U.
S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Corrosion Control for Aircraft Date: 9/11/18 AC No: 43-4B Initiated by: AFS-300 Change:
This advisory circular (AC) is a summary of the current available data regarding identification and treatment of corrosive attack on aircraft structures and engine materials. Corrosion inspection frequency, corrosion identification, and especially corrosion treatment continues to be the responsibility of the operator. These inspections should be accomplished per this AC, the manufacturer’s recommendations, or the operator’s own maintenance program. The procedures in this AC are an acceptable means, but not the only acceptable means, of corrosion treatment.
The information in this AC is applicable to aircraft for which the manufacturer has not published corrosion control information. Where the airframe or engine manufacturer has published a recommended corrosion inspection schedule and treatment program, the applicable program must take precedence over the recommendation of this AC.
ORIGINAL SIGNED by /s/ Rick Domingo Executive Director, Flight Standards Service
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CONTENTS Paragraph Page
Chapter 1. General1-1
1.1 Purpose of This Advisory Circular (AC)1-1
1.2 Audience1-1
1.3 Where You Can Find This AC1-1
1.4 What This AC Cancels1-1
1.5 Related Reading Material1-1
1.6 Background1-1
1.7 Catastrophic Corrosion Events1-2
1.8 Corrosion Control Program1-2
1.9 AC Feedback Form1-3
1.10–1.99 Reserved1-3
Chapter 2. Corrosion Theory2-1
2.1 Introduction2-1
2.2 Background2-1
2.3 Development of Corrosion2-2
2.4 Factors Influencing Corrosion2-2
2.5 Forms of Corrosion2-4
2.6 Corrosion and Mechanical Factors2-10
2.7 Common Corrosive Agents2-14
2.8 Microorganisms2-16
2.9 Metallic Mercury Corrosion on Aluminum Alloys2-17
2.10–2.99 Reserved2-17
Chapter 3. Effects of Co rrosion3-1
3.1 General3-1
3.2 Effects of Corrosion on Metals3-1
3.3–3.99 Reserved3-5
Chapter 4. Corrosion Prone Areas and Preventative Maintenance4-1
4.1 General4-1
4.2 Exhaust Trail Areas4-1
4.3 Battery Compartments and Battery Vent Openings4-1
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4.4 Lavatories, Buffets, and Galleys4-2
4.5 Bilge Areas4-2
4.6 Wheel Wells and Landing Gear4-2
4.7 External Skin Areas4-5
4.8 Water Entrapment Areas4-9
4.9 Engine Frontal Areas and Cooling Air Vents4-9
4.10 Electronic Package Compartments4-11
4.11 Miscellaneous Trouble Areas4-12
4.12 Factors in Corrosion Control4-17
4.13 Preventative Maintenance4-21
4.14–4.99 Reserved4-35
Chapter 5. Inspection Requirements5-1
5.1 General5-1
5.2 Frequency of Inspections5-1
5.3 Recommended Depth of Inspection5-2
5.4 Primary Approach5-3
5.5–5.99 Reserved5-13
Chapter 6. Corrosion Removal Techniques6-1
S
ection 1. Safety Procedures6-1
6.1 General6-1
6.2 General Corrosion Control Work Procedures6-3
6.3–6.9 Reserved6-4
Section 2. Corrosion Removal Techniques6-5
6.10 General6-5
6.11 Standard Methods6-5
6.12 Preparations for Rework6-5
6.13 Paint Removal6-6
6.14 Special Techniques6-6
6.15 Fairing or Blending Reworked Areas6-7
6.16 Chemical Testing6-13
6.17 Chemical Spot Analysis of Magnetic Metals6-14
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6.18 Chemical Spot Analysis of Nonmagnetic Metals6-15
6.19 Surface Treatment Testing6-16
6.20 Post Identification Cleaning and Refinishing6-17
6.21–6.25 Reserved6-17
Sect ion 3 . Mechanical Corrosion Removal by Blasting6-18
6.26 General6-18
6.27 Safety Precautions6-18
6.28–6.39 Reserved6-18
Sect io n 4. Corrosion Damage and Rework Limits6-19
6.40 Discussion6-19
6.41 Removal of Corrosion6-20
6.42 Determining Degree of Corrosion Damage6-21
6.43 Determining Rework Limits6-21
6.44 Determining Material Thickness Reduction After Corrosion Cleanup6-21
6.45–6.49 Reserved6-22
Section 5. Aluminum an d Aluminum Alloys6-23
6.50 Treatment6-23
6.51 Processing of Aluminum Surfaces6-23
6.52 Repair of Aluminum Alloy Sheet Metal6-27
6.53 Corrosion Removal Around Countersunk Fasteners in Aluminum Alloy6-28
6.54 Examples of Removing Corrosion from Aluminum and Aluminum Alloys6-28
6.55–6.59 Reserved6-30
Se ction 6. Magnesium Alloys6-31
6.60 Treatment of Wrought Magnesium Sheets and Forgings6-31
6.61 Repair of Magnesium Sheet Metal After Extensive Corrosion Removal6-31
6.62 In-Place Treatment of Magnesium Castings6-31
6.63 Example of Removing Corrosion from Magnesium6-32
6.64–6.69 Reserved6-34
Se ction 7. Ferrous Metals6-35
6.70 General6-35
6.71 Mechanical Removal of Iron Rust6-35
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6.72 Chemical Surface Treatment of Steel Surfaces6-36
6.73 Removal of Corrosive Products from High-Stressed Steel Parts6-37
6.74 Special Treatment of Stainless Steel Alloys6-37
6.75 Example of Process for Removal of Corrosion from Steel Parts6-37
6.76–6.79 Reserved6-38
Sect ion 8. Plated Parts6-39
6.80 Chromium and Nickel Plated Parts6-39
6.81 Cadmium and Zinc Plated Parts6-39
6.82–6.89 Reserved6-39
Se ction 9. Other Meta ls and Alloys6-40
6.90 Noble Metal Coatings—Cleanup and Restoration6-40
6.91 Copper and Copper Alloys6-40
6.92 Titanium Alloys6-40
6.93–6.99 Reserved6-41
Cha pter 7. Special Problems7-1
7.1 Mercury Spills/Corrosion Damage7-1
7.2 Corrosion Protection for Agricultural Aircraft7-3
7.3–7.99 Reserved7-4
L ist of Figures Figure 2-1. Simplified Corrosion Cell Showing Conditions which Must Exist for
Electrochemical Corrosion2-1
Figure 2-2. Elimination of Corrosion by Application of an Organic Film to Metal Surface2-2
Figure 2-3. Uniform Etch Corrosion2-4
Figure 2-4. P itting Corrosion2-5
Figu re 2 -5. Galvanic Corrosion of Magnesium Adjacent to Steel Fastener2-5
Figure 2-6. C oncentration Cell Corrosion2-7
Figure 2-7. Int ergranular Cracking and Corrosion on a Wing Spar Chord2-7
Figure 2-8. S evere Exfoliation Corrosion of a Seat Track2-8
Figure 2-9. S evere Exfoliation Corrosion2-8
Figure 2-10. Filiform Corrosion2-9
Figure 2-11. Filiform Corrosion Before and After Paint Removal2-10
Figure 2-12. S tress-Corrosion Cracking of 7079 TG Fitting2-11
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Figure 2-13. Stress-Corrosion Cracking Starting at a Pit in CRES Material2-12
Figure 2-14. Stress-Corrosion Cracking of an Extruded Section2-12
Figure 2-15. Fretting Corrosion and Cracking from Cyclic Loading of Lug2-13
Figure 4-1. Exhaust Trail Area Corrosion Control Points4-1
Figure 4-2 . Typical Damage to Flo or Around Lavatories, Buffets, and Galleys4-2
Figure 4-3. Landing Gear and Wheel Corrosion Points4-4
Figure 4-4. Intergranular C orrosion of 7075-T6 Aluminum Adjacent to Steel Fastener4-5
Figure 4-5. C rack and Skin Bulging Caused by Corrosion4-6
Figure 4-6. Spot-Welded Skin Corrosion4-7
Figure 4-7. Hinge Corrosion Points4-8
Figure 4-8. Hinge Failure Caused by Corrosion4-8
Figure 4-9. R eciprocating Engine Frontal Area Corrosion4-10
Figure 4-10. J et Engine Frontal Area Corrosion Points4-11
Figure 4-11. S par Chord Lifting Caused by Corrosion Products4-15
Figure 4-12. S kin Bulging Around Fasteners4-15
Figure 4-13. Opening of a Corroded Lap Joint for Repair4-16
Figure 4-14. C lose-Up View of a Corroded Lap Joint4-16
Figure 4-15. North America Corrosion Severity Map4-22
Figure 4-16. S outh America Corrosion Severity Map4-23
Figure 4-17. Africa Corrosion Severity Map4-24
Figure 4-18. South Pacific Corrosion Severity Map4-25
Figure 4-19. Asia Corrosion Severity Map4-26
Figure 4-20. Europe and Asia Minor Corrosion Severity Map4-27
Figure 4-21. Blocked Drain Passages Resulted in Accumulation of Corrosion Contaminates
and Moisture4-28
Figure 5-1. C orrosion Found After Removing Cargo Door Threshold Covers5-2
Figu re 5 -2. Corrosion Under Chipped and Loose Paint on Wing Skin5-4
Figure 5-3 . Corrosion Indicated by Blistering of Paint in Fuel Cell5-4
Figure 5-4. P opped Rivet Heads Resulting from Corrosion Products5-5
Figure 5-5. S kin Bulging Around Fasteners Caused by Pressure from Corrosion Products5-5
Figure 5-6. Corrosion Cracking Between Fasteners on a Wing Spar5-6
Figure 5-7. Faying Surface Corrosion on Wing Spar Chord with Corrosion Cracking
Also Visible5-6
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Figure 5-8. Severe Crown Stringer Corrosion5-7
Figure 5-9. Spar Chord Corrosion5-7
Figure 5-10. R emoval of Fillet Seal from Internal Edge of Lap Joint Exposed Full Extent
of Corrosion5-8
Figure 5-11. R emoval of Insulation Blankets Exposed Corrosion5-8
Figure 5-12. C orrosion Developed Under Leveling Compound Installed During Previous
Repair of Overwing Exit5-9
Figure 5-13. C orrosion Behind a Structural Component on a Wing Spar5-9
Figure 5-14 . Severely Corroded Lap Joint5-10
Figure 5-1 5. Severely Corro ded Lap Joint5-10
Figure 5-16. Local Paint Removal Required to Expose Full Extent of Corrosion5-11
Figure 5-17. C orroded Area After Paint Removal from Wing Skin5-11
Figure 5-18. Alodine 1200 Treatment Prior to Painting May Highlight Presence of
Remaining Corrosion or Cracks5-12
Figure 6-1. Blendout of Corrosion as Single Depression Width6-9
Figure 6-2. Typical Example of Acceptable Cleanup of Corrosion Pits6-10
Figure 6-3. Blendout of Multiple Pits in Corroded Area6-10
Figure 6-4. P rofile of Reworked Corroded Areas in Regions of Limited Access6-11
Figure 6-5 . Steel Particles from Fasteners or Nutplates May Cau se Future Corrosion
Problems in Aluminum Structure6-11
Figure 6-6. Incomplete Removal of Corrosion Products Resulted in Reoccurrence of
Corrosion6-12
Figure 6-7. Incomplete Removal of Corrosion Products Resulted in Reoccurrence of
Corrosion6-12
Figure 6-8. S evere Corrosion Requiring Trimming Out of Damaged Area6-20
Figure 6-9. Corrosion at Fastener Holes Removed by Spot Facing6-20
Figure 6-10. C orrosion Damage and Rework Measurement Using Depth Dial Gauge6-22
Figure 6-11. R emoving Oil and Surface Dirt6-24
Figure 6-12. P olishing and Brightening Alclad Surfaces6-24
Figure 6-13. C leaning and Stripping Paint6-25
Figure 6-14. C leaning and Inhibiting Corroded Aluminum Surfaces6-26
Figure 6-15. R emoving Excess Inhibitor Solution6-27
Figure 6-16. Applying Wax to Cleaned Surfaces6-27
Figure 6-17. R emoving Corrosion Products from Ordinary Steel Surfaces6-36
Figure 6-18. Removing Corrosion Products from Highly-Stressed Steel Parts6-36
viii List of Tables
Table 3-1. Corrosion of Metals3-4
Table 4-1. Galvanic Series of Metals and Alloys4-19
Table 4-2. Gr ouping of Metals and Alloys4-20
Table 6-1. Ab rasives for Corrosion Removal6-8
1-1 CHAPTER 1. GENERAL
1.1 Purpose of This Advisory Circular (AC). This AC is a summary of the current available data regarding identification and treatment of corrosive attack on aircraft structures and engine materials. Corrosion inspection frequency, corrosion identification, and especially corrosion treatment continues to be the responsibility of the operator.
These inspections should be accomplished per this AC, the manufacturer’s recommendations, or the operator’s own maintenance program. The procedures in this AC are an acceptable means, but not the only acceptable means, of corrosion treatment.
The information in this AC is applicable to aircraft for which the manufacturer has not published corrosion control information. Where the airframe or engine manufacturer has published a recommended corrosion inspection schedule and treatment program, the applicable program must take precedence over the recommendation of this AC.
1.2 Audience. This AC applies to aircraft operators and maintenance, repair, and overhaul (MRO) organizations.
1.3 Where You Can Find This AC. You can find this AC on the Federal Aviation Administration’s (FAA) website at http://www.faa.gov/regulations_policies/advisory_circulars.
1.4 What This AC Cancels. AC 43-4A, Corrosion Control for Aircraft, dated July 25, 1991, is canceled.
1.5 Related Reading Material.
1.5.1 FAA Documents (current editions).
• AC 65-15, Airframe and Powerplant Mechanics Airframe Handbook. You can find this AC at http://rgl.faa.gov/.
• FAA Order 8300.12, Corrosion Prevention and Control Programs.
1.5.2 Other Documents:
• Naval Air Systems Command, NAVAIR 01-1A-509-1, Cleaning and Corrosion Control.
• U.S. Air Force Technical Order TO 1-1-691, Cleaning and Corrosion Prevention and Control, Aerospace and Non-Aerospace Equipment.
1.6 Background. Corrosion is the electrochemical deterioration of a metal because of its chemical reaction with a surrounding environment. While the aerospace industry is continuously developing new and better materials, progress is offset partly by a more aggressive operational environment and by the complexity of the corrosion phenomenon, which can take many different forms. The resistance of aircraft materials to corrosion can drastically change with only a small environmental change.
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1.7 Catastrophic Corrosion Events. Corrosion is most often a slow process of material deterioration, taking place over a long period of time. Examples are general corrosion, pitting, stress-corrosion cracking, environmental embrittlement, and corrosion fatigue and exfoliation. Corrosion degradation can occur very quickly, in days or even hours, and with catastrophic results. Corrosion thrives on both the chemical and mechanical aspects of the environment and can cause catastrophic structural failure without warning.
1.8 Corrosion Control Program.
1.8.1 The possibility of an in-flight mishap or excessive down time for structural repairs mandates an active corrosion prevention and control program (CPCP). The operational environment of the aircraft sets the type and aggressiveness of the program. Aircraft exposed to such environments as salt air, heavy atmospheric industrial pollution, or over-water operations, will need a more stringent CPCP than an aircraft operating in a dry environment.
1.8.2 Operators must follow a constant cycle of cleaning, inspection, operational preservation, and lubrication to prevent corrosion. Prompt detection and removal of corrosion will limit the extent of damage to an aircraft and its components. The basic philosophy of a CPCP should consist of:
1. Personnel adequately trained in recognizing 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. Scheduled corrosion inspections;
5. Aircraft washing at regularly scheduled intervals;
6. Routine cleaning or wipe down of all exposed unpainted surfaces;
7. Keeping drain holes and passages open and functional;
8. Scheduled inspections, removals, and reapplications of preservation compounds;
9. Early detection and repair of damaged protective coatings;
10. Periodic thorough cleaning, lubrication, and preservation;
11. Prompt corrosion treatment after detection;
12. Accurate recordkeeping and reporting of material or design deficiencies; and
13. Use of appropriate materials, equipment, and technical publications.
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1.9 AC Feedback Form. For your convenience, the AC Feedback Form is the last page of this AC. Note any deficiencies found, clarifications needed, or suggested improvements regarding the contents of this AC on the Feedback Form.
1.10–1.99 RESERVED
2-1 CHAPTER 2. CORROSION THEORY
2.1 Introduction. This chapter introduces corrosion theory, causes of corrosion, factors influencing its development, and descriptions of forms of corrosion and common corrosive agents.
2.2 Background.
2.2.1 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 different from erosion, which causes destruction by mechanical action. The corrosion occurs because metals tend 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, Simplified Corrosion Cell Showing Conditions which Must Exist for Electrochemical Corrosion):
1. Presence of an anode, or a metal that will corrode;
2. Presence of a cathode, a dissimilar conductive material which has less tendency to corrode;
3. Presence of an electrolyte, a conductive liquid; and
4. Electrical contact between the anode and cathode, which is usually by metal-to-metal contact, or through a fastener.
Figure 2-1. Simplified Corrosion Cell Showing Conditions which Must Exist for Electrochemical Corrosion
2.2.2 E limination of any one of these conditions will stop corrosion. An example would be a paint film on the metal surface (see Figure 2-2, Elimination of Corrosion by Application of an Organic Film to Metal Surface). Metals such as stainless steel and titanium can produce corrosion products so tightly bound to the corroding metal that they form an