v
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
vi
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
vii
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.
1-2
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.
1-3
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
2-2 invisible oxide, or passive, film, preventing further corrosion. When a 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.
Figure 2-2. Elimination of Corrosion by Application of an Organic Film to Metal Surface
2.3 D evelopment of Corrosion.
2.3.1 All corrosive attack begins on the surface of the metal, and the corrosion process involves two chemical changes: 1) the metal that is attacked or oxidized undergoes an anodic change, and 2) the corrosive agent is reduced or undergoes a cathodic change. The tendency of most metals to corrode creates one of the major problems in aircraft maintenance, particularly where adverse environmental or weather conditions exist.
2.3.2 Paint coatings can mask initial stages of corrosion. Since corrosion products occupy more volume than the original metal, paint surfaces should be inspected often for such irregularities as blisters, flakes, chips, and lumps.
2.4 Factors Influencing Corrosion.
2.4.1 Some factors which influence metal corrosion and the rate of corrosion are the:
1. Type of metal;
2. Heat treatment and grain direction;
3. Presence of a dissimilar, less corrodible metal (galvanic corrosion);
4. Anode and cathode surface areas (in galvanic corrosion);
2-3
5. Temperature;
6. Presence of electrolytes (such as hard water, salt water, or battery fluids);
7. Availability of oxygen;
8. Presence of different concentrations of the same electrolyte;
9. Presence of biological organisms;
10. Mechanical stress on the corroding metal;
11. Time of exposure to a corrosive environment; and
12. Lavatory fluids with hard water, salt water, or battery fluids.
2.4.2 Most pure metals are not suitable for aircraft construction and are used to form alloys.
Most alloys are made up entirely of small, crystalline regions called grains. Corrosion can occur on the less resistant surfaces of these regions and at boundaries between regions, resulting in the formation of pits and intergranular corrosion. Metals have a wide range of corrosion resistance. The most chemically active metals (which tend to lose electrons easily, such as magnesium and aluminum) corrode easily. The most noble metals (which do not lose electrons easily, such as gold and silver) do not corrode easily.
2.4.3 High-temperature environments accelerate corrosion by increasing chemical reaction rates and moisture content in saturated air.
2.4.4 Electrolytes, or electrically conducting solutions, form when condensation, salt spray, rain, or rinse water accumulate on surfaces. Dirt, salt, acidic gases, and engine exhaust gases can dissolve on wet surfaces, increasing electrolyte conductivity and also the rate of corrosion.
2.4.5 When areas such as between faying surfaces or in deep crevices partially confine an electrolyte on a metal surface, that surface corrodes more rapidly than other metal surfaces outside this area. This area of corrosion is called an oxygen concentration cell.
The reduced oxygen content of the confined electrolyte causes adjacent metal to become anodic to other metal surfaces immersed in electrolyte and exposed to air on the same part.
2.4.6 Slimes, molds, fungi, and other living organisms, some of which are microscopic, can grow on damp surfaces. The area where such organisms become established tends to remain damp, increasing the possibility of corrosion.
2.4.7 Manufacturing processes, such as machining, forming, welding, or heat treatment, can leave stresses in aircraft parts. This residual stress can cause cracking in a corrosive environment when the threshold for stress-corrosion is exceeded.
2.4.8 In some cases, corrosion progresses at the same rate no matter how long the metal has been exposed to an environment. In other cases, corrosion can decrease with time because of a barrier formed by corrosion products, or increase with time if a barrier is breaking down.
2-4
2.5 Forms of Corrosion. Different types of corrosive attack will vary with the metal concerned, corrosive media location, and the length of time of exposure. For descriptive purposes, the types are listed under what the FAA considers the most commonly accepted titles.
2.5.1 Uniform Etch Corrosion. Uniform etch corrosion results from a direct chemical attack on a metal surface and involves only the metal surface (see Figure 2-3, Uniform Etch Corrosion). On a polished surface, this type of corrosion may first appear as a general dulling of the surface. If the attack continues, the surface becomes rough and possibly frosted in appearance. However, it should be noted that the discoloration or general dulling of metal created by exposure to elevated temperatures is not uniform etch corrosion.
2.5.2 Pitting Corrosion. This is the most common effect of corrosion on aluminum and magnesium alloys, but can occur in other metal alloys (see Figure 2-4, Pitting Corrosion).
It is first noticeable as a white or gray powdery deposit, similar to dust, which blotches the surface. When the deposit is cleaned away, tiny pits or holes in the surface can be seen. The combination of small active anodes and large passive cathodes causes severe pitting. The principle also applies to metals passivated by chemical treatments or environmental condition.
Figure 2-3. Uniform Etch Corrosion
2-5 Figure 2-4. Pitting Corrosion
2.5.3 Galvanic Corrosion. This corrosion type occurs when two dissimilar metals make electrical contact in the presence of an electrolyte (see Figure 2-5, Galvanic Corrosion of Magnesium Adjacent to Steel Fastener). The rate of corrosion depends on differences in reaction activity between dissimilar metals. The greater the difference, the faster corrosion occurs. For example, aluminum skin panels and stainless steel doublers, riveted together in an aircraft wing, form a galvanic couple if moisture and contamination are present. The rate of galvanic corrosion also depends on the size of the parts in contact. If the surface area of the corroding metal (the anode) is smaller than the surface area of the less active metal (the cathode), corrosion will be rapid and severe. But if the corroding metal is larger than the less active metal, corrosion will be slow and superficial.
Figure 2-5. Galvanic Corrosion of Magnesium Adjacent to Steel Fastener