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How to detect and prevent corrosion on your vessel

The chemical process of corrosion is the natural consequence of mixing metals with salt water, which means getting to grips with it is an essential aspect of boat ownership.

Keeping your vessel corrosion-free requires an understanding of the different types of corrosion, knowing how to spot it - and how to prevent it. So to help you stave off expensive marine repair bills, here’s a guide to the corrosion essentials…

What types of corrosion can affect my boat?

Oxidation

In other words, rust. Most of the time, an aluminium hull is inherently resistant to corrosion, thanks to a naturally occurring protective surface layer of aluminium oxide, preventing the metal below from coming into contact with oxygen. Saltwater, however, can break down this protective oxide layer, which allows oxidation to occur.

For steel hulls, keeping oxidation at bay means priming and painting the surface followed up by re-priming and re-painting on a regular basis. The aim is to keep that all-important outer layer of rust protection in tact.  

Galvanic corrosion

This describes the electrochemical reaction that occurs when a current flows between two dissimilar metals. Take for instance, the example of an aluminium hull boat docked at a pier with steel pilings, or fitted with a stainless steel propeller. When both immersed in a conductive solution (e.g. saltwater), the least chemical active of the two metals - i.e. the steel - becomes a cathode, while the aluminium becomes an anode.

Aluminium electrons flow from the anode via the conductive path to the cathode. Meanwhile, the aluminium atoms (minus their electrons) become ions and break away into the water. In other words, the reaction that occurs causes the aluminium to dissolve.

Stray current corrosion

This can eat away at underwater fittings in the same way as galvanic corrosion. Here though, the electric current causing the reaction comes from a power source - rather than simply a case of having two dissimilar metals in contact with each other. Causes include a badly wired system on a boat, on a neighbouring boat or on the dock. With galvanic corrosion, the effects can take weeks or months to become apparent. By contrast, stray current corrosion can cause significant metal damage within a matter of hours or days.

Signs of corrosion to look out for…

  • Leaks. A leak on an aluminium-hulled vessel can be a sign of advanced galvanic or stray current corrosion.
  • Blistered or peeling paintwork. This is an indicator of corrosion beneath the paintwork. Also, look out for evidence of a whitish powder on an unpainted aluminium hull.
  • Deposits around stainless steel fixtures. Stainless steel maintains its rust-free quality as a result of a thin surface layer of chrome oxide. If this layer breaks down due to the component being deprived of oxygen (through being surrounded by silt or sand, for instance), it will rust in the same way as non-stainless steel. Inspect steel fittings regularly for signs of brownish discolouration and keep them clear from silt and sand build-up.
  • Reference electrode testing. Test meters can be attached to individual items on your boat such as props, shafts and underwater fittings to identify electrochemical reactions. Testing kits can also be used to identify stray currents.

Preventing corrosion

  • Zinc protection. Zinc is a considerably more chemically reactive metal than most other metals used in boat construction. For this reason, fitting a sacrificial anode (usually just referred to as a ‘zinc’) is a standard method of protecting aluminium hulls. In simple terms, the strategically placed zinc will effectively give itself up and become an anode before the aluminium does - thus saving the aluminium.
  • Keeping electrical circuits dry. Wherever there is water between copper wiring and a metallic object, there is scope for corrosion. Use waterproof insulation around cable connections and regularly inspect circuits and connections for signs of fraying or damage.
  • Electrical bonding. A bonding circuit involves ‘tying together’ underwater metal fixtures, bringing them all to the same potential, creating a single, low resistance circuit with electrically tight connections. Any stray currents that may otherwise give rise to galvanic corrosion will instead choose the electric path of least resistance to ground.  
  • Prevent pools of water from forming. Stainless steel fixtures and bodywork can be especially prone to rust when surrounded by pooled water. Effective boat maintenance involves emptying out such pools when they form.

Boat owners should always follow manufacturers’ advice carefully regarding maintenance and the specific measures to put in place to keep on top of corrosion.