I have to agree with Mark Steitle on the control tube corrosion issue. Below
is a sample of a few items published in AC43-16A (Aviation Maintenance
Alerts) from 1989, 1997 and 2000.

If you have tubes that are already installed you might consider attempting
to spray the interior with ACF50 or Corrosion X. As I understand it these
types of materials are sacrificial and must be reapplied periodically.

Ted Stanley - A&P-IA


Cessna, Flap Tube Corrosion, Model 550, Citation
While accomplishing a routine phase 12 inspection, the right wing inboard
flap push-pull tube was found to have bubbled paint near one end. Further
inspection disclosed a hole approximately 5/8 inch in diameter corroded
through the tube. The hole was located 4 1/2 inches from the rod end bolt
hole. Upon inspection of the tube interior, no corrosion preventative was
evident. Part total time - 1212 hours.


Main Landing Gear Torque Tube Corrosion
Cessna - Model 340

   Information for the following article was furnished by Mr. Stephen Smith,
an Airworthiness Inspector, with the FAA's Flight Standards District Office
located in Hillsboro, Oregon.
   The main landing gear had been removed for installation of a Cessna
repair kit (P/N SK414-9E). After installation of the kit, the torque tubes
(P/N's 5045010-18, -19) were being cleaned and inspected for cracks prior to
reinstallation. No cracks were found; however, when the torque tubes were
shaken, something made a "scraping" sound inside both torque tubes.
   Both of the torque tube end plugs were removed, and an alarming amount of
"rust scale" was removed. The interior of the torque tube was inspected with
a borescope, and the walls were found covered with severe corrosion. Cessna
Technical Support was contacted concerning this condition. Their response
was that if the corrosion had consumed more than 10 percent of the torque
tube's wall thickness, the torque tube would no longer be airworthy. An
ultrasonic inspection of the torque tubes revealed one tube was marginal and
the other exceeded the 10 percent limit.
   While attempting to obtain serviceable used parts, all parts supplied by
vendors were determined to be in the same poor condition, and all parts were
rejected.
   Needless to say, that failure of this part would present a very hazardous
situation which could endanger life and property. The torque tubes may
appear to be in good condition from the outside and have a large amount of
corrosion on the inside. Only by removing the end plugs can the torque tube
interior be properly inspected. A search of the FAA's Service Difficulty
Report data base disclosed 40 reported incidents where these torque tubes
cracked or failed. This is a very significant number, and the submitter
suggested each operator establish a periodic, calendar, and flight-time
schedule for inspection of the interior of these torque tubes. The
ultrasonic inspection technique seems to be the most reliable inspection
method for determining tube wall thickness.

Cessna; Model 310K; Flight Control Push-Pull Tube Corrosion; ATA 2700

During an accident investigation, inspectors found several severely corroded
push-pull tubes.
The aileron, elevator, and rudder push-pull tubes exhibited corrosion of the
interior surfaces. These tubes used a forked terminal on each end, and the
tube remained open to the atmosphere. The open tubes invite the migration
and accumulation of water and other contaminates into the interior causing
the propagation of corrosion. The tubes, in this case, did not have internal
corrosion protection.
Part total time-2,000 plus hours.