Anodes & Corrosion Protection
The cheapest insurance on the boat: a few kilograms of sacrificial metal standing between seawater and your shaft, prop, hull and engine. We stock Australian-made CAA anodes — zinc, aluminium and magnesium — for shafts, rudders, hulls, trim tabs and engines.
One metal volunteers
Put two different metals in seawater and connect them, and the more reactive one corrodes to protect the other — that's galvanic corrosion, and it never sleeps. The galvanic series ranks metals by reactivity: magnesium, aluminium and zinc anode alloys sit at the top, above aluminium hulls, cast iron, steel, bronze and stainless. A sacrificial anode is simply the most reactive metal on your boat, placed there on purpose — it corrodes, everything else doesn't.
The effect is dramatic in salt water, where metals corrode roughly ten times faster than in fresh — CAA's catalogue puts seawater's electrical resistivity at around 200 times lower than typical fresh water, which is also why the right anode alloy depends on where the boat lives.
The three anode sins: painting over them (insulated = useless), poor contact (clean to bright metal or bond properly), and the wrong alloy for the water — especially magnesium in the sea, which CAA warns can wreck coatings on steel and alloy hulls.
Alloy × water — the selection table
Distilled from CAA's published selection guidance.
| Anode alloy | Salt water | Brackish | Fresh water | Worth knowing |
|---|---|---|---|---|
| Zinc (Z1 alloy) | Yes | Yes | Limited — can passivate; CAA warns of polarity reversal above 50 °C in hard fresh water | The traditional choice; AS2239-verified alloy matters |
| Aluminium (A1/A2/A6) | Yes — slightly better output and life than zinc | Yes (slightly lower efficiency than in salt) | Usable at reduced efficiency | Lighter, and one anode type can cover mixed cruising waters |
| Magnesium (M-series) | No — never without corrosion-engineer advice | No | Yes — the standard freshwater choice | In salt water it can damage hull coatings on steel and alloy boats |
- Replace at ~50% wasted; size so that's no more than annual
- Fast wastage = look for stray current or new dissimilar metal, don't just fit a bigger anode
- No wastage at all = check the electrical contact
- Mask anodes when antifouling — never paint them
Why we stock CAA
Cathodic Anodes Australasia has made anodes in Queensland since 1984 in what it describes as Australia's only purpose-built large-scale anode factory — an Australian Made licensee with ISO 9001 certification, verifying every alloy by spectrometer against AS2239 and international standards. Alloy purity is the entire product in an anode: contaminated alloys passivate and stop protecting, which is precisely what you can't see from the outside. Buying certified Australian-made is the insurance.
Anode FAQs
Zinc, aluminium or magnesium — which anode do I need?
Match the metal to the water. Salt water: zinc or aluminium both work — CAA's data shows an aluminium anode of equal size slightly outperforms and outlasts zinc, though for most vessels the difference is marginal. Brackish: zinc or aluminium again. Fresh water: magnesium is the standard choice — zinc can passivate in fresh water, and CAA warns that above 50 °C in hard fresh water a zinc anode's polarity can actually reverse. And never fit magnesium in salt water: CAA cautions it can seriously damage coating systems on steel and aluminium hulls.
How do sacrificial anodes actually work?
Galvanic corrosion is electrochemical: connect two different metals in an electrolyte like seawater and the more 'anodic' one corrodes in preference to the other. The galvanic series ranks metals by that tendency — magnesium, aluminium and zinc anode alloys sit at the very top, which is the whole trick: bolted to your hull or shaft and electrically connected, the anode corrodes so the bronze, steel and aluminium below it on the series don't.
When should anodes be replaced?
The working rule: replace at roughly 50% wasted, and size the anodes so that doesn't happen more often than annually. An anode that vanishes in months is telling you something — usually stray current, new dissimilar metal nearby, or an undersized anode for the job. An anode that never wastes at all is telling you something too: it's probably not making electrical contact.
Why can't I paint or antifoul over an anode?
Paint insulates it, and an insulated anode is dead weight. Anodes must stay bare to the water and in clean metal-to-metal contact with what they protect — mounting surfaces cleaned to bright metal, or connected through the boat's bonding system. Mask them when antifouling.
What about the anodes inside my engine?
Many engine makers fit pencil anodes inside raw-water cooling passages and OEM heat exchangers — small zinc or aluminium rods that waste quickly in high-flow conditions and deserve a check whenever the impeller comes out. One exception worth knowing: Bowman states its cupronickel heat exchangers need no anode at all — the alloy protects itself, and adding one can disturb its protective oxide film.
Are CAA anodes actually made in Australia?
Yes — Cathodic Anodes Australasia has manufactured anodes in Queensland since 1984, runs what it describes as the only purpose-built large-scale anode factory in Australia, and is an Australian Made licensee. Composition is verified by spectrometer against Australian Standard AS2239 and international equivalents, which matters: an anode is only as good as its alloy purity.
Anodes for the next haul-out
Tell us the boat, where it lives (salt, brackish, fresh) and what the old anodes were — we'll match the CAA alloys and sizes so the whole set is ready when the boat comes out.
(03) 5973 6444