The metal: Zn Al Mg
Zinc and aluminium anodes are the sea water anodes; magnesium has the most negative potential of the three and is the fresh water anode. Anodes of different metals are not mixed on one hull.
Reference tool — not a protection design
Read a sacrificial anode alloy before it goes in an RFQ.
MIL-DTL-18001 · MIL-DTL-24779 · ASTM B843
The anode, part by part.
Enter the metal, the specification (the former or the current designation) or the water, in English or Russian. Below, an offered anode can be checked against the one wanted: the metal, the specification and, for magnesium, the alloy.
Examples:
Which anode for which water
Hull, shaft, rudder and sea-chest anode shapes and sizes are maker-specific: give the old anode’s maker code and dimensions in the request. The number and placing of anodes come from the vessel’s drawings or a cathodic protection design.
Replacement check
On the metal, the specification and the alloy alone: check the shape, size and weight against the maker code.
All 130 engineering tools
Hull, shaft, rudder and sea-chest anodes are cast in three metals: zinc to MIL-DTL-18001, aluminium (an aluminium-zinc-indium alloy) to MIL-DTL-24779, and magnesium, for example the ASTM B843 alloys. Enter the metal, the specification or the water, and the reader gives the alloy family, its main alloying elements, the water it suits and the design values two sources confirm, checks an offered anode against the one wanted and writes an RFQ line.
This is a reference for reading an anode and writing an RFQ, not a cathodic protection design: the number, mass and placing of the anodes come from the vessel’s drawings or a protection design. Shapes and sizes are maker-specific, so the old anode’s maker code and dimensions go into the request.
How an anode is named
Metal, specification, water.
The specification: MIL-DTL-18001
Zinc anodes follow MIL-DTL-18001 (formerly MIL-A-18001): zinc with small additions of aluminium and cadmium. Aluminium anodes follow MIL-DTL-24779 (formerly MIL-A-24779): aluminium with zinc and indium. Magnesium anodes follow ASTM B843, for example AZ63 (aluminium and zinc) or the high-potential alloy with manganese.
The water: sea · brackish · fresh
Sea water: zinc or aluminium. Brackish water: aluminium. Fresh water: magnesium. An ordinary zinc anode is for water up to about 50 °C; in heated water it can passivate and stop protecting.
| Metal | Specification | Alloying elements | Sea water | Brackish water | Fresh water | Design values |
|---|---|---|---|---|---|---|
| Zinc | MIL-DTL-18001 (formerly MIL-A-18001) | aluminium, cadmium | suits | sources differ | not for it | −1.03 V; 780 Ah/kg |
| Aluminium (Al-Zn-In) | MIL-DTL-24779 (formerly MIL-A-24779) | zinc, indium | suits | suits | sources differ | −1.05 V; 2000 Ah/kg |
| Magnesium | ASTM B843 | AZ63: aluminium, zinc; high-potential alloy (M1C): manganese | not for it | not for it | suits | left out: the sources differ |
Every value is confirmed by two independent sources. The design values are those of DNV-RP-B401 (2021 edition) for sea water up to 30 °C: the closed-circuit potential against a silver/silver chloride reference electrode, and the electrochemical capacity. Left out because the sources differ or only one gives it: zinc in brackish water; aluminium in fresh water; every potential and capacity of magnesium; makers’ laboratory values for aluminium, which run higher than the design values; the 2005 zinc design potential, changed in 2021; silicon in the aluminium alloy; the exact content of each element.
Sources
DNV-RP-B401 Cathodic protection design, editions 2005 and 2021, and a DNV type approval certificate that quotes its design table; MIL-DTL-24779B, the US Navy specification for aluminium anodes; the ASTM B843 abstract; technical datasheets of a Norwegian anode maker (zinc, aluminium, magnesium) and of a US anode maker (zinc); the alloy pages of two further makers, and the anode guide of one of them; the anode guides of two boating magazines and of a US marine retailer; the Wikipedia article on galvanic anodes; a NACE test method summary; a magnesium anode supplier sheet; two US patents on anode alloys. Every value is in at least two of them.
Before you send the request
Questions about sacrificial anodes in an RFQ
Which anode metal for sea, brackish or fresh water?
Sea water: zinc or aluminium (the aluminium-zinc-indium alloy). Brackish water: aluminium; the sources differ on zinc there. Fresh water: magnesium; zinc is not for fresh water, and the sources differ on aluminium. Magnesium is not used on a hull in sea or brackish water: it is consumed in a short time and can overprotect.
Is MIL-A-18001 the same as MIL-DTL-18001?
Yes: MIL-A-18001 is the former designation of the US military specification for zinc anodes, now MIL-DTL-18001; in the same way MIL-A-24779 became MIL-DTL-24779 for aluminium anodes. The letter after the number is the revision, for example MIL-DTL-18001L. Either designation in a request names the same alloy family.
Can zinc and aluminium anodes be fitted together on one hull?
No: the references agree that anodes of different metals are not mixed, and that all anodes should be of the same metal. Replace an anode with one of the same metal; to change the metal, change all the anodes together.
Why are there no figures for magnesium anodes?
The sources give different potentials and capacities for magnesium, measured against different reference electrodes, so none is kept. For zinc and aluminium the page gives the design values of DNV-RP-B401 for sea water up to 30 °C, which a second source confirms.
What should the request include?
The metal and the specification; the anode type (hull weld-on, bolt-on, shaft collar, rudder, sea chest, heat exchanger); the old anode’s maker code or its dimensions, since shapes and sizes are maker-specific; the net weight; the quantity; where on the vessel it is fitted; and a photograph of the old anode.
Anode read?
Send the line with the old anode’s maker code or a photograph.
A reading is a check of the alloy, not a cathodic protection design. Give the type, the maker code or the dimensions, the net weight and the quantity, and Vessel Core checks them before an offer goes back.
Buyer tools
