[VAC] Aluminum, Fasteners and Corrosion

["Arlen & Shirley" <globetrotter64@xxxxxxx>]

I have seen some recent posts regarding screws in aluminum and corrosion,
and wanted to add my opinions to the subject. There was a comment regarding
using aluminum oxide paste under the screw. I do not see how this would be
of any benefit, and would actually be a determent, as it provides place for
moisture to better wick into the joint between the screw and the aluminum.
You will have plenty of aluminum oxide soon enough without adding any more
to start with!

In simple theory, two dissimilar metals in contact in the presence of an
electrolyte (moisture plus pollution, acids, smog) form a battery. The more
noble metal becomes the cathode and the less noble metal becomes the
SACRIFICIAL anode. A galvanized steel screw is less noble than most alloys
of aluminum, and the screw will corrode first. But alas, as the galvanized
plating deteriorates it exposes the steel, and soon both the aluminum and
bare steel are corroding.  One can use stainless steel screws, but most
stainless steel is more noble than most aluminum alloys and the aluminum
becomes sacrificial. What to do?

Fortunately, this is a well know issue with aircraft builders and owners -
they like sound aluminum and pretty screws too!  The practical solution is
to use a thin nylon washer under a stainless steel screw head.  This
isolates the contact point where the screw head contacts the skin and seals
the area where the screw threads go into the aluminum. This is not a perfect
solution as there is still electrical contact, but it provides a practical
compromise. These nylon washers are usually available from the same source
as the nice truss head screws - aircraft hardware supply houses. Aircraft
Spruce and Specialty is one well known supplier.

On the same subject, you should use a barrier between any non-aluminum
fittings or name plates attached to the skin. A piece of Mylar sheet cut to
the shape of the contact area works well.

There is a great deal of both practical and theoretical information at this
NASA site regarding corrosion.  This page includes a nobility chart for
metals.
http://corrosion.ksc.nasa.gov/html/galcorr.htm

One last comment about stainless screws. Most stainless steel screws are
relatively soft, weak, and do not have nice smooth thread form. On new
installations, screw a carbon steel screw into the hole first to form the
threads. Remove it and install the stainless screw with the nylon washer.

Arlen & Shirley Manning Napa CA
1964 Globetrotter
http://globetrotter64.home.att.net/