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.
Chap o Dar 670 191
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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
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.
682.-689. RESERVED.
Chap 6 Par 681 202 through 210
7/25/91 AC 43-4A SECTION 9. OTHER METALS AND ALLOYS gold finishes are generally used in aircraft assembles because of their NOBLE METAL COATINGS - CLEANUP AND RESTORATION. Silver, platinum, and resistance to ordinary surface attack and their improved electrical or heat conductivity. Silver-plated electrodes can be cleaned of brown or black sulfide sheet stock while immersed in a warm water solution of common table salt mixed tarnish, as necessary, by placing them in contact with a piece of magnesium with baking soda or by using a fine grade abrasive mat or pencil eraser followed by solvent cleaning. displacement of water is necessary. In general, cleaning of gold or platinum coatings is not' recommended in the field. If assemblies are involved, careful drying and complete COPPER AND COPPER ALLOYS. Copper and copper alloys are relatively corrosion resistant, and attacks on such components will usually be limited to dangerous and should ordinarily have no effect on the function of the part. staining and tarnish. Generally, such change in surface condition is not 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.
a. The stained part should be immersed in the cold solution. However, surface with a small brush. surfaces can also be treated in place by applying the solution to the stained
b. Care should be exercised to avoid any entrapment of the solution after treatment. The part should be cleaned thoroughly following treatment with all residual solution removed.
blue copper salts on the corroded part. These products should be removed c. Serious copper corrosion is evident by the accumulation of green-tonechanically using a stiff bristle brush, brass wire brush, 400-grit abrasive paper or bead blast with glass beads, Specitication MIL-G-9954, size 13).
bead blasting, air pressure at the nozzle should be 20 to 30 psi for direct pressure machines. Do not bead blast braided copper flexible lines. A surface coating should be reapplied over the reworked area. Again, chromic acid treatment will tend to remove the residual corrosion products.
surface plate. d. Most brass and bronze structural parts will be protected by cadmium The mottling of the protective cadmium coat should not be mechanical removal of the protective cadmium should be held to a minimum and limited to the immediate area of the copper attack. copper corrosion products are beginning to appear. removed, and mechanical surface cleaning should not be attempted unless actual Under these conditions, any
692. IITANIUM ALLOYS. Titanium and its alloys are highly corrosion resistant because a protective 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 Chap 6 Par 690 211
7/25/91 AC 43-4A to 950 °F; and a gray or black oxide at 1,000 °F or higher. titanium surfaces is difficult to detect. It may show deterioration from the Corrosive attack on presence of salt deposits and metal impurities at elevated temperatures so periodic removal of surface deposits is required.
a. Clean titanium surfaces until all traces of corrosion or surface deposits are removed using one of the following:
(1) Stainless steel wire brush;
(2) Hand sand with aluminum oxide abrasive paper or abrasive mat;
(3) Dry blast with glass beads, (Specification MIL-G-9954) sizes 10, 11, 12, or 13, or aluminum oxide (Specification MIL-G-21380 Type I) grades A or B, using 40 to 50 psi air pressure at the nozzle for direct pressure machines;
(4) Hand polish with aluminum polish and soft cloth.
alloys which are subject to elevated temperatures in service. Such solvents can b. Chlorinated Hydrocarbon cleaners should not be used on titanium cause stress-corrosion in titanium.
693.-699. RESERVED.
Chap 6 212 through 220 Par 692
CHAPTER 7. SPECIAL PROBLEMS
700. MERCURY SPILLS/CORROSION DAMAGE.
The presence of mercury and mercury salts in air cargo is a definite possibility. Loading, unloading, and general shifting of such cargo can and does result occasionally in damaged containers, cartons, electronic tubes, etc., with subsequent possible leakage of mercury on aircraft structure.
Spillage of mercury or mercury compounds within an airplane requires immediate action for its isolation and recovery to prevent possible corrosion Lumbuckie arvada sentel. damage and embrittlement of aluminum alloy structural components, stainless steels (300 and 400 series), and unplated brass components such as cable
c. Mercury, by the amalgamation process, can penetrate any break in the finish (anodize/alodine), paint, and seal coating of a metal structural component. Bright, polished, shining, or scratched surface will hasten the process, as well as moisture. Mercury or mercury compounds attack the metal Corrosive attack of freshly scratched aluminum alloy is very rapid, and complete grain boundaries and seriously embrittle and reduce the strength of parts. penetration of sheet material is known to occur within 3 or 4 minutes.
Mercury is highly toxic and spreads very easily from one surface to another. It adheres to hands, shoes, clothes, tools, etc. The following precautions for mercury spills should be strictly followed:
(1) Avoid contact with surfaces suspected of being contaminated.
Use wood or fiber sheets to support body while working in the area.
WARNING: DO NOT ATTEMPT TO PICK UP MERCURY BY HAND.
(2) Wear wing socks (shoe protectors) and protective (disposable) clothing in contaminated area to prevent scratching metal surfaces. Properly dispose of socks after use.
(3) Do not wear clothing used in contaminated areas on jobs in metal container outside of aircraft. similar authority of the local State Health Department for instructions on uncontaminated areas. Dispose of wing socks and protective clothing into unused proper disposal procedures for mercury. Contact the Waste Management Service or Clean all tools that have been used in contaminated area with steam or hot water (4) Have personal clothing cleaned. Wash shoes with soap and water.
and soap. Discard any drill bits used on mercury contaminated areas.
Thoroughly clean vacuum cleaner, if used.
(5) Always wash thoroughly with soap and water after contacting mercury. Keep hands away from mouth. Do not eat, smoke, or blow nose without first washing your hands thoroughly.
Chap 7 Par 700 221