Types of Corrosion
There are no mysteries in corrosion control, since the early 1800's scientists have been trying to solve the problem and it is understood that salt water is one of the most difficult environments in which to control corrosion.
It is important to note that corrosion in aluminium boats start from the inside out, so by the time it shows itself on the outside of the boat it’s usually a lot worse than a pin hole and will require professional repair.
Aluminium naturally develops a hard outer coating that protects itself against corrosion (oxide layer). However, where the stringers and bulk-heads sit on the hull there are crevices that can trap saltwater. This saltwater can break down the protective oxide layer, allowing corrosion to develop.
Lack of oxygen (oxygen starvation) can be another reason aluminium corrodes (ie; leaving saltwater sitting in the hull of your boat over a long period of time can cause tiny areas where the oxide layer is compromised, allowing corrosion that is evident as pitting).
Dissimilar metal fittings incorrectly installed to your hull (stainless rod holders, stainless fasteners, chrome plated brass fittings - even your anchor winch) and stray metallic objects, such as hooks, sinkers, swivels, coins, lying against the aluminium hull, cause corrosion when combined with moisture. Dissimilar metals cause galvanic corrosion when immersed together in saltwater.
Stray current corrosion, where an electrical component or system is incorrectly installed or has a bad earth, will turn your boat into a huge battery, with the saltwater becoming the electrolyte: Aluminium Hull + Current + Electrolyte = Corrosion.
How to Identify Corrosion
If you have a leak on the outside of your boat, there is definitely something more sinister happening on the inside beneath the floor.
If you can, lift up the floor of your boat and flush it with fresh water. Make sure all the internal drain-ways are free from build up of scum, leaves, sinkers, or an escaped pilchard. If you can see a white paste on the surface of the aluminium or a white powder forming in the joints, then it is a sign that corrosion is taking place and requires attention.
If you have a painted hull and the paint is blistering and peeling off around the fittings, or there is evidence of a white powder between the hull and the fittings on a non-painted hull, it is likely that galvanic corrosion is taking place.
If your boat is equipped with sacrificial anodes (zinc blocks) and they are not corroding, your hull more than likely could be!
One of the most important things to do after each fishing trip is to flush out the hull of your boat with fresh water. Remove the bung, let the salt water empty then put a fresh water hose in there and let it fill up. If possible, do this before you take the boat home so it can swish around whilst on the trailer, then open the bung up when you get home to let it drain. Always leave the boat parked bow high and the bungs open - this enables the hull to completely drain and breathe, preventing sweating. Some wooden floors and outboard packers in and on aluminium boats may contribute to advancing the corrosion process, as the copper and chrome used in treatment of some timbers (CCA treated) reacts with the aluminium. Timber also absorbs moisture holding it against the hull, thus causing corrosion.
If your hull is painted, any chips, scratches or holes drilled through the hull will leave the aluminium exposed to the environment which will allow moisture to ingress between the paint and the hull. This will cause the paint to blister and peel off. It is important to seal any exposed areas to prevent corrosion forming.
It is okay to fit stainless steel fittings to your aluminium boat. However, if there is moisture present between the aluminium hull and the stainless fitting, the less noble metal (aluminium) will deteriorate while protecting the more noble metal (stainless steel). This can be overcome easily during the installation process by using a sealing product such as Sikaflex or Duralac to provide a moisture barrier between the two metals.
In summary, there will be no corrosion if there is no moisture present, so keep your boat clean and dry. Lift the floor at least twice a season to check your hull if you can. If in doubt, contact Nautech Marine who specialise in aluminium boat repairs for our professional opinion.