Yes I would definitely alodine aluminium before bonding. Magnesium is a very
active (anodic) metal.

Of course the environment in which dissimilar metals interfaces operate has
a big influence on the likelihood and severity of corrosion. If the metals
interface is dry and sealed then you can get away with a lot ... if exposed
to moisture and or a salty atmosphere then you are in much bigger trouble !



" jls" wrote in message
. ..

"smjmitchell" wrote in message [...] 5.
Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on
the galvanic table. That is a big different. You definitely need
something
to separate the two or the magnesium is going to get gobbled up !!


Interesting comment but the rear case on many A-65 Continentals, and on
most
C-85's, is magnesium bolted to an aluminum alloy crankcase. The bolts,
of
course, are cad-plated steel and there's a fat gasket between the cases.
Whenever I rebuild one of these engines I always use magnedyne to
passivate
the magnesium and alodine for the aluminum case. Magnesium doesn't need
any contact with another metal to corrode like crazy, but it sure does
make
a nice lightweight accessory case, including good bearing pockets for the
oil pump and its shafts. So you magnedyne it and paint it, and of course
paint the crankcase too after passivating it, just like the Continental
overhaul manual advises.

Aileron hinges on the later Taylorcrafts are magnesium, and so are the
yokes, and so are the wheels. Beautiful castings but they must be
treated
and preserved with great care.