'Anode going in - aluminium, zinc or magnesium?'
When it comes to making environmentally friendly choices, the tradeoff is not always paying more for something that doesn't work as well.
Boat owners seeking a 'greener' sacrificial anode have an option that works better than the old, familiar zinc versions while costing less: aluminum anodes.
Aluminum is a far better environmental anode choice than zinc. Though zinc is a heavy metal—not particularly desirable to have in excess quantities in water—the big concern is with the percentage of cadmium that zinc anodes must contain in order to work as anodes.
Aluminium for salt or brackish water (for fresh, use magnesium) - .. .
Cadmium is a nasty heavy metal, with its use and disposal becoming increasingly restricted and regulated. In anodes, it's of considerable concern to scientists studying estuarial pollution and its inevitable presence in aquatic life, especially once in the human food chain.
Authorities in the USA are required to deal with marinas as a source of non-point pollution in accordance with EPA guidelines, with periodic water quality assessments. The path is clearly open for regulating the acceptable amount of cadmium in marina water, which could lead to a ban on zinc anodes. Fortunately, boat owners already have a safe and cost-effective alternative in aluminum anodes.
Leading commercial vessel operators have used aluminum anodes for years. Propulsion system manufacturers such as Mercury Marine standardize aluminum anodes to protect their marine engines, as well as specify them as OEM replacement parts. In the offshore industry, aluminum anodes are the material of choice to protect pipelines and other subsea installations where long term corrosion prevention is essential.
So why do boaters still cling to zincs? Old habits die hard, and most people don't pay anodes much thought, let alone understand how they work. An anode's purpose is to protect expensive metal components from galvanic corrosion. The simple science is that when a boat is in the water, its various metal components are exposed to galvanic corrosion, though not all corrode at the same rate. Those that corrode fastest are 'least noble,' while the 'most noble' corrode most slowly.
This is why relatively low cost, replaceable sacrificial anodes are made from less noble metals. The inexpensive anode is sacrificed, rather than the costly propeller shaft, engine, rudder, engine cooling system, refrigeration condenser, or even the hull.
While anodes may appear to be fairly inert pieces of metal, they are actually sophisticated pieces of anti-corrosion engineering. Their complexity lies in that they are truly alloys, made to an exacting specification in order to sacrifice effectively, but at a controlled rate. Aluminum anodes, for example, contain trace amounts of indium and other components to help them perform properly.
The U.S. Navy has done very extensive work on determining the alloys, composition and, importantly, the production process to create effective sacrificial anodes. Consequently, the Navy's military standard, familiarly known as MILSPEC, for anodes has become 'the global benchmark for shipyards and ship owners,' says John Mitchell, president of leading anode manufacturer CMP Global.
Anodes that wander too far from the specifications won't work, as various forms of surface oxidation will slow, or even halt, the sacrificial process. Anodes have to be cast in a facility capable of maintaining the necessary purity of the materials used and of running the analytic tests needed to make sure that anodes are in spec.
Apart from having the right composition for optimal protection, water salinity will affect the rate at which the anode sacrifices, and its overall effectiveness. In salt or brackish water, the anode material will be zinc or aluminum. While aluminum is superior across a greater range of salinities than zinc, the recommendation for vessels that operate only in fresh water is magnesium.
John Rothermel, VP of sales at Fisheries Supply in Seattle, Washington, USA, says magnesium is the right choice, 'if the boat is always only going to be in fresh water. If the boat is going back and forth, aluminum seems to be the only answer. Aluminum does lose its effectiveness in fresh water but will begin working again once the boat enters salt water. To provide better fresh water protection, a supplemental magnesium anode called a 'grouper' can be hung over the side of the boat.'
But what about the cost of aluminum anodes, since it's a more expensive material than zinc? The rate at which an anode sacrifices is controlled by the surface area it presents to the water, and equivalent aluminum and zinc anodes will have the same dimensions. While pound for pound aluminum costs more than zinc, an aluminum anode will weigh less than half of its zinc counterpart and should therefore cost less.
The big payoff, though, is that aluminum anodes have a longer effective life. Cathodic protection specialist Paul Fleury, an ex-U.S. Navy nuclear plant technician and founder of Marine Services in Earlysville, Virginia, USA, says that in terms of protective power, aluminum anodes have 3.5 times the energy of zinc anodes. They also have 20% better self-cleaning benefits, in his experience.
When it comes to ensuring the quality of any anode type, boat owners need to be confident that the Navy's MILSPEC benchmark is adhered to by the manufacturer. It seems leading industry suppliers take it seriously, as companies such as Fisheries Supply and West Marine only carry MILSPEC anodes, for example.
The market contains many inexpensive anodes but most live up to the saying, 'you get what you pay for.' An anode that does not perform properly leaves metal components at the mercy of galvanic corrosion. The nominal cost of a MILSPEC anode, backed by a manufacturer's quality processes, is a small price to pay to protect expensive investments. And, using aluminum anodes that help preserve the environment gives owners peace of mind, too.
CMP Global offers a wide array of aluminum, magenesium and zinc anodes to protect boats and components in all types of water. Its Martyr II aluminum and Martyr III magnesium anodes contain no cadmium.
To learn more email email@example.com, or visit their website.
by Martin Flory, CMP Global
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8:45 PM Thu 3 Jan 2013GMT
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