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Let us start by saying that there is nothing mystical about anodes. They sacrifice themselves to protect your hull and other immersed or buried metallic fittings against corrosion.
Metals and Alloys that are higher on the Galvanic Table protect those alloys below them.
The Galvanic Series in Seawater table
Decades of research and field testing has been undertaken by engineers and scientists from Universities, Defence Organisations and private companies from around the world. Their combined research has identified the best combinations of elements to create the most effective anodes for various environmental conditions.
Their research and testing has been performed in fully equipped and accredited laboratories and in controlled long term field testing facilities. Research papers are peer reviewed by industry bodies such as the ACA (Australasian Corrosion Association) in Australia/New Zealand and NACE (National Association of Corrosion Engineers) in the US, as well as many similar organisations around the world. After rigorous testing and review, if alloys are proven to be more effective than the alloys before them, eventually they become part of national and international standards. This is the process that has been undertaken to determine the current accepted AS/NZS and ISO standards for Anode Alloys.
The current internationally accepted alloys have been in production for many years; they are cast by anode manufacturing foundries all around the world and they protect billions of dollars’ worth of vessels and other infrastructure such as oil rigs, sheet pile walls, wharves and pipelines. Their strong scientific backing, along with long term data on their in-field performance is what makes them the universally accepted alloys.
Like any industry, anode manufacturing has some inexperienced and unethical suppliers. From the backyarder to the shonky salesman trying to baffle boat owners.
The Inexperienced Manufacturer
Some manufacturers cast their anodes using just Aluminium or Zinc ingot without adding the activating elements required to promote the corrosion process. These anodes can offer limited protection but usually end up becoming passive and non-protective.
Also, we have recently seen imported aluminium anodes cast onto zinc galvanized straps. This combination does not comply with recognised Standards because Aluminium has a melting point of around 660 deg C and Zinc has a melting point of around 420 deg C. When the much hotter Aluminium is cast onto the zinc galvanized strap, it melts the Zinc, which can create voids in the anode adjacent to the strap and also increase the amount of Zinc in the body of the anode, which can in turn reduce their effectiveness.
Some manufacturers use scrap Aluminium or Zinc as the base metal which is often contaminated with other elements such as iron or lead. They melt the scrap and cast it into moulds then visit slipways and chandleries offering cheap anodes that may look great, but do not offer any protection against corrosion.
Impurities in the scrap material can adversely affect the efficiency of the anode. For example a single rusty nail dropped into a furnace of molten Aluminium or Zinc will prevent all of the anodes from working. They will become passive and offer no protection to your vessel.
If the surface of your anodes are black, this can indicate that the anode has a high lead content, which usually leads to the anode becoming passive. Also, if your anodes are bubbling away like a Berocca tablet and disappearing very quickly, this indicates high levels of other impurities which shortens the anodes service life, leaving your vessel vulnerable.
Some years ago a Queensland foundry started selling cheap anodes to the commercial fishing Industry. Subsequent investigations revealed that they were mixing high purity Zinc ingot with scrap zinc previously used for printer type setting. This scrap material was cheap and readily available, the only problem was that it contained high levels of lead and iron. No surprises, the anodes never worked. Unfortunately the vessels owners were not aware of the problem until their vessels came up on the slip 12 to 18 months later. The corrosion damage to hulls and metallic fittings was extensive. Legal action followed.
The Decorative Anode
There are many different Aluminium grades, they are selected for a whole range of characteristics from strength to durability, architectural properties or finished appearance. Aluminium anode alloys are different, their primary objective is to protect against corrosion. Appearance and other characteristics are of secondary importance.
A distributor of ours had a customer who removed an anode from the back of their boat. It had been installed for over 2 years and only showed minimal signs of corrosion. The very low corrosion rate caused some concern as it indicated that the anode was not doing its job.
To find out the chemical composition, we analysed it on our Optical Emission Spectrometer (Figure 3), which is set up to identify the chemical make-up of Aluminium, Magnesium and Zinc based products. This so called anode turned out to be an aluminium based alloy with no resemblance to any recognised anode alloys. In fact it had a very high percentages of Iron, which limits the effectiveness of the anode and explains why it had become passive. The anode was offering very limited corrosion protection, more likely it was offering no protection at all.
The aluminium grade of this so called anode may have been chosen because it extrudes well and the finished product looks great, adding to the illusion that it is a superior anode. Keep in mind that anode alloys are designed to protect against corrosion. Appearance is irrelevant in anodes.
To make matters worse, this suppliers’ website claims that their anode meets the highly respected internationally recognised anode manufacturing standard DNV-RP-B401. Our Spectrometer analysis tells a very different story.
Keep in mind that there is no magic or mystery associated with anodes, just a lot of research and testing by a lot of engineers and scientists over a very long period of time to identify the alloys that really work.
Do not let a supplier tell you that their anodes are made of some mysterious Wiz Bang alloy that has amazing properties that regular anodes cannot achieve!
Some National and International alloys that are universally recognised include:
- Australian Standard AS 2239 – A1, A2 and A6
- Det Norske Veritas DNV RP-B401 – Aluminium
- National Association of Corrosion Engineers NACE TM0190 and NACE SP0607
- NORSOK M-503
- US Military Specification MIL-DTL-24779A(SH)
- ASTM International ASTM B418
- Australian Standard AS 2239 – Z1
- Det Norske Veritas DNV RP-B401 – Zinc
- National Association of Corrosion Engineers NACE SP0607
- US Mil Spec MIL-DTL-18001L
If the anodes on your vessel do not appear to be working, you can analyse their chemical composition and find out what they are made of.
Oil and Gas, Government, Defence and Mining are industries that have strict protocols when it comes to specifying the chemical composition of the anodes that they require, and rightly so. Their assets are valuable and must be suitably protected against corrosion, just like your vessel.
Author: Brent Linde, CAA
Originally published in Corrosion & Materials May 2018