U.S. Department Advisory of transportation Circular Federal Aviation Administration Subject: CORROSION CONTROL FOR AIRCRAFT Initiated by: AFS-340 Date: // 25/91 Change: AC No: 43-4A PURPOSE. This advisory circular (AC) is a summary of current available data regarding identification and treatment of corrosive attack on aircraft structure and engine materials. Corrosion inspection frequency, corrosion identification, and corrosion treatment continues to be the responsibility of the operator and should be accomplished per this AC, the manufacturer's recommendations, or the operator's own maintenance program. The procedures presented in this Ac are an acceptable means, but not the only acceptable means, of corrosion treatment. The information contained in this AC is applicable to aircraft for which the manufacturer has not published corrosion control information. Where the aircraft manufacturer has published a recommended corrosion inspection schedule and treatment program for a particular aircraft, that program should take precedence over the recommendations of this AC.
2. CANCELLATION. AC 43-4, Corrosion Control for Aircraft, dated 5/15/73.
3. RELATED READING MATERIAL.
a. Federal Aviation Administration (FAA) documents:
(1) Advisory Circular 65-9A, Airframe and Powerplant Mechanics General Handbook. Copies may be obtained from U.S. Department of 20590. Transportation, Distribution Requirements Section, M-443.2, Washington, D.C.
(2) Advisory Circular 65-12A, Airframe and Powerplant Mechanics Powerplant Handbook. Copies may be obtained from U.S. Department of 20590. Transportation, Distribution Requirements Section, M-443.2, Washington, D.C.
Airframe Handbook. (3) Advisory Circular 65-15A, Airframe and Powerplant Mechanics Copies may be obtained from U.S. Department of Transportation, Distribution Requirements Section, M-453.2, washington, D.C.
7/25/91 AC 43-4A
b. Other documents:
(1) Naval Air Systems Command, NAVAIR 01-1A-509, Aircraft Weapons Systems Cleaning and Corrosion Control. Requests for this document must be referred to Naval Publications and Forms, Navy Aviation Supply Office, A.S.O. Code 10, 5801 Tabor Avenue, Philadelphia, PA 19120-5099.
(2) U.S. Air Force Technical Order 1-1-2, Corrosion Control and Treatment for Aerospace Equipment. Requests for this document must be referred to Warner Robins ALC/MMEDT, Robins AFB GA 31098-5609.
Milliam, A Mite william J. White Acting Director, Flight Standards Service page ii
CONTENTS Paragraph Page No.
CHAPTER 1. GENERAL
2. 1. BACKGROUND CATASTROPHIC CORROSION EVENTS 1
3. CORROSION CONTROL PROGRAM
4.-199. RESERVED 2 CHAPTER 2. CORROSION THEORY 11
200. INTRODUCTION 11
201. BACKGROUND 11
202. DEVELOPMENT OF CORROSION 12
203. FACTORS INFLUENCING CORROSION 12
205. CORROSION AND MECHANICAL FACTORS 204. FORMS OF CORROSION 14 22
206. COMMON CORROSIVE AGENTS 26
207. MICRO-ORGANISMS 27
208. METALLIC MERCURY CORROSION ON ALUMINUM ALLOYS 29
209.-299. RESERVED 29 CHAPTER 3. EFFECTS OF CORROSION 41
300. GENERAL 41
301. EFFECTS OF CORROSION ON METALS 41
302.-399. RESERVED 46 CHAPTER 4. CORROSION PRONE AREAS AND PREVENTATIVE MAINTENANCE 51
400. GENERAL 51
401. EXHAUST TRAIL AREAS . 51
402. BATTERY COMPARTMENTS AND BATTERY VENT OPENINGS 51
403. LAVATORIES, BUFFETS, AND GALLEYS 52
404. BILGE AREAS . 53
405. WHEEL WELLS AND LANDING GEAR 53
406. 407. WATER ENTRAPMENT AREAS EXTERNAL SKIN AREAS 55 59
408. ENGINE FRONTAL AREAS AND COOLING AIR VENTS 59
409. ELECTRONIC PACKAGE COMPARTMENTS 60
410. MISCELLANEOUS TROUBLE AREAS . 62
411. FACTORS IN CORROSION CONTROL 66
412. PREVENTATIVE MAINTENANCE 70
413.-499. RESERVED . . . 85 CHAPTER 5. INSPECTION REQUIREMENTS 91
500. GENERAL 91
Paragraph
501. FREQUENCY OF INSPECTION 91
502. RECOMMENDED DEPTH OF INSPECTION 92
503. PRIMARY APPROACH 93
504. NONDESTRUCTIVE INSPECTION (NDI) 93
505.-599. RESERVED. 104 CHAPTER 6. CORROSION REMOVAL TECHNIQUES 111 SECTION 1. SAFETY PROCEDURES 111
600. GENERAL 111
601. GENERAL CORROSION CONTROL WORK PROCEDURES EIL
602.-609. RESERVED . . 114 SECTION 2. CORROSION REMOVAL TECHNIQUES 121
610. GENERAL 121
611. STANDARD METHODS 121
612. PREPARATIONS FOR REWORK 121
613. PAINT REMOVAL 122
614. SPECIAL TECHNIQUES 122
615. FAIRING OR BLENDING REWORKED AREAS 123
616. CHEMICAL TESTING 128 CHEMICAL SPOT ANALYSIS OF MAGNETIC METALS 129 CHEMICAL SPOT ANALYSIS OF NONMAGNETIC METALS 130 SURFACE TREATMENT TESTING 131 POST IDENTIFICATION CLEANING AND REFINISHING 132
621.-625. RESERVED • 132 SECTION 3. MECHANICAL CORROSION REMOVAL BY BLASTING 141
626. GENERAL 141
627. SAFETY PRECAUTIONS 141
628.-639. RESERVED . 141 SECTION 4. CORROSION DAMAGE AND REWORK LIMITS 151
640. DISCUSSION 151 REMOVAL OF CORROSION 152 DETERMINING DEGREE OF CORROSION DAMAGE 152 DETERMINING REWORK LIMITS 153
DETERMINING MATERIAL THICKNESS REDUCTION AFTER CORROSION CLEANUP 153
645.-649. RESERVED • 154 SECTION 5. ALUMINUM AND ALUMINUM ALLOYS 161
650. TREATMENT 161 TV
Paragraph Page No.
651. PROCESSING OF ALUMINUM SURFACES 161
652. REPAIR OF ALUMINUM ALLOY SHEET METAL 165
653. CORROSION REMOVAL AROUND COUNTERSUNK FASTENERS IN ALUMINUM
654. ALLOY EXAMPLES OF REMOVING CORROSION FROM ALUMINUM AND ALUMINUM 166 ALLOYS 167
655.-659. RESERVED 169 SECTION 6. MAGNESIUM ALLOYS 181
660. TREATMENT OF WROUGHT MAGNESIUM SHEETS AND FORGINGS 181
661. REPAIR OF MAGNESIUM SHEET METAL AFTER EXTENSIVE CORROSION
662. IN-PLACE TREATMENT OF MAGNESIUM CASTINGS REMOVAL . 181 182 EXAMPLE OF REMOVING CORROSION FROM MAGNESIUM 182 -669. RESERVED . 185 SECTION 7. FERROUS METALS 191
670. 191
671. MECHANICAL REMOVAL OF IRON RUST 191
672. CHEMICAL SURFACE TREATMENT OF STEEL SURFACES 193
673. REMOVAL OF CORROSIVE PRODUCTS FROM HIGH-STRESSED STEEL PARTS . 193
674. 675. EXAMPLE OF PROCESS FOR REMOVAL OF CORROSION FROM STEEL SPECIAL TREATMENT OF STAINLESS STEEL ALLOYS 193 194
676.-679. RESERVED 195 SECTION 8. PLATED PARTS .
680. CHROMIUM AND NICKEL PLATED PARTS 681 CADMIUM AND ZINC PLATED PARTS
682. -689. RESERVED .
SECTION 9. OTHER METALS AND ALLOYS
690. NOBLE METAL COATINGS - CLEANUP AND RESTORATION
691. COPPER AND COPPER ALLOYS
692. TITANIUM ALLOYS
693.-699. RESERVED ..
CHAPTER 7. SPECIAL PROBLEMS
700. MERCURY SPILLS/CORROSION DAMAGE
701. CORROSION PROTECTION FOR AGRICULTURAL AIRCRAFT
702.-799. RESERVED . .
LIST OF ILLUSTRATIONS Page No.
FIGURE 2-1. SIMPLIFIED CORROSION CELL SHOWING CONDITIONS WHICH MUST EXIST FOR ELECTROCHEMICAL CORROSION. 11 FIGURE 2-2. ELIMINATION OF CORROSION BY APPLICATION OF AN ORGANIC FILM TO METAL SURFACE. 12 FIGURE 2-3. UNIFORM ETCH CORROSION. 15 FIGURE 2-4. PITTING CORROSION. 15 FIGURE 2-5. GALVANIC CORROSION OF MAGNESIUM ADJACENT TO STEEL FASTENER 16 FIGURE 2-6. CONCENTRATION CELL CORROSION 18 FIGURE 2-7. INTERGRANULAR CRACKING AND CORROSION ON A WINGSPAR CHORD 18 FIGURE 2-8. SEVERE EXFOLIATION CORROSION OF A SEAT TRACK 19 FIGURE 2-9. SEVERE EXFOLIATION CORROSION 20 FIGURE 2-10. FILIFORM CORROSION . . . 21 FIGURE 2-11. FILIFORM CORROSION BEFORE AND AFTER PAINT. REMOVAL. 21 FIGURE 2-12. STRESS CORROSION CRACKING OF 7079-16 FITTING . 23 FIGURE 2-13. STRESS CORROSION CRACKING STARTING AT A PIT IN CRES MATERIAL 23 FIGURE 2-14. STRESS CORROSION CRACKING OF AN EXTRUDED SECTION 24 FIGURE 2-15. FRETTING CORROSION AND CRACKING FROM CYCLIC LOADING OF LUG 25 TABLE 3-1. CORROSION OF METALS. • • 45 FIGURE 4-1. EXHAUST TRAIL AREA CORROSION CONTROL POINTS. 51 FIGURE 4-2. TYPICAL DAMAGE TO FLOOR AROUND LAVATORIES, BUFFEIS, AND GALLEYS . . 52 FIGURE 4-3. LANDING GEAR AND WHEEL CORROSION POINTS. 54 FIGURE 4-4. INTERGRANULAR CORROSION OF 7075-T6 ALUMINUM ADJACENT TO STEEL FASTENER. • 55 FIGURE 4-5. CRACK AND SKIN BULGING CAUSED BY CORROSION 56 FIGURE 4-6. SPOT-WELDED SKIN CORROSION 57 FIGURE 4-7. HINGE CORROSION POINTS • 58 FIGURE 4-8. HINGE FAILURE CAUSED BY CORROSION. 58 FIGURE 4-9. RECIPROCATING ENGINE FRONTAL AREA CORROSION. 60 FIGURE 4-10. FIGURE 4-11. JET ENGINE FRONTAL AREA CORROSION POINTS SPAR CHORD LIFTING CAUSED BY CORROSION PRODUCTS. 60 64 FIGURE 4-12. SKIN BULGING AROUND FASTENERS. 65 FIGURE 4-13. OPENING OF A CORRODED LAP JOINT FOR REPAIR 65 FIGURE 4-14. CLOSEUP VIEW OF A CORRODED LAP JOINT 66 TABLE 4-1. GALVANIC SERIES OF METALS AND ALLOYS 68 TABLE 4-2. GROUPING OF METALS AND ALLOYS. 69 FIGURE 4-15. NORTH AMERICA CORROSION SEVERITY MAP 72 FIGURE 4-16. SOUTH AMERICA CORROSION SEVERITY MAP 73 FIGURE 4-17. AFRICA CORROSION SEVERITY MAP. 74 FIGURE 4-18. SOUTH PACIFIC CORROSION SEVERITY MAP 75 FIGURE 4-19. ASIA CORROSION SEVERITY MAP. 76 FIGURE 4-20. EUROPE AND ASIA MINOR CORROSION SEVERITY MAP 77 FIGURE 4-21. BLOCKED DRAIN PASSAGES RESULTED IN ACCUMULATION OF CORROSION CONTAMINATES AND MOISTURE. 78 FIGURE 5-1. CORROSION FOUND AFTER REMOVING CARGO DOOR THRESHOLD COVERS 93 FIGURE 5-2. CORROSION UNDER CHIPPED AND LOOSE PAINT ON WING SKIN 94 FIGURE 5-3. CORROSION INDICATED BY BLISTERING OF PAINT IN FUEL CELL. 95 FIGURE 5-4. POPPED RIVET HEADS RESULTING FROM CORROSION PRODUCTS . 95 FIGURE 5-5. SKIN BULGING AROUND FASTENERS CAUSED BY PRESSURE FROM CORROSION PRODUCTS. 96 vi
FIGURE 5-6. CORROSION CRACKING BETWEEN FASTENERS ON A WING SPAR.
FIGURE 5-7. FAYING SURFACE CORROSION ON WING SPAR CHORD 96 FIGURE 5-9. FIGURE 5-8. FIGURE 5-10. SEVERE CROWN STRINGER CORROSION. SPAR CHORD CORROSION. WITH CORROSION CRACKING ALSO VISIBLE REMOVAL OF FILLET SEAL FROM INTERNAL EDGE OF LAP JOINT 98 97 97 FIGURE 5-12. FIGURE 5-11. CORROSION DEVELOPED UNDER LEVELING COMPOUND EXPOSED FULL EXTENT OF CORROSION REMOVAL OF INSULATION BLANKETS EXPOSED CORROSION 98 99 FIGURE 5-14. FIGURE 5-13. CORROSION DAMAGE AROUND FASTENER HOLES INSTALLED DURING PREVIOUS REPAIR OF OVERWING EXIT. CORROSION BEHIND A STRUCTURAL COMPONENT ON A WING SPAR 100 99 FIGURE 5-16. FIGURE 5-15. AND IN FASTENER HOLES. SEVERELY CORRODED LAP JOINT. LOCAL PAINT REMOVAL REQUIRED TO EXPOSE FULL 100 101 FIGURE 5-17. EXTENT OF CORROSION. CORRODED AREA AFTER PAINT REMOVAL FROM WING SKIN 101 FIGURE 5-18. ALODINE 1200 TREATMENT PRIOR TO PAINTING MAY HIGHLIGHT 102 TABLE 6-1. FIGURE 6-2. FIGURE 6-1. ABRASIVES FOR CORROSION REMOVAL. TYPICAL EXAMPLE OF ACCEPTABLE CLEANUP OF CORROSION PITS. BLENDOUT OF CORROSION AS SINGLE DEPRESSION PRESENCE OF REMAINING CORROSION OR CRACKS. 102 123 124 FIGURE 6-3. FIGURE 6-4. PROFILE OF REWORKED CORRODED AREAS IN REGIONS BLENDOUT OF MULTIPLE PITS IN CORRODED AREA 125 125 FIGURE 6-5. OF LIMITED ACCESS. STEEL PARTICLES FROM FASTENERS OR NUTPLATES MAY CAUSE 126 FIGURE 6-6. FUTURE CORROSION PROBLEMS IN ALUMINUM STRUCTURE. INCOMPLETE REMOVAL OF CORROSION PRODUCTS RESULTED 126 FIGURE 6-7. IN REOCCURRENCE OF CORROSION. INCOMPLETE REMOVAL OF CORROSION PRODUCTS RESULTED 127 FIGURE 6-8. SEVERE CORROSION REQUIRING TRIMMING OUT OF DAMAGED AREA. IN REOCCURRENCE OF CORROSION 127 FIGURE 6-9. FIGURE 6-10. CORROSION AT FASTENER HOLES REMOVED BY SPOT FACING CORROSION DAMAGE AND REWORK MEASUREMENT USING 151 152 FIGURE 6-11. FIGURE 6-12. POLISHING AND BRIGHTENING ALCLAD SURFACES. DEPTH DIAL GAUGE REMOVING OIL AND SURFACE DIRT. 162 154 FIGURE 6-13. CLEANING AND STRIPPING PAINT . 162 FIGURE 6-18. FIGURE 6-14. FIGURE 6-16. FIGURE 6-17. FIGURE 6-15. REMOVING CORROSION PRODUCTS FROM REMOVING EXCESS INHIBITOR SOLUTION APPLYING WAX TO CLEANED SURFACES . CLEANING AND INHIBITING CORRODED ALUMINUM SURFACES REMOVING CORROSION PRODUCTS FROM ORDINARY STEEL SURFACES 165 192 164 163 165 HIGHLY STRESSED STEEL PARTS. 192
CHAPTER 1. GENERAL BACKGROUND. Corrosion is the electrochemical deterioration of a metal because of its chemical reaction with the surrounding environment. while new and better materials are continuously being developed, this progress is offset, in part, by a more aggressive operational environment. This problem is compounded by the fact that corrosion is a complex phenomenon. It can take many ° Fact eat correston to a different forms and the resistance of aircraft materials to corrosion can drastically change with only a small environmental change.
CATASTROPHIC CORROSION EVENTS. Corrosion is most often thought of as a slow process of material deterioration, taking place over a signiticant period of time (examples being general corrosion, pitting, exfoliation, etc.). Other forms of corrosion degradation can occur very quickly, in days or even hours, with catastrophic results. These forms (such as stress corrosion cracking, environmental embrittlement, and corrosion fatigue) depend on both the chemical and mechanical aspects of the environment and can cause catastrophic structural failure without warning.
3. CORROSION CONTROL PROGRAM.
The possibility of an in-flight mishap or excessive down time for structural repairs necessitates an active corrosion prevention and control program. The type and aggressiveness of the corrosion prevention and control program depend on the operational environment of the aircraft. Aircraft exposed to salt air, heavy atmospheric industrial pollution, and/or over water operations will require a more stringent corrosion prevention and control program than an aircraft that is operated in a dry environment.
b. In order to prevent corrosion, a constant cycle of cleaning, inspection, operational preservation, and lubrication must be followed. Prompt detection and removal of corrosion will limit the extent of damage to aircraft and aircraft components. The basic philosophy of a corrosion prevention and control program should consist of the following:
(1) Adequately trained personnel in the recognition of corrosion including conditions, detection and identification, cleaning, treating, and preservation;
(2) Thorough knowledge of corrosion identification techniques;
(3) Proper emphasis on the concept of all hands responsibility for corrosion control;
(4) Inspection for corrosion on a scheduled basis;
(5) Aircraft washing at regularly scheduled intervals;
(6) Routine cleaning or wipe down of all exposed unpainted surfaces;
Chap 1 Par 1
(7) Keeping drain holes and passages open and functional;
(8) Inspection, removal, and reapplication of preservation compounds on a scheduled basis;
(9) Early detection and repair of damaged protective coatings.
(10) Thorough cleaning, lubrication, and preservation at prescribed intervals;
(11) Prompt corrosion treatment after detection;
(12) Accurate record keeping and reporting of material or design deficiencies; and
(13) Use of appropriate materials, equipment, and technical publications.
4.-199. RESERVED Chap 1 2 through 10 Par 3
CHAPTER 2. CORROSION THEORY causes of corrosion, and the factors which influence its development. INTRODUCTION. This chapter briefly describes correver pment. The various forms of corrosion and common corrosive agents are also described.
201. BACKGROUND.
a. Corrosion is a natural phenomenon which attacks metal by chemical or electrochemical action and converts it into a metallic compound, such as an oxide, hydroxide, or sulfate. Corrosion is to be distinguished from erosion, which is primarily destruction by mechanical action. The corrosion occurs because of the tendency for metals to return to their natural state. Noble metals, such as gold and platinum, do not corrode since they are chemically uncombined in their natural state. Four conditions must exist before corrosion can occur (see Figure 2-1):
(1) Presence of a metal that will corrode (anode);
(2) Presence of a dissimilar conductive material (cathode) which has less tendency to corrode;
(3) Presence of a conductive liquid (electrolyte); and
(4) Electrical contact between the anode and cathode (usually metalto-metal contact, or a fastener.
///"CONTINUOUS LIQUID
PATH (ELECTROLYTE)
CURRENT FLOW
ANODIC CATHODIC
AREA AREA 2
NITRON LON
ELECTRON CONDUCTOR META FIGURE 2-1. SIMPLIFIED CORROSION CELL SHOWING CONDITIONS WHICH MUST EXIST FOR ELECTROCHEMICAL CORROSION.
b. Elimination of any one of these conditions will stop corrosion. An example would be a paint film on the metal surface (see Figure 2-2).
metals (such as stainless steel and titanium), under the right conditions, Chap 2 Par 200 11
produce corrosion products that are so tightly bound to the corroding metal that they form an invisible oxide film (called a passive film), which prevents further corrosion. When the film of corrosion products is loose and porous (such as those of aluminum and magnesium), an electrolyte can easily penetrate and continue the corrosion process, producing more extensive damage than surface appearance would show.
• NO CONTACT BETWEEN
UNBROKEN PAINT ELECTROLYTE AND ANODE
FILM AND CATHODE ››››>››**›>>››››››››>›
CONTINUOUS LIQUID
PATH (ELECTROLYTE)
ANODIC CATHODIC
AREA AREA
ELECTRON
CONDUCTOR METAL FIGURE 2-2. ELIMINATION OF CORROSION BY APPLICATION OF AN ORGANIC FILM TO METAL SURFACE.
202. DEVELOPMENT OF CORROSION.
a. All corrosive attack begins on the surface of the metal. The corrosion process involves two chemical changes. The metal that is attacked or oxidized undergoes an anodic change, with the corrosive agent being reduced and undergoing a cathodic change. The tendency of most metals to corrode creates one of the major problems in the maintenance of the aircraft, particularly in areas where adverse environmental or weather conditions exist.
b. Paint coatings can mask the initial stages of corrosion. Since corrosion products occupy more volume than the original metal, paint surfaces should be inspected often for irregularities such as blisters, flakes, chips, and lumps.
203. FACTORS INFLUENCING CORROSION.
a. Some factors which influence metal corrosion and the rate of corrosion are the:
Chap 2 12 Par 201