Metal Finishing
Aircraft parts are almost always provided with some type of surface finish by the manufacturer. The primary purpose of these finishes is to provide corrosion resistance; however, they may also be applied to improve wear resistance or to provide a suitable base for paint application.
In most cases, the original finishes described in the following paragraphs cannot be fully restored in the field due to limitations in equipment or facilities. Nevertheless, an understanding of these metal finishes is essential for proper maintenance and for the effective application of corrosion control and partial restoration techniques.
Surface Preparation
Original surface treatments for steel parts typically begin with thorough cleaning to remove all traces of dirt, oil, grease, oxides, and moisture. This step is essential to ensure a strong bond between the metal surface and the applied finish. Cleaning methods may be either mechanical or chemical.
Mechanical cleaning methods include the use of wire brushes, steel wool, emery cloth, sandblasting, or vapor blasting.
Chemical cleaning is generally preferred over mechanical methods because it does not remove any base metal. Various chemical processes are used depending on the type of material and the nature of the contaminants being removed.
Steel parts are often pickled to remove scale, rust, or other foreign matter, especially prior to plating. Pickling solutions typically consist of muriatic (hydrochloric) acid or sulfuric acid. While sulfuric acid is more economical, muriatic acid is more effective in removing certain types of scale. The pickling solution is usually contained in a stoneware tank and heated using a steam coil. Parts that are not to be electroplated after pickling are immersed in a lime bath to neutralize any residual acid.
Electrocleaning is another chemical cleaning method used to remove grease, oil, or organic contaminants. In this process, the metal is suspended in a hot alkaline solution containing wetting agents, inhibitors, and conductive materials. An electric current is then passed through the solution, similar to the electroplating process.
Aluminum and magnesium parts may also be cleaned using some of the methods described above. However, abrasive blast cleaning is not suitable for thin aluminum sheets, particularly Alclad materials. Steel grit must never be used on aluminum or corrosion-resistant metals, as it may cause contamination and subsequent corrosion.
Polishing, buffing, and coloring are important finishing processes used to improve surface condition and appearance. These processes are often used prior to electroplating, and in some cases, to achieve a high-luster surface finish.