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MG Duff MD77 Magnesium (Fresh Water) Hull Anodes

A bolt-on magnesium hull anode for freshwater cathodic protection.

MG Duff MD77 Magnesium (Fresh Water) Hull Anode

By MG Duff : MD77
Part #: SG040A
Availability: In Stock
SRP £39.43
You Save £6.93 (18%)

MG Duff MD77 Magnesium (Fresh Water) Hull Anodes

Our thoughts...

It is important to remember NOT to paint anodes when antifouling. Doing so will prevent the anode from working and may cause expensive problems.

This MG Duff magnesium bolt-on hull anode for use in freshwater conditions is designed to help guard against corrosion of underwater gear such as propellers, shafts, and rudders which are vital to the vessel and are expensive items to replace; the failure of a propeller or rudder could have disastrous consequences.

The bolt-on anodes fit on the hull of the boat with the two fixing holes.

If corrosion is allowed to persist the effects can vary from the pitting of propellers and shafts to the decomposition of the alloy. The failure of something as small and inexpensive as a split pin can result in the loss of the propeller.

Underwater corrosion control is an essential maintenance task and something to consider. It is relatively easy to replace existing anodes and install new cathodic protection systems if required.

What is cathodic protection?

Cathodic protection is an electrochemical process that halts the natural reaction (corrosion) of metals in a particular environment by superimposing an electrochemical cell more powerful than the corrosion cell.

Sacrificial Anodes are fitted or bonded to the metal to be protected which in turn as it has a greater electrical potential than the anode material becomes cathodic and causes the anode to waste instead of itself. In a correctly installed cathodic protection system, the only corrosion occurs to the sacrificial anode which is replaceable.

The number and size of anodes are determined by the type of material and the surface area being protected.

Which anode should I choose?

As a general rule owners should fit the anodes suitable for the environment they most regularly berth in and the following list provides a useful guide:

  • Freshwater - Fit Magnesium anodes
  • Brackish water - Fit Aluminium anodes (Brackish water is a mixture of fresh water and saltwater, commonly associated with estuary waters).
  • Saltwater - Fit Zinc or Aluminium anodes

Not all anodes are suitable for every environment, for example, the surface of zinc or aluminium anodes will if left in freshwater for some time become covered with an off-white crust of oxide which effectively seals the anode and stops it from working even when returned to saltwater.

Zinc anodes suffer a similar problem even in brackish conditions whereas aluminium will continue to operate effectively in river estuaries and other areas of brackish water indefinitely. The consequences of this passivation of the anode are that the next most anodic item within the anode bonding system will start to sacrifice itself which could of course be very serious.

Owners must be aware of the effects that this may have on their boats and fit the correct cathodic protection system to avoid corrosion.

Our thoughts...

It is important to remember NOT to paint anodes when antifouling. Doing so will prevent the anode from working and may cause expensive problems.


Width 83mm
Depth 41mm
Length 229mm
Fixing Holes 20 x 14mm at 200mm hole centres
Stud Size M10
Please Note

Magnesium anodes are not suitable for wooden hull vessels as they can damage the timber. Please contact us for further information.

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