Types of Corrosion
Oxygen Starvation
As described earlier, lack of oxygen (oxygen starvation) is where the natural hard outer coating (oxide layer) of the aluminium has not been allowed to breath (or dry out) and over time has become compromised allowing corrosion to form. Some examples of corrosion caused by Oxygen Styarvation follow;
From left: Once the floor of this boat was lifted, and the fuel tank removed, it was evident signs of corrosion were well underway. Build up of gunge under the floor boards had allowed the salt water to sit in the hull, not allowing the hull to breath. White paste had formed (this is where the oxide layer is breaking down) and underneath was an urgent repair awaiting.
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From left: At first glance there is no apparent corrosion present where this outboard is bolted onto the bracket. With the wooden backing board removed, we can clearly see extensive pitting and corrosion, compromising the integrity of the bracket. A reaction to contact with the preservative chemicals in the timber and the dislike between stainless steel bolts and alloy plate all probably contributed to the problem.
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Stray Current Corrosion
This type of corrosion is not overly common, however does happen as seen in the images below. Stray Current Corrosoin is where an electrical component or system is incorrectly installed or has a bad earth on the boat. It will simply turn the boat into a huge battery, with the salt water becoming the electrolyte. Aluminium hull, plus current, plus electrolyte, equals corrosion.
An incorrectly fitted earth resulted in the images below. The pitting you can see on the aluminium is evidence of where the outer layer of the aluminium has been compromised by the saltwater acting as the electrolyte.
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Galvanic (Dissimilar Metal) Corrosion
Galvanic corrosion is caused by two dissimilar metals immersed together in saltwater and have no sealing product between them. As described earlier, the less noble metal (aluminium) will deteriorate, protecting the more noble metal (stainless) if there is moisture between them. Images of Galvanic Corrosion are to follow soon.