Protecting your assets: Boat Corrosion 101, Vessel Corrosion
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Electralogic

New or old, plastic, steel, carbon, wooden, alloy, fibreglass or concrete vessels all differ and require different styles of protection. All vessels require protection, inside and out, topsides and bottom. It is the individual nature of every single vessel that indicates the importance of understanding what corrosion is, types of corrosion and the various methods that can be implemented in order to save your vessel from certain deterioration. When in doubt, contract someone who does. The initial outlay for facilitating a professional will save you heartache and cold hard cash in the long run.

Understanding what can affect your vessels protection levels is paramount in determining the level of action that is required: Shore power and on board power influences affecting boats in many ways and raises numerous questions:

• What anodes are required for your boat? Have you made the correct choice?
• What determines a good Cathodic Protection system from a bad system?
• DC (Battery Power) power systems affects vessels exceedingly worse than any stray shore power. Are you using the correct systems, is your wiring up to scratch?
• Faulty and household electrical appliances installed onboard, how can these affect your vessel?
• What types of corrosion is taking place on your vessel now?
• Aluminium Oxide
• Ferric Oxide
• Osmosis
• Dezincification
• Atmospheric corrosion
• Galvanic corrosion
• Electrolysis
• Stray currents
• Crevasse corrosion

The Basics
All metals want to revert back to their original state which is originally dirt (ore). Without the correct protection regarding both Cathodic Protection Systems and surface coatings – one thing is certain – your vessel will suffer and you will pay the price.

Galvanic corrosion occurs when two dissimilar metals form a natural battery when placed in contact with each other while partially or completely immersed in an electrolyte. Seawater is a perfect electrolyte. Metals vary in their reaction to being part of this natural battery. Metals that are more active are less noble or anodic (creating a more negative voltage). In a galvanic cell a less noble metal like alloy, zinc or aluminium, will dissolve sooner than a more noble metal like brass or stainless steel.

 

Galvanic Corrosion on Propeller

Failures in seacocks and through hull fittings can be linked to being galvanically coupled between stainless and bronze or between different grades of similar metals with no cathodic protection. Some signs are when the skin fitting is turning light or dark green, then going black then to white which means the fitting is highly active and the material is gassing (dissolving).

Galvanic Corrosion a sinking threat

Avoid using bottom paint that is incompatible with the vessel hull material, keel, or drive leg material. A good example is copper-based bottom paint on an aluminium hulled boat or on alloy drive legs, which will sacrifice to protect the copper-based anti foul. A good primer will create a barrier between the hull and the antifoul – stopping osmosis, under film corrosion and reduce anode dissipation.

Underfilm Oxidation

Crevasse corrosion is where a shaft and shaft gland or rudder and rudder gland that are not electrically connected together and have different ‘galvanic’ values due to differing metallic composition (can be small or large differences). Where crevasse corrosion establishes, the water turns acidic and corrosion can be aggressive. This can be passified by electrically connecting the shaft and shaft gland electrically together to a hull anode.

Protecting bronze and stainless steel on wooden a wooden boat is a fine art, using zinc and you run the risk of Alki attack or wood rot below the water line, and worse corroding your fasteners in the wood. An article dedicated to protecting your wooden vessel will be released in the coming editions.

Much like you service your engine or replace anodes, your cathodic protection system should be considered as an asset to your vessel – a way of safeguarding from unwarranted damage. Without a correctly functioning cathodic protection system your vessel is at risk from costly and potentially catastrophic damage. Electralogic are specialists in all things to do with cathodic protection. No matter what your issue or concern, contact Electralogic on (02) 4381 0790 or info@electralogic.com.au for more information.

Look out for further articles by Electralogic on “Protecting Your Assets”:
“Cathodic Protection: What you need to know”
“Small electrical faults … massive effects”
“Performance Coatings”
“Cathodic Protection & wooden vessels – can it be done?”

Brian & Jessica Gatt
Electralogic Pty Ltd
www.electralogic.com.au