How Impressed Current Anodes Are Used for Immersed Concrete Structures?

Aug 31, 2022

How Impressed Current Anodes Are Used for Immersed Concrete Structures?

Both Galvanic anodes and Impressed current anodes are applied for immersed concrete structure cathodic protections. However, there are some differences between these two types of anodes.

Galvanic Anodes
Galvanic anodes for immersed structures will normally be slender stand-off or hull-mounted type anodes and will normally be installed by direct welding of the anode steel core to the embedded steel. This will require breakout of the concrete to expose embedded steel, welding of the steel core (or welding of a stand-off support), and making good the concrete. An alternative method of fixing connects the anode to electrical contact points (which themselves have been connected to the embedded steel) either directly (using lugs/nuts) or via an integrate anode cable.

Galvanic anode alloys suitable for immersed concrete structures are varies by working conditions.   For seawater and saline waters, anodes of aluminium-zinc-indium, zinc or magnesium can be used. For fresh water applications, only zinc or magnesium anodes are applicable.

Impressed Current Anodes

Impressed current anodes suitable for immersed concrete structures typically comprise high-silicon cast iron (with chrome in chloride environments), lead silver alloys (very seldom used in recent years), mixed metal-oxide-coated titanium, platinized titanium or platinized niobium. Impressed current anodes can operate at high-output current densities, ranging from 10 A/m2 to 30 A/m2 for silicon-ironchrome, 200 A/m2 to 300 A/m2 for lead silver and up to 1 000 A/m2 for mixed metal-oxide-coated titanium anodes and up to 3 000 A/m2 for platinized titanium or niobium anodes.

Impressed current anodes are available in rod, tube or strip form and can be fabricated in a wide range of sizes and shapes. Small rod or single tube anodes rated at 2 A to 5 A each can be fixed directly on or adjacent to a structure to provide localized current, as necessary. Larger anodes (comprising multiple tubes connected together or large-size rods/wires) can be assembled to form remote anodes rated at 50 A to 500 A each, to provide general protection to a large surface area.

The consumption rate for impressed current anodes ranges from 250 g to 500 g a year for high-silicon iron, to 25 g to 100 g a year for lead silver, 0,6 mg to 6 mg a year for mixed metal oxide titanium and 4 mg to 10 mg a year for platinum-based anodes. The consumption rate for platinum anodes is also dependent on salinity, with the rate being approximately an order of magnitude (10×) higher in fresh water compared to seawater conditions.

Cables for impressed current anodes should be suitable for exposed to immersed conditions, be capable of carrying high currents and, at the cable-to-anode interface, be capable of withstanding the aggressive/oxidizing conditions created.