7/25/91 AC 43-4A 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 areas. Engine cases are among the most common IN-PLACE TREATMENT OF MAGNESIUM CASTINGS. Magnesium castings, in examples of cast magnesium encountered in modern aircraft. Bellcranks, fittings, and numerous covers, plates, and handles may also be magnesium castings. When attack occurs on a casting, the earliest practicable treatment in salt water can develop "moth holes" and complete penetration overnight. is required if dangerous corrosive penetration is to be avoided. Engine cases separated in order to effectively treat the existing attack and prevent its If it is at all practicable, faying surfaces involved should be further progress. The same general treatment sequence detailed for magnesium skin should be followed. where engine cases are concerned, . baked enamel overcoats are usually involved rather than other topcoat 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 may be necessary in order manuals usually include dimensional tolerance limits for critical structural to evaluate the adequacy of structural strength remaining. Structural repair members. The FAA should be consulted if any questions of safety are involved.
magnesium parts should be removed from aircraft. When impossible to remove the part, make aircraft preparations detailed in paragraph 612. When using that procedure, observe the safety precautions and procedures of paragraph 600. EXAMPLE OF REMOVING CORROSION FROM MAGNESIUM. If possible, corroded
a. Positively identify metal as magnesium. (Refer to paragraph 618)
b. Clean area to be reworked.
c. Strip paint if required.
To remove light corrosion, proceed with paragraph 663.e. To remove moderate or Determine extent of corrosion damage as detailed in paragraph 642.
severe corrosion, proceed with paragraph 663.f.
follows. assemblies, areas where the brush-on solution might become lodged, or local e° Do not use the following procedure for adhesive bonded parts or Remove light corrosion by light hand-sanding or chemically, as areas bared specifically for grounding or electrical bonding purpose.
(1) Remove loose corrosion with aluminum wool or abrasive mat, paper, or cloth.
(2) Mask off other materials and parts, especially rubber parts, bearings, and cast or pressed inserts to prevent contact with the treating solution or its fumes.
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(3) Prepare corrosion treating solution in the following proportions: 1-1/2 pounds of sodium dichromate and 1-1/2 pints of concentrated nitric acid (HNO,) per gallon of water. Mix as follows, but prepare and store the solution in clean polyethylene or glass containers:
Fill a suitable container with a volume of water equal to 1/4 the desired total quantity of solution.
(ii) Add full quantity of sodium dichromate in proportions indicated and agitate solution until the chemical is dissolved.
(iii) Add water until quantity of solution is equal to approximately 2/3 the desired total quantity.
and mix thoroughly. (iv) Slowly add total volume of nitric acid (HNO,) to solution solution is reached and stir until entire solution concentration is equal. Add remaining water until total desired quantity of
(4) Remove remaining corrosion by swabbing the corroded surface 1 to 2 minutes with the nitric acid (HNO,) solution, then wipe dry.
brush, or abrasive mat and wipe dry. (5) Rinse thoroughly with clean water while scrubbing with a mop, has been removed. (6) Repeat the preceding sequence, as necessary, until all corrosion paragraph 663.g. (7) After all corrosion has been removed, proceed with
f. Mechanically remove moderate or severe corrosion. Wear goggles or a face shield to preclude injury from corrosion particles breaking loose and flying off. Protect adjacent areas to prevent additional damage from corrosion products removed when using this procedure.
NOTE: DO NOT USE CARBON STEEL WIRE BRUSHES OR SILICONE CARBIDE ABRASIVES ON MAGNESIUM.
(1) Remove heavy corrosion products by hand brushing with a stainless steel or fiber brush followed by vacuum abrasive blasting with glass to 35 psi should be used for direct pressure machines. For suction type blast Specification MIL-G-5634 lypes 1 or Il1. (Specification MIL-G-9954) sizes 10, 11, 12, 13; or grain abrasive An air pressure at the nozzle of 10 equipment, use 50 percent higher pressure.
operated power tool. (2) Remove residual corrosion by hand sanding or with approved handapply corrosion treating solution. (3) After removing all corrosion visible through a magnifying glass, Chap 6 Par 663 183
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g. 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 final polish with 400 grit abrasive paper.
h. Determine depth of faired depressions to ensure that rework limits have not been exceeded.
i. Clean reworked area using a solvent to provide a water breakfree surface. Do not use kerosene.
Prepare and apply magnesium conversion coat conforming to MIL-M-3171, TYPE VI (DOW-19) as follows:
(1) Measure 1 gallon of distilled water into a clean polyethylene or glass container.
(2) Add 1.3 ounces (dry) of chromium trioxide or 1.3 ounces of technical grade chromic acid.
(3) Add 1 ounce of calcium sulfate dehydrate (CaS04.2H,0).
(4) Vigorously stir for at least 15 minutes to ensure that the solution is saturated with calcium sulfate. (Let chromate solution stand for 15 minutes prior to decanting.)
(5) Prior to use, decant solution (avoid transfer of undissolved calcium sulfate) into suitable usage containers (polyethylene or glass).
(6) Apply solution by swabbing until the metal surface becomes a dull color (the color can vary from green-brown, brassy, yellow-brown to dark brown). Under optimum conditions of temperature at 70 °F or above and fresh materials, the time required to properly apply magnesium pretreatment is usually produce a brassy film, and 3 to 5 minutes a dark brown coating. Under adverse 1 to 5 minutes. Under these conditions, 1 to 2 minutes of treatment should conditions, and if the desired specified finish color is not produced in the specified time, the treatment may have to be prolonged up to 20 to 30 minutes in some instances until the proper finish is effected. For good paint adhesion, a dark brown color, free of powder, is considered best. The color may vary in using different vendors' materials. Too long exposure to the brush-on solution produces a coating which will powder and impair adhesion of applied paint finish/films. Use caution in swabbing on the solution. Severe rubbing of the wet surface will damage the coating.
NOTE: HIGH PRESSURE SPRAYING OR RUBBING ABRASION WILL DAMAGE THE FRESH COATING.
Rinse with clean water, then allow to dry at ambient temperature for a minimum of 1 hour (more in high humidity areas).
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k. Apply primer and topcoat finish.
1. Remove masking and protective covering.
664.-669. RESERVED.
Chap 6 Par 663 185 through 190
7/25/91 AC 43-4A SECTION 7. FERROUS METALS GENERAL. One of the most familiar kinds of corrosion is red iron rust, generally resulting 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 acting as a catalyst in causing additional corrosion to take place.
unprotected aircraft hardware. are secured to steel parts. dangerous and has no immediate effect on the structural strength of any major a. Red rust first shows on bolt heads, hold-down nuts, and other Its presence in these areas is generally not Red rust will often occur under nameplates which components. However, it is indicative of a general lack of maintenance and possible 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 in these areas and should be removed and controlled.
MECHANICAL REMOVAL OF IRON RUST. The most practicable means of controlling the corrosion of steel is the complete removal of corrosion products not recommended on high strength steel because of the danger of local overheating and the formation of notches which could lead to failure. However, it should be recognized that in any such use of abrasives, residual iron rust by mechanical means (see Figures 6-17 and 6-18). On high strength steel, corrosion removal by hand sanding is recommended. The use of powered tools is usually remains in the bottom of small pits and other crevices.
The best method to use on exterior surfaces is abrasive blasting which has the ability of removing nearly all rust.
removal, and in any event do not allow the surface to become wet before b. Paint the cleaned metal surface as soon as possible after corrosion painting.
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WIRE BUFFING WHEEL FIGURE 6-17. REMOVING CORROSION PRODUCTS
FROM ORDINARY STEEL SURFACES
CLOTH BUFFING
WHEEL WITH MILD
ABRASIVE
CAUTION- DO NOT OVERHEAT
OR SCRATCH FIGURE 6-18. REMOVING CORROSION PRODUCTS FROM HIGHLY
STRESSED STEEL PARTS Chap 6 Par 671 192
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672. CHEMICAL SURFACE TREATMENT OF STEEL SURFACES.
NOTE: CHEMICAL SURFACE TREATMENT SHALL NOT BE USED ON HIGH STRENGTH STEEL PARTS WITHOUT ENGINEERING AUTHORIZATION.
There are acceptable methods for converting iron rust to phosphates and other protective coatings. Parco Lubrizing and use of phosphoric zinc preparations are examples of such treatment. shop installed hot tanks and are impracticable for use in the field. However, these processes require effective rust converters where tolerances are not critical and where thorough rinsing and neutralizing of residual acid is also possible. b. Other preparations (i.e. phosphoric acid, naval jelly, etc.) are and use of chemical inhibitors on installed steel parts is not only undesirable, resulting in uncontrolled attack which could occur when such materials are used but very dangerous. The possibility of entrapping of corrosive solutions, c. These situations are generally not applicable to assembled aircraft under field conditions, outweighs any advantage to be gained from their use.
highly stressed steel part is potentially dangerous, and the careful removal of wire brushes on high-stressed steel parts. Any corrosion on the surface of a corrosion products is mandatory. Surface scratches or change in surface structure from overheating can cause sudden failure of these parts. Removal of 673. REMOVAL OF CORROSIVE PRODUCTS FORM HIGH-STRESSED STEEL PARTS. Do not use corrosion products may be accomplished by careful use of mild abrasive papers, cloth buffing wheels. such as fine grit aluminum oxide, or tine butting compounds, such as rouge, or
a. It is essential that steel surfaces not be overheated.
high-strength steel located on aircraft exteriors. b. Abrasive blasting is also a satisfactory corrosion removal method for should be applied immediately. c. After careful removal of surface corrosion, protective paint finishes identified by numbers in the 400-series. Corrosion often occurs on 400-series on stainless steels. nonmagnetic. Magnetic steels are of the ferritic or martensitic types and are stainless steels and treatment is the same as specified in paragraph 673. 674. SPECIAL TREATMENT OF STAINLESS STEEL ALLOYS. Do not use chemical cleaners Stainless steels are of two general types: magnetic and Nonmagnetic 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.
a. Austenitic steels develop corrosion resistance by an oxide film which broken accidentally or by abrasion, it may not restore itself without should not be removed even though the surface is discolored. The original oxide film is normally formed at time of fabrication by passivation. repassivation. If this film is Chap 6 Par 672 193
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b. 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.
675. EXAMPLE OF PROCESS FOR REMOVAL OF CORROSION FROM STEEL PARTS. If possible, corroded steel parts should be removed from the aircraft. When impossible to remove the part, observe the aircraft preparations and safety precautions in paragraph 601, 602, and 603. Chemical removal or chemical conversion coatings are not allowed on steel parts.
Positively identify the metal as steel as detailed in paragraph 617 and establish its heat-treated value.
b. Clean area to be reworked.
CAUTION: DO NOT USE ACID BASED STRIPPERS ON HIGH STRENGTH STEEL PARTS.
c. Strip paint if required.
d. Remove all degrees of corrosion damage from steels heat-treated below 220,000 psi as required by paragraph 655.e. Corrosion removal on steels treated to 220,000 psi and above should be accomplished only by hand sanding or dry abrasive blasting. (Refer to section 6 of this chapter.)
Mechanically remove all degrees of corrosion from steel parts heattreated below 220,000 psi as follows.
CAUTION: USE GOGGLES OR FACE SHIELD TO PRECLUDE INJURY FROM FLYING PARTICLES. PROTECT ADJACENT AREAS TO PREVENT
ADDITIONAL DAMAGE FROM CORROSION PRODUCTS REMOVED BY MECHANICAL PROCESS.
Remove heavy deposits of corrosion products using a stainless steel hand brush. Abrasive blasting may be used as an alternate method of corrosion removal. Abrasive blasting should be accomplished using aluminum oxide (Specification MIL-G-21380) Type I, Grades A or B, grit sizes 25, 50, or 120; or No. 13 glass beads (Specification MIL-G-9954). An air pressure at the nozzle of 40 to 50 psi should be used for direct pressure machines when removing corrosion by abrasive blasting. Engineering approval from the aircraft manufacturer should be obtained prior to abrasive blasting metal thinner than
0.0625 inch.
(2) Remove residual corrosion by hand sanding or with approved hand-operated power tool.
(3) The surface is highly reactive immediately following corrosion removal; consequently, primer coats should be applied within 1 hour after sanding. After removing all corrosion visible through a magnifying glass, continue with paragraph 675. f.
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f. 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 final polish with 400 grit abrasive paper.
rework limits have not been exceeded. Determine depth of faired depressions as required to ensure that
h. Clean reworked area with dry, cleaning solvent. Do not use kerosene.
i. Apply protective finish or specific organic finish as required.
j. Remove masking and protective covering.
676.-679. RESERVED.
Chap 6 Par 675 195 through 200
7/25/91 AC 43-4A SECTION 8. PLATED PARTS CHROMIUM AND NICKEL PLATED PARTS. Nickel and chromium platings are used extensively as protective and wear resistant coatings over high strength steel arts (landing gear journals, shock strut pistons, etc.). chromium and nicke late provide protection by forming a somewhat impervious physical coat over the destroyed. underlying base metal. When breaks occur in the surface, the protection is
a. The amount of reworking that can be performed on chromium and nickel plated components is limited. This is due to the critical requirements to which such components are subjected.
b. The rework should consist of light buffing to remove corrosion products and produce the required smoothness. buffing does not take the plating below the minimum allowable thickness. This is permissible, provided the the area with a corrosion preventive compound, if possible. c. Whenever a chromium or nickel plated component requires buffing, coat
d. When buffing exceeds the minimum thickness, or the base metal has sustained corrosive attack, the component should be removed and replaced.
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.
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 on a sacrificial basis in which the cadmium is attacked rather than the underlying base material. Properly functioning cadmium surface coatings may well show mottling, ranging from white to brown to black spots on their surfaces. These are indicative of the sacrificial protection being surfaces appear. even when actual breaks in the coating develop and bare steel or exposed copper offered by the cadmium coat, and under no condition should such spotting be removed merely for appearance's sake. In fact, cadmium will continue to protect cleaning of the area may be necessary but should be limited to removal of the appearance of corrosion products of the base metal develops, some mechanical corrosion products from the underlying base material. a. Where actual failures of the cadmium plate occur and the initial
b. Under no condition should such a coating be cleaned with a wire brush. If protection is needed, a touch-up with primer or a temporary preservative coating should be applied. Restoration of the plate coating is impracticable in the field.
Chap 6 Par 680 201