Quick Tips: Stainless steel fasteners and aluminum aircraft.
Stainless steel fasteners are a great alternative to cad-plated steel fasteners. Many owners and pilots opt for this change-out to prevent rust colored stains all over their aircraft. However, some important precautions should be taken to avoid even more expensive repairs due to galvanic corrosion caused by mixing stainless steel and aluminum - otherwise known as dissimilar metal corrosion. This problem is compounded when an aircraft is tied down outside where moisture - an electrolyte - is abundant.
"Aluminum is the anode and stainless is the cathode. In the presence of an electrolyte, the stainless steel is protected and the aluminum is corroded. Vibration and rubbing between the screw head and the aluminum destroys the naturally occuring oxide film that normally protects aluminum. The protective oxide film on aluminum is normally self-renewing, however, under a screw head the absense of oxygen prevents it from renewing." -Mechanic's Toolbox on Galvanic Corrosion
As shown in the slideshow above, dissimilar metal corrosion begins with the aluminum corroding underneath the paint, crawling outward. Once it had grown far enough and expanded, the paint will likely begin to flake off. This process continues, all the while eating away the thickness of the underlying aluminum. Eventually, for sheet aluminum, the corroded circle (as seen in the slideshow) has been known to reach 4-5" in diameter from one fastener. At this point, the aluminum is often so thin that you can poke thru it quite easily.
Also pictured is a location where a flap hinge meets a flap stop set screw. In this instance, the 1/4" aluminum stock "bubbles" much more prominently and usually leads to "exfoliation" corrosion where the aluminum peels off in thin wafers.
Lastly pictured is an easy, inexpensive solution to a potentially devastating problem. Installing nylon washers under the heads of all stainless steel fasteners not only helps to insulate the two metals from each other, but provides a cushion when installing the fasteners. This "cushion" keeps the fastener from spinning against the paint, thus keeping the paint intact and further protecting against corrosion. Nylon washers can be purchased in a variety of sizes from most parts distributors. Aircraft spruce sells them online for pennies a piece.
Charles Stence is an A&P/IA with many years experience inspecting, repairing and maintaining South Florida aircraft.