When Sir Humphry Davy first outlined the cathodic protection process in 1824, he set the stage for today’s go-to method for protecting metals in nearly every environment.
For cathodic protection to work, you need four things: an anode, cathode, electrolyte, and a pathway to complete the circuit. The anode and cathode are added to the electrolyte and connected using a metallic wire to create a circuit. Once the circuit is complete, current flows from the anode to the surface of the cathode, protecting it from corrosion.
Corrosion happens for many reasons, including contact with oxygen and water. When unprotected metals encounter them, chemical reactions slowly eat away at the metal.
But water and air aren’t the only enemies to worry about. Common corroders include salts, warm temperatures, and acids, which can quickly chew through unprotected structures.
This is where cathodic protection steps in to lend a friendly hand.
Despite land and water being completely different environments, the cathodic protection process doesn’t change much.
For example, soil can be incredibly corrosive to metals, especially clay soils, which hold moisture and have poor drainage. Alternatively, gravel and sand are more forgiving because they hold less moisture and drain well, reducing corrosion.
The same principles are true for water environments. Seawater is incredibly corrosive because of its high salt content, while freshwater is less damaging to metal surfaces.
Seawater is unique because it’s highly conductive, meaning current flows well through it. This is because of the high concentrations of salt in the water.
When setting up a seawater CP system, there are a few variables to watch. Ocean water is a consistent and even conductor, but overall salinity can impact conductivity.
Sea applications also call for different sacrificial anodes. Soil CP systems work well with zinc or magnesium alloys, which are slow-corroding metals. Ocean CP systems rely on sacrificial metals like graphite, cast iron, titanium alloys, and other coated metals to form circuits.
Once the anode is installed into the CP system, it’s critical to monitor it regularly. Anode placement and the water’s current density can create situations where cathodes are over or under-protected. Both can reduce performance and even lead to damaged structures.
Underwater projects are also subject to biofouling. Biofouling is when organisms and debris attach to the anode and create a biofilm, eventually reducing anode performance. To prevent growth, check and clean the anodes regularly.
In a cathodic protection system, current flows from the anode to the cathode. This allows the metal structure to remain safe while the anode corrodes.
Two types of cathodic protection systems are available; Impressed Current Cathodic Protection (ICCP) and Sacrificial Anode Cathodic Protection. Both options are fully capable of protecting structures but have specific strengths and weaknesses.
For ICCP to work, the anode and cathode must be in the same electrolyte and connected to an external power supply.
In an underwater CP system, seawater is the electrolyte. A DC power source connects an insoluble anode and the cathode to create a circuit. Current flows from the power source to the anode, then to the cathode through the water.
ICCP systems are a long-term solution that, if done correctly, protects both the anode and cathode from corrosion. DC current in the system moves opposite the corrosion path, protecting the cathode. If the DC current equals or slightly exceeds the corrosion current, it also protects the anode.
This form of CP doesn’t require power to prevent corrosion. Instead, a metal with a more negative electrochemical potential (more willing to oxidize or corrode) protects metal structures.
For a sacrificial anode system to work, the anode (usually zinc or aluminum) and cathode must be in the same electrolyte. The two metals in the system are either attached together or connected using a CP cable to complete the circuit.
The system works without power and is typically easier to control than an ICCP system. This is because the circuit self-regulates in its environment rather than constantly needing adjustment.
This sounds easy, so you might ask why this seemingly foolproof system is less preferred than ICCP. Unlike ICCP, a sacrificial anode system adds weight to metal structures. This can be a problem for ships and boats moving through the water. The anodes also have a short lifespan because they corrode quickly and can sometimes release heavy metals as they dissolve.
Additionally, though the system self-regulates, sacrificial anodes struggle with high resistivity. Higher resistivity reduces the anode’s ability to protect the cathode, affecting performance.
In either CP system, the process works best when combined with protective coatings for added support.
While any metal structure can be a candidate for protection, a few common ones include:
Ship Hulls – Ships are constantly under attack by corrosion because they spend so much time in the water. Corrosion rates increase as temperatures, water velocity, oxygen content, and contaminants rise.
Offshore Drilling and Wind Installations – Saltwater constantly pounds these structures on their exposed metal sections and underwater pieces. A DC power system mounted on the structure above the water line offers excellent protection against rust and corrosion.
Pipelines and Steel Storage Tanks – ICCP should be avoided for some pipeline installations, especially if chlorine gas or hydrogen could build up. The current from the DC power source could ignite the gas, causing an explosion or fire risk.
Welds, grout lines, and other pipeline/tank parts should be tight and clean. Weld systems to the structure for better contact and performance.
Piers, Bulkheads, Pilings – Though these structures use concrete, the concrete has steel rebar for reinforcement. Steel reacts to many environmental conditions, so engineers prefer using CP to maintain the structure’s strength.
What qualities should you look for in cathodic protection cable?
It should perform well in even the harshest environments, offering abrasion, crushing, chemical, oil, sunlight, and moisture protection. Most CP cable sizes have an ultra-thick .110-inch HMWPE insulation, though wires larger than #1 AWG can have up to .140-inch insulation.
If HMWPE insulation isn’t enough, Halar® and Kynar are premium options. Halar is preferred for cathodic protection in saltwater because it is highly resistant to seawater’s chloride ions. Kynar also works well, protecting conductors from brackish water, hydrochloric acid, sulfuric acid, and chlorine.
No matter what, make sure to do your research to understand what insulation will suit your project’s needs best.
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