Corrosion of Ferrous Metals
One of the most familiar types of corrosion is ferrous oxide (rust), generally resulting from atmospheric oxidation of steel surfaces. Some metal oxides protect the underlying base metal, but rust is not a protective coating in any sense of the word. Its presence actually promotes additional attack by attracting moisture from the air and acting as a catalyst for additional corrosion. If complete control of the corrosive attack is to be realized, all rust must be removed from steel surfaces.
Rust first appears on bolt heads, hold-down nuts, or other unprotected aircraft hardware. [Figure 6-14] Its presence in these areas is generally not dangerous and has no immediate effect on the structural strength of any major components. The residue from the rust may also contaminate other ferrous components, promoting corrosion of those parts. The rust is indicative of a need for maintenance and of possible corrosive attack in more critical areas. It is also a factor in the general appearance of the equipment. When paint failures occur or mechanical damage exposes highly stressed steel surfaces to the atmosphere, even the smallest amount of rusting is potentially dangerous in these areas and must be removed and controlled.
Rust removal from structural components, followed by an inspection and damage assessment, must be done as soon as feasible. [Figure 6-15]
Mechanical Removal of Iron Rust
The most practicable means of controlling the corrosion of steel is the complete removal of corrosion products by mechanical means and restoring corrosion preventive coatings. Except on highly stressed steel surfaces, the use of abrasive papers and compounds, small power buffers and buffing compounds, hand wire brushing, or steel wool are all acceptable cleanup procedures. However, it should be recognized that in any such use of abrasives, residual rust usually remains in the bottom of small pits and other crevices. It is practically impossible to remove all corrosion products by abrasive or polishing methods alone. As a result, once a part cleaned in such a manner has rusted, it usually corrodes again more easily than it did the first time.
The introduction of variations of the nonwoven abrasive pad has also increased the options available for the removal of surface rust. [Figure 6-16] Flap wheels, pads intended for use with rotary or oscillating power tools, and hand-held nonwoven abrasive pads all can be used alone or with light oils to remove corrosion from ferrous components.
Chemical Removal of Rust
As environmental concerns have been addressed in recent years, interest in noncaustic chemical rust removal has increased. A variety of commercial products, which actively remove the iron oxide without chemically etching the base metal, are available and should be considered for use. Generally speaking, if at all possible, the steel part should be removed from the airframe for treatment, as it can be nearly impossible to remove all residues. The use of any caustic rust removal product will require the isolation of the part from any nonferrous metals during treatment, and will probably require inspection for proper dimensions.
Chemical Surface Treatment of Steel
There are approved methods for converting active rust to phosphates and other protective coatings. Other commercial preparations are effective rust converters where tolerances are not critical and where thorough rinsing and neutralizing of residual acid is possible. These situations are generally not applicable to assembled aircraft, and the use of chemical inhibitors on installed steel parts is not only undesirable but also very dangerous. The danger of entrapment of corrosive solutions and the resulting uncontrolled attack, which could occur when such materials are used under field conditions, outweigh any advantages to be gained from their use.
Removal of Corrosion from Highly Stressed Steel Parts
Any corrosion on the surface of a highly stressed steel part is potentially dangerous, and the careful removal of corrosion products is required. Surface scratches or change in surface structure from overheating can also cause sudden failure of these parts. Corrosion products must be removed by careful processing, using mild abrasive papers such as rouge or fine grit aluminum oxide, or fine buffing compounds on cloth buffing wheels. Nonwoven abrasive pads can also be used. It is essential that steel surfaces not be overheated during buffing. After careful removal of surface corrosion, reapply protective paint finishes immediately.