----[Please read http://ercoupers.com/disclaimer.htm before following any advice in this forum.]----Warren is referring to the fact that zinc chromate on the surface of an
aluminum wing spar cap doesn't protect against intergranular corrosion. This
type of corrosion gets started within the aluminum itself. Some
metallurgists believe the source is an impurity in the bauxite that was used
to make the aluminum coupled with the practice of laying hot extrusions (spar
caps) on cold concrete floors to cool at the factory. Whatever the cause,
the boundaries between layers of the metal grain begin to separate and
corrosion fills in the void. Thus, the corrosion works its way to the
surface and, without using X-ray equipment, it isn't detected until it forms
a bubble or blister on the surface or, until it breaks through completely.
Other types of corrosion, particularly surface corrosion, can be protected
against with thorough cleaning and then covering with zinc chromate or some
other zinc-based primer. Dissimilar metal is another form of corrosion and
sometimes can be found inside the Ercoupe aileron and at the hinges where
steel is riveted to aluminum or on early Coupes that had steel main gear
struts bolted to the aluminum center section main spar.
Jim
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