Par 6-150 Page 6-36 points where repairs are made. It is recommended that only noncorrosive-type sealant be used, since it serves a dual purpose of material separation and sealing.
6-151. IN-PLACE TREATMENT OF MAGNESIUM CASTINGS. Magnesium castings, in general, are more porous and more prone to penetrating attack than wrought magnesium skin.
However, treatment in the field is, for all practical purposes, the same for all magnesium.
Bellcranks, fittings, and numerous covers, plates, and handles may also be magnesium castings.
When attack occurs on a casting, the earliest practical treatment is required to prevent dangerous corrosive penetration. Engine cases in saltwater can develop moth holes and complete penetration overnight.
a. If at all practical, faying surfaces involved shall be separated to treat the existing attack effectively and prevent its further progress. The same general treatment sequence as detailed for magnesium skin should be followed. Where engine cases are concerned, baked enamel overcoats are usually involved rather than other top coat finishes. A good air-drying enamel can be used to restore protection.
b. If extensive removal of corrosion products from a structural casting is involved, a decision from the aircraft manufacturer or a DER may be necessary to evaluate the adequacy of structural strength remaining. Refer to the aircraft manufacturer if any questions of safety are involved.
6-152. EXAMPLE OF REMOVING CORROSION FROM MAGNESIUM. If possible, corroded magnesium parts shall be removed from aircraft. When impossible to remove the part, the following procedure will be used.
a. Positively identify metal as magnesium.
b. Clean area to be reworked.
c. Strip paint if required.
d. Determine the extent of corrosion damage.
e. Remove light to moderate corrosion by one of the following means:
(1) Nonpowered Corrosion Removal.
(a) Nonpowered removal can be accomplished using abrasive mats, cloth, and paper with aluminum oxide grit (do not use silicon carbide abrasive). Metallic wools and hand brushes compatible with magnesium, such as stainless steel and aluminum, may be used.
(b) When a brush is used, the bristles should not exceed 0.010 inch in diameter.
After using a brush, the surface should be polished with 400-grit aluminum oxide abrasive paper, then with 600-grit aluminum oxide abrasive paper.
(c) Pumice powder may be used to remove stains or to remove corrosion on thin metal surfaces where minimum metal removal is allowed.
Par 6-152 Page 6-37
(2) Chemical Corrosion Removal.
(a) Chemical corrosion removal on magnesium alloys is usually done with a chromic acid pickle solution. Chemical corrosion removal methods are not considered adequate for areas that have:
1. Deep pitting,
2. Heavy corrosion and corrosion byproducts,
3. Previously had corrosion removed by mechanical means, or
4. Previously been sandblasted.
(b) Do not use this method for parts containing copper and steel-based inserts (unless the inserts are masked off) and where it might come into contact with adhesive-bonded skins or parts.
(3) The following solution may be used to remove surface oxidation and light corrosion products from magnesium surfaces.
(a) Solution Composition and Operation:
1. Chromium Trioxide. 24 oz.
2. (A-A-55827, Type II). Water to Make 1 gal. Reaction Time 1 to 15 min.
3. Operation Temperature. (Solution can be operated at room temperature for a longer reaction time if desired.) 190 to 202 °F.
4. Container Construction. Lead-lined steel, stainless steel, or 1100 aluminum.
(b) Mask off nearby operating mechanisms, cracks, and plated steel to keep the solution from attacking them.
(c) Apply chromic acid solution carefully to the corroded area with an acid-resistant brush.
(d) Allow the solution to remain on the surface for approximately 15 minutes.
Agitation may be required.
(e) Thoroughly rinse the solution from the surface with plenty of clean water.
(f) Repeat the preceding sequence as necessary until all corrosion products have been removed and the metal is a bright metallic color.
f. Remove moderate to heavy corrosion by one of the following means:
Par 6-152 Page 6-38
(1) Powered Corrosion Removal.
(a) Powered corrosion removal can be accomplished using a pneumatic drill motor with either an aluminum-oxide-impregnated abrasive wheel, flap brush, or rubber grinding wheel with an abrasive value to approximately 120-grain size.
(b) Also, a rotary file with fine flutes can be used for severe or heavy corrosion product buildup on metals thicker than 0.0625 inch. If a flap brush or rotary file is used, it should only be used on one type of metal. Do not use either a hand or rotary carbon steel brush on magnesium.
(c) Pneumatic sanders are acceptable if used with disc or paper of aluminum oxide.
When using sanders, use extra care to avoid overheating aircraft skins thinner than 0.0625 inch.
(d) Do not use rotary wire brushes on magnesium.
WARNING: Cuttings and small shavings from magnesium can ignite easily and are an extreme fire hazard. Fires of this metal must be extinguished with absolutely dry talc, calcium carbonate, sand, or graphite by applying the powder to a depth of 1/2 inch over the metal.
(2) Blasting. Abrasive blasting is an approved method of corrosion removal on magnesium alloys of a thickness greater than 0.0625 inch. Remove heavy corrosion products by hand brushing with a stainless steel or fiber brush, followed by vacuum abrasive blasting with glass beads (Specification MIL-G-9954), sizes 10–13, or grain abrasive (Specification A-A-1722), type I or III, at an air pressure of 10–35 psi (if suction equipment is used, use a 50 percent higher pressure). Upon completion of blasting, inspect for the presence of corrosion in the blast area. Give particular attention to areas where pitting has progressed into intergranular attack. This is necessary because abrasive blasting has a tendency to close up streaks of intergranular corrosion rather than remove them if the operator uses an improper impingement angle. If the corrosion has not been removed in a total blasting time of 60 seconds on any one specific area, other mechanical methods of removal should be utilized.
CAUTION: When blasting magnesium alloys, do not allow the blast stream to dwell on the same spot longer than 15 seconds. Longer dwell times will cause excessive metal removal.
g. Inspect the reworked area to ensure that no corrosion products remain. If corrosion products are found, repeat method used and reinspect.
h. Fair depressions resulting from rework using a blend ratio of 20:1. Clean rework area using 240-grit abrasive paper. Smooth with 300 grit and finally polish with 400-grit abrasive paper.
i. Determine depth of faired depressions to ensure that rework limits have not been exceeded. Refer to the manufacturer’s specifications.
Par 6-152 Page 6-39
j. Clean reworked area using a solvent to provide a water-break-free surface. Do not use kerosene or another petroleum base fuel as a cleaning solvent.
k. Apply chromic acid brush-on pretreatment.
(1) Chemical pretreatment such as the following chromic acid solution (conversion coat conforming to Specification SAE-AMS-M-3171, type VI) provides a passive surface layer with an inhibitive characteristic that resists corrosive attack and also provides a bond for subsequent coatings. Properly applied magnesium pretreatment tends to neutralize corrosion media in contact with the surface.
(2) The chromic acid brush-on pretreatment may be applied to all magnesium parts that require touchup. This treatment is generally used in refinishing procedures or when parts and assemblies are too large to be immersed. This treatment is less critical to apply than the other brush-on treatments. It is relatively inexpensive and not as harmful when trapped in faying surfaces.
(a) Solution Composition and Operation:
1. Distilled Water 1 gal.
2. Chromic Acid (CrO3) 1.3 oz.
3. (99.5 pure), Calcium Sulfate 1 oz. (CaSo4.2H2O)
4. Operating Temp. 70–90 °F.
5. Container: Stainless Steel, Aluminum, Vinyl, Polyethylene, or Rubber.
NOTE: Good application requires proper preparation of the chromic acid coating solution and cleaning of the surface where the solution will be applied. A water-break test is recommended if the cleanliness of the surface is in doubt.
(b) Add chemicals to water in the order shown.
(c) Stir vigorously for at least 15 minutes, either mechanically or by air agitation, to ensure that the solution is saturated with calcium sulfate. (Let solution stand for 15 minutes before decanting.)
(d) Prior to use, decant solution (avoid transfer of undissolved calcium sulfate) into suitable containers (polyethylene or glass).
(e) Apply solution by brush, swab, or flow on using low-pressure spray (nonatomizing) until the metal surface becomes a dull color (the color can vary from green-brown, brassy, yellow-brown to dark-brown). For good paint adhesion, a dark-brown color free of powder is considered best. The color may vary in using different vendors’ materials.
Par 6-152 Page 6-40 NOTE: Too long an exposure to the brush-on solution produces coatings that will powder and impair adhesion of applied paint finish/films.
(f) Observe the coating closely during the treatment for color changes, rinsed with cold running water when the desired condition/color is reached and air-dried. Preparation and use of test panels made of the same material and under the same conditions before starting the actual treating operation may be used as to determine the application time required to produce the required coating. A good coating is uniform in color/density, adheres well, and is free of loose powder.
l. Apply primer and top coat finish.
m. Remove masking and protective coverings.
6-153.–6-163. [RESERVED.]
Par 6-164 Page 6-41 SECTION 10. FERROUS METALS
6-164. GENERAL. One of the most familiar kinds of corrosion is red iron rust. Red iron rust results from atmospheric oxidation of steel surfaces. Some metal oxides protect the underlying base metal, but red rust is not a protective coating. Its presence actually promotes additional attack by attracting moisture from the air and acts as a catalyst to promote additional corrosion.
a. Red rust first shows on bolt heads, holddown nuts, and other unprotected aircraft hardware. Red rust will often occur under nameplates that are secured to steel parts. Its presence in these areas is generally not dangerous. It has no immediate effect on the structural strength of any major components. However, it shows a general lack of maintenance and may indicate attack in more critical areas.
b. When paint failures occur or mechanical damage exposes highly stressed steel surfaces to the atmosphere, even the smallest amount of rusting is potentially dangerous and should be removed immediately.
6-165. SPECIAL TREATMENT OF HIGH-STRENGTH STEEL. (High-strength steels heat-treated above Rockwell C40, 180,000 psi tensile strength.) Any corrosion on the surface of a highly stressed steel part is potentially dangerous, and the careful removal of corrosion products is mandatory. Surface scratches or change in surface structure from overheating can cause sudden failure of these parts. The removal of corrosion products is required and will be performed carefully and completely.
a. Acceptable methods include careful use of mild abrasive mats, cloths, and papers, such as fine grit aluminum oxide, metallic wool, or fine buffing compounds.
b. Undesirable methods include the use of any power tool because the danger of local overheating and the formation of notches that could lead to failure. The use of chemical corrosion removers is prohibited, without engineering authorization, because high-strength steel parts are subject to hydrogen embrittlement.
6-166. SPECIAL TREATMENT OF STAINLESS STEEL. Stainless steels are of two general types: magnetic and nonmagnetic.
a. Magnetic steels are of the ferritic or martensitic types and are identified by numbers in the 400 series. Corrosion often occurs on 400-series stainless steels, and treatment is the same as specified in high-strength steels. (See paragraph 6-165.)
b. Nonmagnetic stainless steels are of the austenitic type and are identified by numbers in the 300 series. They are much more corrosion resistant than the 400-series steels, particularly in a marine environment.
(1) Austenitic steels develop corrosion resistance by an oxide film, which should not be removed even though the surface is discolored. The original oxide film is normally formed at time of fabrication by passivation. If this film is broken accidentally or by abrasion, it may not restore itself without repassivation.
Par 6-166 Page 6-42
(2) If any deterioration or corrosion does occur on austenitic steels, and the structural integrity or serviceability of the part is affected, it will be necessary to remove the part.
6-167. EXAMPLE OF REMOVING CORROSION FROM FERROUS METALS. If possible, corroded steel parts should be removed from the aircraft. When impractical to remove the part, follow the procedure below.
a. Prepare the area for rework.
b. Positively identify the metal as steel and establish its heat-treated value.
c. Clean the area and strip paint if required.
NOTE: Use of acid-based strippers, chemical removers, or chemical conversion coatings is not permitted on steel parts without engineering authorization.
d. Determine extent of corrosion damage.
e. Remove residual corrosion by hand sanding with mild abrasive mats, cloths, and papers, such as fine aluminum oxide grit.
f. Remove heavy deposits of corrosion products by approved mechanical methods for that particular form of steel and/or stainless steel.
g. Inspect the area for remaining corrosion. Repeat procedure if any corrosion remains and the structural integrity of the part is not in danger, and the part meets the rework limits established by the manufacturer or FAA-authorized DER.
h. Fair depressions using a blend ratio of 20:1. Clean area using 240-grit paper. Smooth area with 300-grit paper and give final polish with 400-grit paper.
i. Determine depth of faired depression to ensure that rework limits have not been exceeded.
j. Clean reworked area with dry cleaning solvent. Do not use kerosene.
k. Apply protective finish or specific organic finish as required.
NOTE: Steel surfaces are highly reactive immediately following corrosion removal; consequently, primer coats should be applied within 1 hour after sanding.
l. Remove masking and protective coverings.
6-168.–6-178. [RESERVED.]
Par 6-179 Page 6-43 SECTION 11. OTHER METALS AND ALLOYS
6-179. NOBLE METAL COATINGS CLEANUP AND RESTORATION. Silver, platinum, and gold finishes are used in aircraft assemblies because of their resistance to ordinary surface attack and their improved electrical or heat conductivity. Silver-plated electrodes can be cleaned of brown or black sulfide tarnish by placing them in contact with a piece of magnesium sheet stock while immersed in a warm water solution of common table salt mixed with baking soda, or by using a fine grade abrasive mat or pencil eraser followed by solvent cleaning. If assemblies are involved, careful drying and complete displacement of water is necessary. In general, cleaning of gold or platinum coatings is not recommended in the field.
6-180. COPPER AND COPPER ALLOYS are relatively corrosion resistant, and attack on such components will usually be limited to staining and tarnish. Such change in surface condition is not dangerous and should ordinarily have no effect on the function of the part.
However, if it is necessary to remove such staining, a chromic acid solution of 8–24 ounces per gallon of water containing a small amount of battery electrolyte (not to exceed 50 drops per gallon) is an effective brightening bath. Staining may also be removed using a fine grade abrasive mat or pencil eraser followed by solvent cleaning.
a. Immerse the stained part in the cold solution. Surfaces can also be treated in place by applying the solution to the stained surface with a small brush.
b. Avoid any entrapment of the solution after treatment. Clean the part thoroughly following treatment with all residual solution removed.
c. Serious copper corrosion is evident by the accumulation of green-to-blue copper salts on the corroded part. Remove these products mechanically using a stiff bristle brush, brass wire brush, 400-grit abrasive paper, or bead blast with glass beads (Specification MIL-G-9954, size 13). Air pressure when blasting should be 20–30 psi for direct pressure machines. Do not bead blast braided copper flexible lines. Reapply a surface coating over the reworked area.
Chromic acid treatment will tend to remove the residual corrosion products.
WARNING: Brushing, sanding, and abrasive blasting of copper and copper alloys can be dangerous due to the creation of toxic airborne particles. Take necessary precautions to ensure safety.
6-181. TITANIUM AND TITANIUM ALLOYS. Titanium and titanium alloys are highly corrosion resistant because an oxide film forms on their surfaces upon contact with air.
a. When titanium is heated, different oxides having different colors form on the surface.
A blue oxide coating will form at 700–800 °F, a purple oxide at 800–950 °F, and a gray or black oxide at 1,000 °F or higher. These coatings are protective discolorations and should not be removed.
b. Corrosive attack on titanium surfaces is difficult to detect. It may show deterioration from the presence of salt deposits and metal impurities at elevated temperatures, so periodic removal of surface deposits is required. However, if corrosion develops on titanium, it usually occurs as pitting. Acceptable methods for corrosion removal are:
Par 6-181 Page 6-44
(1) Stainless steel wool or hand brush.
(2) Abrasive mats, cloths, and papers with either aluminum oxide or silicon carbide grit.
(3) Dry abrasive blasting using glass beads (Specification MIL-G-9954), sizes 10–13, or aluminum oxide (Specification A-A-59316, type I, grade A or B) at a blast pressure of 40–50 psi (if using suction equipment, use 50 percent higher pressure).
WARNING: Dry abrasive blasting of titanium alloys creates sparking.
Ensure that hazardous concentrations of flammable vapors are not present.
(4) Hand polish with aluminum polish and soft cloth.
c. Titanium surfaces are susceptible to hydrogen embrittlement that can induce stress corrosion and associated pitting. Therefore, chemicals such as fire-resistant hydraulic fluids must be controlled. Chlorinated hydrocarbon solvents and chemical corrosion removers are prohibited from use on titanium and titanium alloys.
6-182.–6-192. [RESERVED.]
Par 6-193 Page 6-45 SECTION 12. PLATED PARTS
6-193. CHROMIUM AND NICKEL-PLATED PARTS. Nickel and chromium platings are used extensively as protective and wear-resistant coatings over high-strength steel parts (landing gear journals, shock strut pistons, etc.). Chromium and nickel plate provide protection by forming a somewhat impervious physical coat over the underlying base metal. When breaks occur in the surface, the protection is destroyed.
a. The amount of reworking that can be performed on chromium and nickel-plated components is limited.
b. The rework should consist of light buffing to remove corrosion products and produce the required smoothness. The buffing should not take the plating below the minimum allowable thickness.
c. Whenever a chromium or nickel-plated component requires buffing, coat the area with a corrosion-preventive compound, if possible.
d. When buffing exceeds the minimum thickness of the plating, or the base metal has sustained corrosive attack, the component should be removed and replaced.
e. The removed component can be restored to serviceable condition by having the old plating completely stripped and replated in accordance with acceptable methods and specifications.
6-194. CADMIUM AND ZINC-PLATED PARTS. Cadmium plating is used extensively in aircraft construction as a protective finish over both steel and copper alloys. Protection is provided by a sacrificial process in which the cadmium is attacked rather than the underlying base material. Properly functioning cadmium surface coatings may show mottling, ranging from white to brown to black spots on their surfaces. These show the sacrificial protection being offered by the cadmium coat, and under no condition should such spotting be removed merely for appearance’ sake. In fact, cadmium will continue to protect even when actual breaks in the coating develop and bare steel or exposed copper surfaces appear.
a. When the breakdown of the cadmium plating occurs and the initial appearance of corrosion products on the base metal develops, some mechanical cleaning of the area may be necessary but shall be limited to removal of the corrosion products from the underlying base material.
b. Under no condition should such a coating be cleaned with a wire brush. If protection is needed, a touchup with primer or a temporary preservative coating should be applied.
Restoration of the plate coating cannot be done in the field.
c. Zinc coatings offer protection in an identical manner to cadmium, and the corrective treatment for failure is generally the same as for cadmium-plated parts. However, the amount of zinc on aircraft structures is very limited and usually does not present a maintenance problem.
6-195–6-205. [RESERVED.]
Par 6-206 Page 6-46 SECTION 13. CORROSION PROOFING OF LAND PLANES CONVERTED TO
SEAPLANES
6-206. GENERAL. A special problem is encountered in the conversion of land planes to seaplanes. In general, land planes do not receive corrosion proofing to the same extent as do seaplanes. Corrosion-proofing standards for land planes converted to seaplanes are divided into two classes: necessary minimum precautions and recommended precautions. Regardless of such precautions, it is imperative that the exterior surfaces of seaplanes be washed with clear freshwater immediately following extended water operation, or at least once a day when operated in salty or brackish water. Wash interior surfaces of seaplanes exposed to spray, taking care to prevent damage to electrical circuits or other items subject to injury.
6-207. NECESSARY MINIMUM PRECAUTIONS. The following procedures are considered the minimum to safeguard the airworthiness of converted aircraft and are not in themselves intended to maintain airworthiness for an indefinite period.
a. Unless already protected, treat exposed fittings or fittings that can be reached through inspection openings with two coats of zinc-chromate primer, paralketone, non-water-soluble heavy grease, or comparable materials. This applies to items such as wing-root fittings, wing-strut fittings, control-surface hinges, horns, mating edges of fittings, and attached bolts.
b. Coat nonstainless control cables with grease, paralketone, or other comparable protective coating, if not replaced with corrosion-resistant cables.
c. Inspect all accessible sections of aircraft structure. Clean structural parts showing corrosion and refinish if corrosion attack is superficial. If a part is severely corroded, replace with an adequately corrosion-proofed part.
6-208. RECOMMENDED PRECAUTIONS. Recommended precautions are those which are suggested as a means of maintaining such aircraft in condition for safe operation over extended periods.
a. Provide additional inspection openings to assist in detecting corrosion. Experience has shown openings to allow inspection of the lower and rearward portion of the fuselage to be particularly desirable.
b. Incorporate additional provisions for free drainage and ventilation of all interiors to prevent collection of moisture (scoop-type marine drain grommets).
c. Protect the interior of structural steel tubing. This may be done by air and watertight sealing or by flushing with hot linseed oil and plugging the openings. Inspect tubing for missing sealing screws, presence of entrapped water, local corrosion around sealing screws, welded clusters, and bolted fittings, which may be indicative of entrapped moisture.
d. Slit the fabric of fabric-covered aircraft longitudinally on the bottom of the fuselage and tail structure for access to these sections. Coat the lower structural members with zinc-chromate primer (two coats); follow with a coat of dope-proof paint or wrap with cellophane tape and
Par 6-208 Page 6-47 rejoin the fabric. This precaution is advisable within a few months after start of operation as a seaplane.
e. Spray the interior of metal-covered wings and fuselages with an adherent corrosion inhibitor.
f. Place bags of potassium or sodium dichromate in the bottom of floats and boat hulls to inhibit corrosion.
g. Prevent the entry of water by sealing, as completely as possible, all openings in wings, fuselage, control-surface members, openings for control cables, tail-wheel wells, etc.
6-209.–6-219. [RESERVED.]