First of all, HOW rusty are the tubes? Some surface rust is pretty
common inside an axle housing, especially one that has been sitting for
a long time. If the pitting is deep enough to cause structural problems,
I'd suggest getting a new or good used housing.
If it's just some minor surface rust and thin flakes, you don't need to
do anything quite so "eccentric" as using a cylinder ridge reamer...much
ado about nothing. There is also the chance that a reamer might create
some radial grooves in the inside surface that could create stress
points for future failure. Here's something else to consider: The inside
of axle tubes are seldom, if ever, perfectly round. You may end up
taking too much metal off in places and weakening the tubes. A good
stiff wire brush and a pressure washer would remove the loose flakes.
The solid scale material won't hurt anything, and may actually help
protect the metal underneath.
If you remove all of the loose rust and clean off all of the oil, so
that the metal is solid, dry, and oil free, you could paint the inside
of the housing with a quality internal engine paint like Glyptol, which
is designed to be used inside engine blocks and is unaffected by hot
oil.
As to chemical pickling, just plain acid (such as phosphoric acid) won't
help at all. There are some steel prep solutions that will etch the
surface AND deposit an extremely thin conversion coating that will give
some SHORT-TERM protection against flash rusting, but absolutely will
NOT provide any LONG-TERM protection. These solutions were intended
primarily to enhance paint adhesion and provide some protection against
rust for a short period of time between stripping/blasting and priming.
If you had aluminum housings, that would be different, since there are
several processes that create corrosion-resistant conversion coatings on
aluminum alloys that will last many times longer than any such
chemically-applied coatings on steel (at least that I'm aware of).
Even gun blueing techniques only provide a limited amount of corrosion
protection to steel. I've used several different blueing and browning
techniques, both hot and cold, on gun barrels and other gun components.
While these do provide a measure of protection, it is still necessary to
keep the steel oiled to maintain long-term protection. In order to
create long-term corrosion protection on a steel surface, you'd have to
resort to a process like plating or parkerizing...or paint.
During assembly, it wouldn't hurt to give the insides of the housing a
heavy coating of gear lube. Synthetic lubes will adhere better and leave
a much longer-lasting film. The insides of the axle tubes do not get
much oil splashed on them, since there is normally not enough oil in the
tubes for the shafts to pick it up and fling it around. Actually, on
some axles, this is not true, but most axles usually only have a shallow
pool of lube along the bottom of the tube during normal operation.
If you're serious about future rust prevention, I'd scrape/brush out the
loose rust, pressure wash the tubes, rinse them with solvent, and paint
them. After the paint is good and dry, I'd swab the inside with a heavy
synthetic gear lube. By the way, since the axle won't be experiencing
the high temperatures of an engine block, you could use a wider range of
paints...self-etching primer, primer-sealer, and then a top coat if you
really want to get radical. Make sure you use a paint that has high oil
and solvent resistance.
Regards,
Alan
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