The boater’s information to corrosion


This information is meant to provide you a elementary understanding of what corrosion is, what causes and have an effect on it, and provide help to decide which anode materials is correct to your boat.

Of all of the difficult topics a ship proprietor must know, corrosion might be probably the most complicated. Corrosion could cause implications in each boat upkeep and boat security. Sure sorts of corrosion may even devalue your funding in a matter of months and even days.


What’s corrosion? 

Corrosion is an electrochemical course of of degradation of metallic parts when uncovered to an aqueous atmosphere, equivalent to water. This happens each underwater and within the environment.

Deterioration is the method of which the metallic is turning into its oxide kind. Metal, for instance, will degrade (oxidize) again to its pure steady state, specifically rust.


Corrosion mechanism

The metallic atoms on the floor surrender electrons and switch into positively charged ions, which dissolve into the water or electrolyte (a liquid that may conduct electrical energy). Electrons movement via the metallic from the corrosion space to different areas shut by, the place they kind adverse ions within the water. The optimistic ions movement via the water and mix with the adverse ions flowing in the wrong way.

Corrosion Mechanism

Illustration of corrosion mechanism 

So, you’ll be able to see that an electrical present is ready up between localized areas on the metallic floor, ensuing within the metallic loss (corrosion) on the anodic areas. Solely electrons are given up on the cathodic areas, so no metallic is misplaced in these areas. They’re, in actual fact, protected.



Galvanic corrosion

When two completely different metals (copper and metal within the instance) are in touch, electrons will movement from the extra negatively charged metallic (anode) to the extra optimistic metallic (cathode). The voltage generated between copper and metal could be 0.3 volts. The circuit is accomplished by the lack of positively charged ions, from the anode into the electrolyte, and the negatively charged ions on the cathode.

Galvanic Corrosion

Illustration of galvanic corrosion

This launch of small particles (ions) into the water is far more fast than with one metallic alone and is proscribed to the metal’s corrosion. The cathode materials (copper) is protected.


Sacrificial anode

If you wish to shield each kinds of metallic, you will need to ass a 3rd extra lively metallic. The most typical metallic is zinc, though magnesium and aluminum are additionally used. This lively metallic turns into the anode for each metals.

Sacrificial Anode

Illustration of sacrificial anode

The zinc or aluminum sacrifices itself to guard the opposite two metals, therefore the time period sacrificial anode.


Why do some metals corrode greater than others?

All metals are usually oxidized (corrode), some extra simply than others. The relative fee might be plotted on the galvanic collection:

Galvanc Series

Illustration of the galvanic collection 

What components have an effect on corrosion?

Remember that a few of these components can fluctuate microscopically on the floor of the metallic.

  • Conductivity of electrolyte – seawater is an effective conductor and freshwater a foul conductor, so corrosion is worse in seawater.
  • Quantity of oxygen – typically, corrosion charges improve in proportion to the quantity of oxygen within the water. Nonetheless, cracks and crevices, that are areas starved of oxygen, turn into anodic and corrode additionally.
  • Presence of pollution – will increase corrosion.
  • Circulation fee – will improve corrosion charges. Pitting in chrome steel is diminished, nevertheless.
  • Temperature – larger temperature will increase corrosion charges, roughly doubling for each 10°C/18°F
  • Stress – metallic underneath tensile stress (stretched) together with corrosion can undergo sudden failure attributable to stress cracking.
  • Presence of bio-organisms – there are numerous kinds of microorganisms that may contribute to corrosion, both by eradicating safety or inflicting a corrosive atmosphere.


Space of weight and anodes

The floor space of the sacrificial anodes determines how a lot safety (amperage) you get. The burden determines how lengthy they may final. Completely different anodes have completely different capacities measured in amp-hours per pound.


Cathode to anode ratio

The ratio of the cathodic (protected) space to the anodic (corroding) floor is crucial in galvanic corrosion. The smaller the world the place the anode is giving up materials, the sooner it is going to happen. Ideally, the anodic space needs to be a lot larger than the cathodic space. This ratio might be improved by portray the cathodic floor.


Galvanic corrosion by way of the bottom wire

Connecting into shore energy connects your floor to the neighboring boats. If appropriate anodes don’t shield these boats, you’ll be the one shield them – inflicting fast carrying of your anodes. See the diagram beneath:

Galvanic Corrosion via the Ground Wire

Illustration of corrosion by way of the bottom wire


Galvanic isolator

A galvanic isolator is a tool that’s put in within the inexperienced floor wire to dam direct galvanic currents however nonetheless enable AC to go. Beware: Make certain your galvanic isolator is rated for the ability you utilize, e.g., 30A or 50A. Poor high quality galvanic isolators have been recognized to start out fires, so it is a good suggestion to get one, which is ABYC acknowledged or UL listed to make sure that it has been correctly examined.


Stray present (electrolytic) corrosion

This corrosion is attributable to an exterior present flowing from a battery or different DC supply. This present flows out of the metallic into the water and causes lack of materials or corrosion within the course of. Widespread causes embrace a naked wire within the bilge on incorrectly wired or put in tools.


Impressed present system

As an alternative of utilizing a sacrificial anode to generate a protecting voltage, a DC energy supply can be utilized. The precept is similar, however the present is monitored and adjusted by the system. A non-corroding materials is used for the anode.

Impressed Current System

Illustration of impressed present system

The benefit of an impressed present system is that it could possibly develop larger voltages than a sacrificial anode. The drawback is that it could possibly “over-protect.” Impressed present programs are used on all kinds of boats and sterndrives.



All electrical tools and underwater metallic fittings needs to be linked to the identical floor level (linked to the battery adverse terminal). This ensures that every one parts are on the similar voltage, stopping any stray currents occurring.


Sacrificial anode supplies

  Zinc Aluminium Magnesium
(in seawater)
-1.03 V -1.1 V -1.5 V
Relative Life
(Zinc=100 similar dimension)
100 150 30
Relative Density
100 42 27
Mil. Spec MIL-A-18001 MIL-A-24779 MIL-A-21412

Zinc anodes

Zinc is the most typical materials used. Zinc anodes will not be very helpful in freshwater and may cease working after only some months if not made to millimeter specs. It’s a good coverage to alter them frequently, even when they appear okay. Bear in mind, if an anode does not put on away, it isn’t working!

Aluminum anodes

The aluminum alloy utilized in anodes could be very completely different from extraordinary aluminum. It consists of about 5 % zinc and a hint of Indium, which prevents an oxide layer’s build-up. Aluminum anode alloy supplies extra safety and lasts longer than zinc (see chart). It should proceed to work in freshwater and is secure to be used in saltwater. Aluminum is the one anode that’s secure for all purposes.

Magnesium anodes

Magnesium is probably the most lively metallic on the galvanic scale. It may be utilized in freshwater, however care have to be exercised. Magnesium can over-protect aluminum hulls or outdrives in salt or brackish water and even polluted freshwater, inflicting the paint to be lifted with ensuing corrosion. Even a number of hours of immersion could cause extreme injury.


Which anode materials is correct to your boat? 






Wooden Fiberglass Aluminium Metal All


Alum Alum/Magazine Alum Alum/Magazine Alum/Magazine
Freshwater (Polluted) Alum Alum Alum Alum Alum


Alum/Zinc Alum/Zinc Alum/Zinc Alum/Zinc Alum


Alum/Zinc Alum/Zinc Alum/Zinc Alum/Zinc Alum


Anode do’s and don’ts


  • Change your anodes when they’re 50 % corroded. a ‘put on indicator’ anode will assist inform you when to alter.
  • Make certain they make good electrical contact – take away paint and clear the mounting floor.
  • Defend trim tabs individually (don’t bond). Though they’re often created from chrome steel, they will nonetheless corrode and wish sacrificial anodes.
  • On sterndrives, be sure you use new fasteners (often provided with the anode) – even stainless bolts fail attributable to corrosion.
  • Hold a sterndrive immersed within the water in order that the anodes can work. 

Do not

  • Don’t paint anodes. They won’t work!
  • Don’t combine anode sorts – aluminum anodes will attempt to shield zinc anodes on the identical bonding circuit.
  • Don’t use zinc anodes on aluminum outdrives – they won’t present the correct safety.
  • Don’t use magnesium anodes on outdrives in salt or brackish water as they may ‘overprotect’ the aluminum.



Widespread marine supplies information


Aluminum is a superb materials for marine use. Aluminum is a lightweight, sturdy metallic that’s straightforward to work with. It has wonderful corrosion resistance attributable to its skill to kind a protecting oxide floor movie quickly. Unprotected, it could turn into pitted or coated with a gritty white powder, however these are often superficial and never dangerous. Anodizing eliminates this.

Nonetheless, it is rather lively on the galvanic collection (-.76 to -1.00 volts), which makes it vulnerable to galvanic corrosion when in touch with extra noble metals. Bronze, brass, or monel fittings needs to be averted or insulated to stop galvanic motion. Chrome steel (316) fasteners are advisable. Aluminum might be overprotected by an excessive amount of voltage from magnesium anodes in salt, brackish or polluted freshwater.


Brass is an alloy of copper and zinc. Usually not advisable for uncovered use. Brass suffers from dezincification, which is the galvanic corrosion of the zinc from the alloy, leaving a spongy brittle part. Remember that manganese bronze is brass, not a real bronze, and desires galvanic safety if used underwater.


Bronze alloys of copper with little or no zinc. Genuine bronzes are strong and very immune to corrosion, each within the environment and immersed. Bronzes might comprise tin, aluminum, nickel, or phosphorus, however the perfect and most generally used is silicon bronze. Extensively utilized in fittings and fasteners.

Chrome steel

Chrome steel is a extensively used sturdy corrosion-resistant materials. Stainless owes its corrosion resistance to its chromium content material, which types an oxide movie resistant to connect (materials is then known as passive). Nickel improves welding properties. 18 % chromium and eight % nickel is the minimal grade, 304. Even higher is 316 grade, which has molybdenum, which boosts corrosion resistance.

If stainless is starved of oxygen (e.g., underneath seals or barnacles), it loses its protecting oxide movie and turns into lively. It should then corrode readily. This could additionally happen in microscopic crevises leading to nearly invisible corrosion, which might trigger sudden failure. Appropriate for deck fittings, it isn’t advisable to be used underwater, besides when galvanically protected as, for instance, a fastener in an aluminum outdrive.

Wooden hulls

Wooden hulls are very vulnerable to deterioration attributable to numerous kinds of wooden rot and corrosion attributable to metallic fittings and fastenings. Silicon bronze fasteners are advisable. Don’t use stainless beneath the waterline.

Fiberglass/composite hulls

With fiberglass/composite hulls, silicone bronze fasteners are advisable beneath the waterline. Warning: Carbon (graphite) fibers are electrically conductive and may trigger galvanic corrosion between metallic parts within the construction.


Helpful hyperlinks

Reference: Efficiency Metals

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