Corrosion
They say that the only thing that doesn’t want your boat to sink is you. And while this might be a slight dramatization, any boat owner will tell you that to keep your boat in pristine condition, you must wage a constant war with the elements. One of the primary enemies in that battle is corrosion - particularly if you are operating in salt or brackish waters.
There are two primary kinds of corrosion to worry about. The “regular” kind and dissimilar metal corrosion. The “regular” kind is just your good old oxidation or “rust” (though technically rust is iron oxide only). Some metals are more corrosion resistant than others, which is why most modern boats use primarily stainless steel and aluminum. Some metals such as regular steel or (in the old days iron) require a constant protective coating to prevent them rusting away. Others, like brass and bronze develop a protective patina if left untreated.
All of these are much more labor intensive than the main two mentioned above.
The best way to prevent corrosion in these metals is to ensure that the surfaces remain free of blemishes and scratches. This is more critical for stainless which must be re-polished to regain its corrosion resistant properties. If this is not done, the rough/damaged area will start to corrode and will eventually lead to structural failure if left unchecked. Areas where this is particularly troublesome are welds and fittings, so additional caution is needed in these areas. Mitigation strategies to prevent damaging the stainless and starting the corrosion process is using protective coatings, and inserts (rubber/plastic/teflon etc).
If left untreated, aluminum has the wonderful property of self protection. The metal will oxidize, and that outer layer of oxidation seals off the underlying metal from the oxygen in the atmosphere. So long as that seal remains intact, no additional oxidation occurs. If the surface is scratched, or otherwise damaged, the newly exposed metal will oxidize and re-establish that seal. Even with this wonderful quality, it is still best to avoid damaging surfaces unnecessarily, and some will choose to apply protective coatings/paint to further protect the underlying metal. The tradeoff of this added protection is that if corrosion does manage to infiltrate this layer (as it does so often around fittings etc), the byproduct of that corrosion will separate the paint from the aluminum and bubbles will develop - the bane of any painted aluminum structure.
This leads us neatly to dissimilar metal corrosion - usually stainless bolts into aluminum housings. Without getting too deep into the chemistry of this reaction, suffice it to say that when metals have dissimilar numbers of electrons, the anode gives up its electrons to the cathode and simply dissolves into nothing. The way to prevent this from happening is to inhibit the electrons from migrating between the metals. This is usually done by coating the two surfaces with products like Tef-Gel or Duralac. It is for this exact reason that you should NEVER use electrically isolating coatings with your sacrificial anodes (zincs). They should be in direct contact with the metals they are supposed to protect. But sacrificial anodes are a topic for another time.
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