Cathodic Anode Protection is a powerful anti-corrosion weapon but has costly drawbacks when protecting more considerable steel assets.
Four elements essentially are present for corrosion to occur: a host site from which current flows, a destination site from which no current flows, a medium capable of conducting current (such as water, concrete, or soil), and a metal path connecting the host and destination sites.
The process by which ions on the surface of metal are transferred to another substance is known as electrochemical corrosion (a depolarizer or less active substance or metal). Oxygen, acids, or cations of more passive metals are examples of depolarizers.
What Is Cathodic Protection Used For?
Cathodic protection is frequently used to prevent corrosion on active metal surfaces. It protects pipelines, water treatment plants, offshore production platforms, reinforcement bars in concrete structures and piers, underwater storage tanks, ships, boat hulls, and other structures worldwide.
To protect steel from corrosion, a cathodic system is frequently used. Corrosion occurs when two dissimilar metals are immersed in an electrolytic substance such as water, soil, or concrete. This type of metal conducting path between two different metals provides a pathway for free electrons to move from the more active metal (anode) to the less active metal (cathode) (cathode). If free electrons from the anode do not reach active sites on the cathode before oxygen arrives, ions at the active sites can recombine to form the ferrous hydroxide, also known as rust.
How Does Cathodic Protection Work?
Cathodic protection, in essence, connects the base metal at risk (steel) to a sacrificial metal that corrodes in place of the base metal. The cathodic protection of steel technique preserves the metal by providing a highly active metal that can act as an anode and provide free electrons. The active metal sacrifices its ions by introducing these free electrons, preventing the less active steel from rusting.
The Two Types of Cathodic Protection
Cathodic protection is classified into the galvanic anode and impressed current cathodic protection.
Both provide a cathodic protection current flow from anodes placed in the same electrolyte as the metal to be protected. The anode conducts current into the electrolyte. It discharges onto the metal, preventing corrosion. To complete the circuit, it must flow through the metallic circuit (metal plus cables) and back to the anode.
Galvanic Anode Cathodic Protection (GACP)
As previously stated, galvanic anode cathodic protection works. The anode materials are zinc, aluminum, or magnesium alloys, which are all more active metals than carbon steel. When these more active metals are metallically connected to steel in an electrolyte, they corrode it preferentially.
The cathodic protection current for steel is the corrosion current of the anode material. The current flow through the electrolyte and onto the steel, preventing corrosion. In the metallic circuit, the present returns to the anode.
You’ve probably heard sacrificial anodes.’ Even though this terminology describes the anode materials and how they act (the galvanic anode corrodes preferentially to steel), it was change to ‘galvanic anodes’ in Europe in the 1980s.
How is Galvanic Anode Cathodic Protection Use?
Anodes are typically cast offshore onto structural tubular cores weld to the offshore structure during onshore construction. Offshore oil and gas pipelines are protect by anodes made of aluminum alloy or zinc clamp over the protective coating and connect to the pipeline via short cables or weld connections. Such protection should last for at least 30 years.
Short pipelines on land are frequently protect with magnesium anodes. These are cast onto steel cores and are cable-connect to the pipeline. Extrude or continuously release and hot-rolled zinc ribbon is use in soils with low electrical resistivity. Zinc ribbon is use as an earthing electrode on bury pipelines to reduce induce alternating current (AC).
Impressed Current Cathodic Protection (ICCP)
By connecting a DC power source between the metal is protect and the cathodic protection anodes, impressed current cathodic protection is provide. In contrast to GACP, the cathodic protection current is supply by the DC power source rather than by anode corrosion. The transformer rectifiers (confusing acronyms include TR, TRU, and T/R) that convert mains electricity to low voltage DC are typically use as DC power supplies. Solar panels and batteries are commonly use (and steal) in remote areas; thermo-electric DC generators and diesel and gas engine driving generators is also use.
The negative pole is link to the protect metal (the ‘negative drain point,’ while the positive pole is connect to the anode. Cathodic protection current flows from the anode, through the electrolyte, and onto the protect metal, just like GACP.
Scrap steel (a fairly common practice in France, where old railway rails are frequently use in such applications), high silicon iron, or sophisticate mix metal oxides coated onto titanium is use as anodes. Other materials, such as graphite, magnetite, lead, platinum-coated titanium, and niobium, have use, but their use is reduce due to performance and cost.
How is Impress Current Cathodic Protection Use?
Anodes offshore are typically mixed metal oxide-coated that is titanium cathodic anode (MMO/Ti). These is use in seawater and saline mud, though their consumption rate is higher in the latter. Most impressed current systems for steel in concrete use MMO/Ti anodes in mesh, strip, or tubular form. There is a tubular anode made of MMOs conductive ceramic.
Onshore, anode groups are typically use in ‘grounds,’ which can take the form of a long horizontal trench with multiple anodes bury in a carbonaceous backfill. This increases surface area, decreases electrical resistance to the round, and increases anode life. Anodes and “coke” can use in deep boreholes or multiple shorter boreholes.
Which Form of Cathodic Protection is Best?
Both galvanic anode and impress current cathodic protection can equally effective if the cathodic protection system is well design, install, operate, and maintain. GACP, on the other hand, is more straightforward and has prove more reliable offshore.
Onshore, ICCP systems are easier to maintain, and their components are not subject to the challenges of offshore environments once installed. ICCP can protect many kilometers of well-coated pipelines if adequately designed.
ICCP is also beneficial for bare or poorly coated steel because it can deliver hundreds of amps of low voltage direct current. In contrast, a typical galvanic anode can rarely have more than five amps.
Cathodic Protection | a Specialist Operation
Cathodic protection is widely use to prevent corrosion of critical infrastructure. As an example: It is require by law for gas and oil pipelines to operate safely. Their certification bodies mandate offshore gas and renewable energy structures to receive effective cathodic protection. Ships with practical cathodic protection benefit from extend dry-docking rules.
Cathodic protection extends the life of concrete bridges and structures affected by chlorides from de-icing salts or marine exposure. When cathodic protection is use, new reinforce concrete structures in harsh exposure conditions have a longer life.
Drawbacks Of Cathodic Protection
Cathodic protection is use for many years to protect structures expose to corrosive environments for an extend period. However, the installation of cathodic protection can be costly. The specifics of how networks are build can also add to their complexity. As a result, the cost of cathodic protection. In addition to this cost, the system necessitates routine maintenance, including a regular visual inspection. There is also the ongoing cost of electricity in the case of impressed current cathodic protection. Sacrificial anodes, particularly, have a limited supply and are prone to rapid corrosion. That is, they have a finite lifespan.
This protection is ineffective on large metal surfaces that lack a barrier coating. The bottom of large, welded tanks is a prime example. This is because even a well-design cathodic system will struggle to maintain the proper voltage across a long metal span that is not insulate. It is due to the natural voltage drop that occurs when current flows, and current will undoubtedly flow when a surface is expose to the ground and not insulate. While cathodic protection works well on pipelines with an epoxy barrier coat, it has significant limitations on uncoat surfaces. Cathodic safety, according to experts, is secondary to barrier coating.