Galvanic anodes, or sacrificial anodes, are used in galvanic cathodic protection systems to extend the life of the steel structure they are put in place to protect. This guide explains what they are, how they work and the different types available.
WHAT ARE SACRIFICIAL ANODES?
Galvanic anodes (also commonly known as sacrificial anodes) are highly active metals that are used within cathodic protection systems. They are used to efficiently prevent or dramatically slow down the oxidation reaction of the parent steel material, which reduces the degradation (or rust) of the structure.
Galvanic anodes are created from a metal alloy with a more negative electrochemical potential than the metal it will be used to protect. This is why they are often known as sacrificial anodes, as the anode metal is ¡°sacrificed¡± and so the anodes corrode instead of the parent metal.
HOW DO SACRIFICIAL ANODES WORK?
Galvanic or sacrificial anodes work because they have a stronger or more negative electrochemical potential than the potential of the metal that they are protecting.
These sacrificial anodes are said to behave anodically while the parent metal structure is said to behave as a cathode as the anode is slowly consumed while the metal structure remains protected.
The flow of electrons moves in one direction, from anode to cathode, meaning the conventional current flows in the opposite direction from cathode to anode.
TYPES OF SACRIFICIAL ANODES
There are a variety of different types of galvanic anodes available, suitable for a range of applications. At Jennings Anodes we manufacture four main types of sacrificial anode, using three main alloys, that have been specifically developed for use in different environments: magnesium anodes, aluminium anodes, water box anodes and zinc anodes.
Magnesium anodes are highly electro-negative, making them a good option for the protection of steel structures that are more electro-positive, such as pipelines, towers, pilings and bridges. Historically, they have been the most popular anode choice for onshore protection of oil, gas and water pipelines.
There are two main grades of magnesium that have different purities and slightly varying electrochemical outputs. The purer the grade of magnesium used the more problematic it is to cast and manufacture due to its reactivity. Ideally this requires noble-gas shrouding and other practices to ensure the anode is free from any inclusions, slag, pitting or other casting defects.
More than one million of our magnesium anodes have been installed onto structures across the USA and Canada, and we continue to offer the highest quality anodes on the market.
Aluminium anodes have a slightly lower driving voltage than their magnesium counterparts, but their slower consumption rate makes them ideally suited to seawater (and sometimes also freshwater) applications, such as seagoing vessels, marine engineering, harbor facilities, offshore pipelines, ship hulls, ballast tanks, drilling platforms, and wind turbine foundations.
Aluminium anodes also benefit from being very lightweight, with a higher capacity than zinc and a relatively low cost per pound. This means they typically have a longer lifespan so are a popular choice for most offshore applications.
As aluminium is not considered a toxic pollutant, they are less harmful to the aqueous environment than other options which makes them well suited for use in almost all aquatic applications.
Available in both magnesium and aluminium types (depending on the desired application), water box anodes are vital component in the protection of almost any enclosed water tank or pressurised vessel.
Such tanks, including all domestic hot water tanks, are prone to accelerated corrosion within the steel tank and require relatively small anodes to slow down this internal corrosion.
These anodes are often extruded cylinders of either aluminium or magnesium, and are commonly inserted into the tank using a threaded end-cap.
Zinc ribbon anodes offer a flexible, simple, cost-effective and low-maintenance solution to corrosion control.
As little to no maintenance is required, zinc ribbon anodes are a good choice for unattended or hard to reach applications, and are often used in underground situations where the anode can be positioned very close or next to the structure it is protecting. More recently, zinc ribbon anodes have been used in the control of AC mitigation.
Comprising of an extended ribbon wrapped around a steel wire core, the zinc ribbon anode offers more than 95% current efficiency, making them a highly reliable option. Our zinc ribbon anodes are tested to temperatures as low as 45 degrees below freezing, as well as temperatures that ensure they can operate effectively in the hottest Saudi Arabian deserts.