attack, deep pitting, stress corrosion, intergranular, and galvanic corrosion. Corrosion of magnesium or its alloys forms white, powdery, snowlike mounds. The deposits tend to raise slightly. and the corrosion spreads rapidly. When magnesium corrosion is detected, it requires immediate attention or the corrosion will spread throughout the entire structure. Magnesium corrosion reprotection involves the maximum removal of corrosion products, the partial restoration of surface coatings by chemical treatment, and a reapplication of protective coatings. After maintenance personnel clean the surface and strip the paint, (if any,) they break loose and remove as much of the corrosion products as possible. They do this by using a pneumatic drill with an abrasive wheel or a Vacu-Blast Dry Honing Machine with glass beads. Steel wire brushes. Carborundum™ abrasives, or steel-cutting tools should NOT be used. After corrosion removal, maintenance personnel treat the surface with specification MIL-M-3171 (type VI) chemical treatment solution, as outlined in the NAVAIR 01-1A-509. Then restore the protective paint film. If extensive removal of corrosion products from a structural casting was involved, a decision from a structural engineer may be necessary to evaluate the adequacy of the structural strength remaining. Structural repair manuals for the aircraft models involved usually include tolerance limits for dimensions of critical structural members. They should be referred to if any question of safety of flight is involved. Copper and Copper Alloys Copper and its alloys are generally corrosion resistant, although the products of corrosive attack on copper are commonly known. Sometimes copper or copper-alloy surfaces will tarnish to a dull gray-green color, and the surface may still be smooth. This discoloration is the result of the formation of a fine-grained, copper oxide crust called “patina.” The patina, in itself, offers good protection for the underlying metal in ordinary situations. However, exposure of copper and copper alloys to moisture or salt spray causes the formation of blue or green salts, indicating active corrosion. These salts form over the patina since this crust is not totally moistureproof. Copper alloys used in aircraft have a cadmium-plated finish to prevent surface straining and decay. Copper and copper-based alloys are used in avionic systems as contacts, springs, connectors, printed circuit board runs, and wires. Copper and copper-based alloys (brass and bronze) are resistant to atmospheric corrosion. However, copper is cathodic to iron, steel, aluminum, and magnesium when in electrical contact with these metals. Maintenance personnel can remove corrosion products by using a pneumatic drill with an abrasive wheel or, as an alternate method, a typewriter eraser (ZZ-E-661. type I or III), depending upon the situation. Copper and copper alloys used in avionic equipment are not usually painted. Cadmium and Zinc Cadmium is used as a coating to protect the part to which it is applied. It also provides a compatible surface when the part is in contact with other materials. The cadmium plate supplies sacrificial protection to the underlying metal because of its greater activity. That is, during the time it is protecting the base metal, the cadmium is intentionally being consumed. It functions in the same way that an active magnesium rod inserted in the water system protects the piping of a hot-water heater. The cadmium becomes anodic and is attacked first, leaving the base metal free of corrosion. Zinc coatings are used for the same purpose, but to a lesser extent in aircraft. Attack is evident by white-to-brown-to-black mottling of the cadmium surfaces. These indications DO NOT indicate decay of the base metal and should NEVER be removed for appearance sake alone. Until the characteristic colors peculiar to corrosion of the base metal appear, no steps should be taken, Cadmium is usually used on bolts as a sacrificial metal to protect the base metal. Zinc is used in avionic/electronic equipment for the same general purpose. Maintenance personne1 remove corrosion products by rubbing lightly with stainless steel wool, abrasive impregnated webbing, or 320-grit or finer aluminum oxide abrasive paper. They do not remove the undamaged cadmium plate adjacent to the corroded area; this will reduce the amount of protection for the underlying base metal. Wire brushes are not used on cadmium-plated surfaces since they will remove more plating than corrosion. After removing corrosion products from cadmium-plated surfaces, maintenance personnel apply a protective coating to retard the corrosive attack. 4-38