHN protects pipeline systems through practice-proven products and innovative concepts. This includes isolating spark gaps for use in hazardous areas to protect insulating joints. Choose from our combined arrestors which protect both the anode and cathode side of cathodic protection systems.