Aluminum is fairly corrosion prone. It’s nearly at the bottom of the galvanic scale, making it less noble than most other metals. This makes aluminum susceptible to corrosion when it is in contact with dissimilar and incompatible metals. These primarily take the form of copper alloys, such as bronze and brass, which are found in winch bases, fasteners, tracks and pad eyes, among other hardware items. These metals are to aluminum what Kryptonite is to Superman, so you should make every effort to keep aluminum from coming into contact with them.
Many older aluminum spars were fit using this bronze or brass hardware, without the benefit of insulating materials. Because of this, corrosion often runs rampant beneath and around these items. In fact, you are nearly certain to find corrosion when these items are removed, which is precisely why they must be removed.
When replacing this hardware, use only stainless fasteners (machine screws, tapping screws or rivets; depending on the loads that are to be imparted, aluminum rivets may be used in select applications) whose threads have been coated liberally with bedding compound, as is the case for every fastener used on the spar.
Additionally, copper-alloy components must never be allowed to come in direct contact with an aluminum spar. This may require the fabrication of insulating pads, strips or washers made from nylon, UV-stable Garolite or other similar nonmetallic materials. The latter material is reinforced and thus not prone to splitting or cracking when used in high-stress applications.
Related to dissimilar-metal, or galvanic, corrosion is stray-current corrosion. Spars must never be used as current-carrying conductors. This often occurs – sometimes by default, other times by misguided design – when the spar takes the place of the ground leg or when a high-resistance short occurs, allowing positive current to return to ground through the spar. In either case, the results are always undesirable. In some instances, the spar may be so badly damaged by stray-current corrosion that it must be condemned.