Par 6-148 Page 6-273 Inspect the area for remaining corrosion. Repeat procedure if any corrosion remains.
NOTE: If corrosion remains after the second att empt, use a stronger method, e.g., chemical to mechanical.
Using a blend ratio of 20:1 (length to depth) blend and finish th e corrosion rework area with progressively finer abrasive paper until 400-grit paper is used.
Clean reworked area using dry cleaning solvent. Do not use ke rosene or any other petroleum base fuel as a cleaning solvent.
Determine depth of faired depressions to ensure that rework limits have not been exceeded.
Apply chemical conversion coating, MIL-C-81706, immediately after reworking. If 48 hours or more have elapsed since the conversion coating was first applied and the primer or final paint system has not yet been applied, then reapply the conversion coating before continuing.
NOTE: These solutions should not be allowed to co me in contact with magnesium or high-strength steels (180,000 psi). Do not permit solutions or materials to contact paint thinner, acetone or other combustible material: FIRE MAY RESULT.
Apply paint finish to area.
6-137.6-147. [RESERVED.]
Par 6-148 Page 6-274 SECTION 9. MAGNESIUM AND MAGNESIUM ALLOYS
GENERAL. Magnesium and magnesium alloys are the most chemically active of the metals used in aircraft construction and are the most difficult to protect. However, corrosion on magnesium surfaces is probably the easiest to detect in its early stages. Since magnesium corrosion products occupy several times the volume of the original magnesium metal destroyed, initial signs show a lifting of the paint films and white spots on the magnesium surface. These rapidly develop into snow-like mounds or even white whiskers. The prompt and complete correction of the coating failure is imperative if serious structural damage is to be avoided.
TREATMENT OF WROUGHT MAGNESIUM SHEETS AND FORGINGS. Corrosive attack on magnesium skins will usually occur around the edges of skin panels, underneath hold-down washers, or in areas physically damaged by shearing, drilling, abrasion, or impact.
Entrapment of moisture under and behind skin crevices is frequently a contributing factor. If the skin section can be easily removed, this should be accomplished to ensure complete inhibition and treatment.
Complete mechanical removal of corrosion products should be practiced when practical. Mechanical cleaning should normally be limited to the use of stiff bristle brushes and similar nonmetallic cleaning tools.
Any entrapment of steel particles from steel wire brushes, steel tools, or contamination of treated surfaces, or dirty abrasives, can cause more trouble than the initial corrosive attack. The following procedural summary is recommended for treatment of corroded magnesium areas when accomplished under most field conditions.
When aluminum insulating washers are used and they no longer fasten tightly to magnesium panels, corrosion is likely to occur under the washers if corrective measures are not taken.
When machine screw fasteners are used, aluminum insulating washers must be removed from all locations to surface treat the magnesium panel.
Where permanent fasteners other than machine screws are used, the insulating washer and fastener must be removed.
When located so water can be trapped in the counter-bored area where the washer was located, use sealants to fill the counterbore. If necessary, fill several areas adjacent to each other. It may be advantageous to cover the entire row of fasteners with a strip of sealant.
REPAIR OF MAGNESIUM SHEET METAL AFTER EXTENSIVE CORROSION REMOVAL. The same general instructions apply when making repairs in magnesium as in aluminum alloy skin, except that two coats of epoxy primer may be required on both the doubler and skin being patched instead of one. Where it is difficult to form magnesium alloys in the contour, aluminum alloy may be utilized. When this is done, it is necessary to ensure effective dissimilar metal insulation. Vinyl tape will ensure positive separation of dissimilar metals, but edges will still have to be sealed to prevent entrance of moisture between mating surfaces at all points where repairs are made. It is recommended that only non-corrosive type sealant be used, since it serves a dual purpose of material separation and sealing.
Par 6-148 Page 6-275
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 salt water can develop moth holes and complete penetration overnight.
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.
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 stru ctural strength remaining.
Refer to the aircraft manufacturer if any questions of safety are involved.
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.
Positively identify metal as magnesium.
Clean area to be reworked.
Strip paint if required.
Determine the extent of corrosion damage.
Remove light to moderate corrosion by one of the following means.
Non-Powered Corrosion Removal.
Non-powered 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.
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.
Pumice powder may be used to remove stains or to remove corrosion on thin metal surfaces where minimum metal removal is allowed.
Chemical Corrosion Removal.
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:
Deep pitting,
Heavy corrosion and corrosion by products,
Previously had corrosion removed by mechanical means, or
Previously been sand blasted.
Par 6-152 Page 6-276 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.
The following solution may be used to remove surface oxidation and light corrosion products from magnesium surfaces.
Solution Composition and Operation:
Chromium Trioxide. 24 oz.
(O-C-303, Type II). Water to Make 1 gal.
Reaction Time 1 to 15 min.
Operation Temperature. (Solu-tion can be operated at room temperature for a longer reaction time if desired.) 190 to 202 °F.
Container Construction. Lead-lined steel, stainless steel, or 1100 aluminum.
Mask off nearby operating mechanisms, cracks and plated steel to keep the solution from attacking them.
Apply chromic acid solution carefully to the corroded area with an acid-resistant brush.
Allow the solution to remain on the surface for approximately 15 minutes. Agitation may be required.
Thoroughly rinse the solution from the surface with plenty of clean water.
Repeat the preceding sequence as necessary until all corrosion products have been removed and the metal is a bright metallic color.
Remove moderate to heavy corrosion by one of the following means.
Powered Corrosion Removal.
Powered corrosion removal can be accomplished using 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.
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 ro tary 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.
Pneumatic sanders are acceptable if used with disk or paper of aluminum oxide.
When using sanders, use extra care to avoid over heating aircraft skins thinner than
0.0625 inch.
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.
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
Par 6-152 Page 6-277 blasting with glass beads, (Spec. MIL-G-9954) sizes 10-13; or grain abrasive (Spec. MIL-G-5634), types I or III at an air pressure of 10 to 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.
Inspect the reworked area to ensure that no corrosion products remain. If corrosion products are found, repeat method used and re-inspect.
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.
Determine depth of faired depressions to ensure that rework limits have not been exceeded. Refer to the manufacture’s specifications.
Clean reworked area using a solvent to provide a waterbreak-free surface. Do not use kerosene or another petroleum base fuel as a cleaning solvent.
Apply Chromic Acid Brush-on Pretreatment.
Chemical pretreatment such as the following chromic acid solution (Conversion coat conforming to Spec. MIL-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 tend to neutralize corrosion media in contact with the surface.
The chromic acid brush-on pretreatment may be applied to all magnesium parts that require touch-up. 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.
Solution Composition and Operation:
Distilled Water 1 gal.
Chromic Acid (CrO3) 1.3 oz.
(99.5 pure), Calcium Sulfate 1 oz.
(CaSo4.2H2O)
Operating Temp. 70-90 °F.
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.
Par 6-152 Page 6-278 (and 6-34) Add chemicals to water in the order shown.
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.)
Prior to use, decant solution (avoid transfer of undissolved calcium sulfate) into suitable containers (polyethylene or glass).
Apply solution by brush, swab, or flow on using low-pressure spray (non-atomizing) 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.
NOTE: Too long an exposure to the brush-on solution produces coatings that will powder and impair adhesion of applied paint finish/films.
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.
Apply primer and top coat finish
Remove masking and protective coverings.
6-153.6-163. [RESERVED.]
Par 6-164 Page 6-279
SECTION 10. FERROUS METALS
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.
Red rust first shows on bolt heads, hold down 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.
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.
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.
Acceptable methods include careful use of mild abrasive mats, cloths, and papers, such as fine grit aluminum oxide, metallic wool, or fine buffing compounds.
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.
SPECIAL TREATMENT OF STAINLESS STEEL.
Stainless steels are of two general types:
magnetic and nonmagnetic.
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.)
Non-magnetic 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.
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.
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.
Par 6-179 Page 6-280 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.
Prepare the area for rework.
Positively identify the metal as steel and establish its heat-treated value.
Clean the area and strip paint if required.
NOTE: Use of acid-based strippers, chemical rem overs, or chemical conversion coatings are not permitted on steel parts without engineering authorization.
Determine extent of corrosion damage.
Remove residual corrosion by hand sanding with mild abrasive ma ts, cloths, and papers, such as fine aluminum oxide grit.
Remove heavy deposits of corrosion products by approved mechanical methods for that particular form of steel and/or stainless steel.
Par 6-179 Page 6-281 Inspect the area for remaining corrosion. Rep eat 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.
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.
Determine depth of faired depression to ensure that rework limits have not been exceeded.
Clean reworked area with dry cleaning solvent. Do not use kerosene.
Apply protective finish or specific organic finish as required.
NOTE: Steel surfaces are highly-rea ctive immediately following corrosion removal; consequently, primer coats should be applied within 1 hour after sanding.
Remove masking and protective coverings.
6-168.6-178. [RESERVED.]
Par 6-179 Page 6-282 SECTION 11. OTHER METALS AND ALLOYS
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.
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 to 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.
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.
Avoid any entrapment of the solution after treatment.
Clean the part thoroughly following treatment with all residual solution removed.
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 to 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.
TITANIUM AND TITANIUM ALLOYS are highly corrosion resistant because an oxide film forms on their surfaces upon contact with air.
When titanium is heated, different oxides having different colors form on the surface. A blue oxide coating will form at 700 to 800 °F; a purple oxide at 800 to 950 °F; and a gray or black oxide at 1000 °F or higher. These coatings are protective discolorations and should not be removed.
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 6-193 Page 6-283 (and 6-40) Stainless steel wool or hand brush.
Abrasive mats, cloths, and papers with either aluminum oxide or silicon carbide grit.
Dry abrasive blasting using glass beads (spec. MIL-G-9954) sizes 10-13 or Aluminum oxide (spec.
MIL-G-21380, type I, grades A or B) at a blast pressure of 40 to 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.
Par 6-193 Page 6-284 (and 6-40) Hand polish with aluminum polish and soft cloth.
Titanium surfaces are susceptible to hydrogen embr ittlement that can induce stress co rrosion 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.]