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