In this blog, we take a look at Sacrificial Anodes and the differences between Zinc based and Aluminium Anodes. Read on to find out more...
All metal industrial and marine structures submerged in water will inevitably corrode over time. If left untreated and unprotected, these refineries, storage tanks, bridges, pipelines, platforms and vessels will all experience a loss of structural integrity, usually to the point of failure. This is because water acts as an electrolyte causing a transfer of electrons from the metal structure through oxidation and leading to rust.
How can we effectively stop this oxidation reaction in the metal? The answer is actually a neat trick using physics and something called sacrificial anodes!
A sacrificial anode is made from a metal alloy that features a more "active" voltage (i.e. more negative electrochemical potential) than the metal it is going to protect. The metals used for anodes are chosen because they oxidize more easily than the metals used for the structure. This means that they will turn the structure you want to protect into a cathode, while acting as the anode, of an electrochemical cell.
How do anodes work? As the electrons leave the structure through these sacrificial anodes, the rods will slowly dissolve over time instead of degrading the main structure. Why do anodes burn away? In other words, this form of sacrificial anodes cathodic protection uses the naturally occurring electrochemical potential difference between different metallic elements to provide unrivalled, long-lasting protection for the primary metal structure.
This form of cathodic protection is tried and tested across many industries for preventing rust on submerged and underground metallic structures. The most popular metal alloys for making these anodes are magnesium, aluminium and zinc. What is the difference between using each of these for the rods? Well, each alloy actually has a very specific application.
Magnesium anodes are the most active. This means that they are the only ones that work really well in the low conductivity of fresh water due to their high negative electrode potential. Often referred to as the “soft water” anode, magnesium is also popular for cathodic protection in domestic or commercial hot water systems. Being so active, magnesium doesn’t last as long as the other two metal alloys in salt or brackish water. (basically used in fresh water)
These kinds of metal alloys are both suitable for use in salt water where resistivity is generally lower. Aluminium is highly resistant to hard water making it perfect for these types of applications and in older tanks. It also does carry a better environmental footprint than zinc anodes as they do not contain cadmium, which is harmful to marine ecologies.
Aluminium possesses a lower voltage rate than its magnesium counterpart and also offers a far slower consumption rate than either magnesium or zinc. Generally, you will consider this alloy when magnesium anode rods wouldn’t last a year. It’s also immune to calcareous coating. Typical uses are for the hulls of ships and boats, offshore pipelines and production platforms, in salt-water-cooled marine engines, as well as small boat propellers and rudders.
On the other hand, Zinc-based sacrificial anodes are also suitable for salt water applications and are deployed to protect marine assets. However, it’s also particularly effective in mining applications too. Zinc is not suitable for use at high temperatures though, as it tends to passivate. These types of anodes can be produced in rather complex geometry, as opposed to aluminium, which can be useful in more streamlined designs e.g. rope guard anode rings.
Ultimately, you can rest assured that Zinc (Z1) protects well in salt, fair in brackish and just a little in fresh, this is why it is mainly used in marine environments with sea water. However, you will also find it applied in brackish and freshwater, as well as some low resistivity soils.
Aluminium protects well in salt, good in the upper levels of brackish waters, but not so great in the lower levels of brackish, as well as offering decent performance in fresh water. This is why you will often see it applied to seawater, brackish water and fresh water too. However, Aluminium anodes exhibit a lower rate of efficiency in brackish and fresh water when compared to their zinc counterparts.
It is important to take care of and replace your anodes frequently. You should always monitor it for replacement when it reaches around half its original size. If you fail to do so, it will eventually reach the end of its useful life and the cathodic protection you’re relying on to protect your asset will cease to activate. If you’d like to know more about sacrificial anodes, don’t hesitate to read through our handy FAQs.
Want to know even more? As a leading supplier of anodes in Australia, we’re able to assist with any of your queries!
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