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The marine corrosion resistrance of copper-, iron-, nickel-, aluminum-, and titanium-based alloy fasteners are reviewed. Coated-steel systems are also included. Several new alloys are characterized as candidate materials. Galvanic compatibility is identified as the most critical consideration for marine fasteners. Guidelines are presented to assist designers in selection of appropriate marine fasteners materials.
Fastener materials can be subjected to five primary conditions or zones in the marine environment: marine atmosphere, splash, tidal, full-immersion, and mud zones. Each of these environments provides different degrees of corrosiveness depending on the material. In addition to these corrosion zones, other seawater conditions greatly affect fastener corrosion resistance: seawater velocity, pollutants (industrial airborne and dissolved gases from decaying organisms), temperature, and hydrogen evolution from electrochemical reactions in the marine environment.
The first fastener failure was apparently documented in 1761. Iron nails corroded from a copper hull sheathing with partial loss of the sheathing from the H.M.S. Alarm. This classic example of galvanic corrosion in seawater was one of the first forms of marine corrosion identified.
Today, galvanic corrosion is one of the most important forms of corrosion to consider when designing a fastener system, because by its nature, a fastener system is normally made up of two or more different metals in a metallic couple. One or more of the metals acts as an anode (corrodes), and the other metal acts as a cathode (protected against corrosion).
Galvanic effects are of primary importance in selecting fasteners. The fastener should be cathodic to (more noble than) the base plate material. It should never be the anode (less noble) material. The use of stainless-steel fasteners in aluminum structures is common and follows the galvanic guidelines just mentioned.
However, in aluminum structures exposed in the marine atmosphere, galvanic corrosion of the aluminum causes enlarging of the fastener hole allowing the uncorroded stainless-steel fastener to drop out. The use of copper alloy fasteners in aluminum results in severe pitting and corrosion of the aluminum, and should be avoided.
Using fastener systems of aluminum to aluminum, stainless to stainless, stainless to fiberreinforced plastics (FRP), and stainless to wood limits the usefulness of these combinations and requires special precautions for satisfactory service. Packing these fastener joints with most greases is not effective, since moisture wicks...