Technical Feature - Sometimes, anodes ain’t anodes. Understanding the science and materials of anodes

Sometimes, anodes ain’t anodes

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By Brent Linde

There is nothing mystical about anodes. They sacrifice themselves to protect the hull and other metallic fittings of your motor yacht against corrosion.

Metals and alloys that are higher on the Galvanic Table protect those alloys below them on the table (see Figure 1).

Figure 1 – Galvanic Table

Decades of research and field testing have 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 they eventually become part of national and international standards. This is the process that has been undertaken to determine the current accepted national and international standards for anode alloys.

The current internationally accepted alloys have been in production for many years. They are cast by reputable 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 only 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.

We have recently seen imported aluminium anodes cast onto a galvanised strap. 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 galvanised 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 reduce its effectiveness.

The Backyarder

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 anodes have a high lead content, which usually leads to the anode becoming passive. On the other hand, if your anodes are bubbling away and disappearing very quickly, this indicates high levels of other impurities which shorten the anodes’ service life, leaving your vessel vulnerable.

Some years ago a Queensland foundry began 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 typesetting. This scrap material was cheap and readily available. The problem was that it contained high levels of lead and iron. Not surprisingly, the anodes never worked. Unfortunately the vessel owners were not aware of the problem until their vessels were slipped 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 grades of aluminium. They are selected for a wide range of characteristics, from strength to durability, architectural properties and finished appearance.

Aluminium anode alloys are different. Their primary objective is to protect against corrosion. Appearance and other characteristics are of secondary importance.

We recently heard about a vessel owner who had a fancy-looking anode installed on the stern of his boat. After two years it showed only minimal signs of corrosion. The very low corrosion rate caused some concern as it indicated that the anode was not doing its job. The chemical composition of the anode was analysed using an Optical Emission Spectrometer, which identifies the chemical make-up of aluminium, magnesium and zinc based products. This so called anode was found to be an aluminium-based alloy with no resemblance to any Australian/NZ or internationally recognised anode alloys. In fact it had a very high percentage of iron, which limits the effectiveness of the anode and explains why it had become passive. The anode was offering very limited protection against corrosion. More likely it was offering no protection at all.

Optical Emission Spectrometer

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 secondary.

To make matters worse, the supplier’s website claims that their anodes meet the highly respected internationally recognised Anode Manufacturing Standard DNV-RP-B401. Spectrometer analysis of the chemical composition tells a very different story.

Conclusion

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 time to identify the alloys that really work.

Do not let a supplier tell you that their anodes are made of some mysterious alloy that has amazing properties that regular anodes cannot achieve!

A simple way to ensure that your vessel is adequately protected against corrosion is to install Riviera branded anodes that may be purchased from your Riviera and Belize dealership or from the Riviera Genuine Parts web site. Click here to visit the website.

Brent Linde is Business Development Manager at Cathodic Anodes Australia.

 

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