FIGURE 4-14. CLOSEUP VIEW OF A CORRODED LAP JOINT
411. FACTORS IN CORROSION CONTROL.
Corrosion Factors. The degree of severity, the cause, and the type of corrosion depend on many factors, including the size or thickness of the part, the material, heat treatment of the material, protective finishes, environmental conditions, preventative measures, and design.
(1) Thick structural sections are generally more susceptible to corrosive attack because of variations in their composition, particularly if the sections are heat-treated during fabrication. When large sections are machined or chem-milled after heat treatment, the corrosion characteristics of thinner sections may be different from those of thicker areas. Section size is based on Contre ind on structural requirements and cannot be changed for the purpose of controlling corrosion. From a maintenance standpoint, the correct approach is one of recognizing the need to ensure the integrity and strength of major structural parts and maintaining permanent protection over such areas at all times.
(2) In-service stresses and field repairs may affect the rates and types of corrosion. Aircraft structure under high cyclic stresses, such as helicopter main rotors, are particularly subject to stress-corrosion cracking.
Also areas adjacent to weld repaired items often have corrosion due to insufficient removal of the weld flux, or, for some steels, buildup of a magnetic field. Areas such as these should be closely inspected for signs of corrosion and, when found, proper treatment accomplished.
Chap 4 66 Par 410
b. Corrosion Control in Design. Since corrosion is the deterioration of metals resulting from reactions between metals and their environment, some corrosion control or means to minimize corrosion when the aircraft enters operational service should be introduced during the design phase. The corrosion issues discussed in this AC provide information to reduce the rate of corrosive attack by corrosion control measures introduced early in design.
(1) The nature of the material is a fundamental factor in corrosion.
High-strength, heat-treatable aluminum and magnesium alloys are very susceptible to corrosion, while titanium and some stainless steel alloys are less susceptible in atmospheric environment. The aircraft manufacturer selects material for the aircraft based on material strength, weight, and cost, while corrosion resistance is often a secondary consideration. However, corrosion control should be considered as early as possible during the preliminary design
(2) The use of more corrosion resistant materials in any design normally involves additional weight to achieve required strength. Since weight consideration is a major factor in the construction of airframes, the primary means of preventing corrosion is by use of protective coatings and proper maintenance procedures.
The use of corrosion resistant alloys is not a cure-all for corrosion prevention. A common mistake is to replace a corroded part with a corrosion resistant alloy only to find that the corrosion has now shifted to another part and increased in severity.
(4) The problem of protection against corrosion is minimized if the material to be protected is intrinsically resistant to corrosion. Aluminum copper alloys are known to have better stress-corrosion resistance and better fatigue strength properties than aluminum zinc alloys; therefore, they are often used as the primary structural materials.
(5) Galvanic corrosion is created by dissimilar metals being in contact with each other. The galvanic series of metals and alloys (Table 4-1) is a factor that should be considered in the repair of aircraft. The further galvanic corrosion. apart the metals listed in Table 4-1 are, the greater the tendency will be for The metals grouped together in Table 4-2 have little differences in electrical potential; thus they are relatively safe to use in contact with one another. However, the coupling of metals from different groups will result in corrosion of the group having a lower number.
Chap 4 Par 411 67
TABLE 4-1. GALVANIC SERIES OF METALS AND ALLOYS Electrode Potential of Various Metals and Alloys (a) Metal or Alloy (b) Potential, volts
0.1 N calomel scale (c) Magnesium. -1.73 Zinc 7072, Alciad 3003, Aiciad .... -1.10 6061, Alclad 7075 -0.96 5056, 7079-T6, 5456, 5083, 214, 218. • • . . • -0.87 5052, 5652, 5086, 1099 -0.85 3004, B214, 1185, 1060, 1260, 5050. • -0.84 1100, 3003, 615, 6053, 6061-T6, 6062-T6, 6063, 6363, Alclad 2014, Alclad 2024 -0.83 Cadmium . -0.82 7075-76, 356-76, 360 -0.81 2024-781, 6061-T4, 6062-T4 -0.80 355-T6 -0.79 2014-T6, 113, 750-T5 -0.78 2014-T4, 2017-T4, 2024-T3, and T4 -0.68 to -0.70 (d) Mild steel -0.58 Lead -0.55 Tin . -0.49 Copper -0.20 Bismuth -0.18 Stainless steel (series 300, type 430) -0.09 Silver . . -0.08 Nickel -0.07 Chromium -0.49 to +0.018
(a) Data from Alcoa Research Laboratories.
(b) The potential of all tempers is the same unless temper is designated.
(c) Measured in an aqueous solution of 53g of sodium chloride + 3g hydrogen peroxide per liter at 25 degree C.
(d) The potential varies with quenching rate.
Chap 4 68 Par 411
TABLE 4-2. GROUPING OF METALS AND ALLOYS GROUP I Magnesium and Magnesium Alloys.
GROUP II Aluminum, Aluminum Alloys, Zinc, Cadium and Cadium-Titanium Plate.
Group III Iron, Steels - Except Stainless Steels;
Lead, Tin and their Alloys.
GROUP IV copper, Brass, Bronze, Copper-Berylium, Copper-Nickel Chromium, Nickel, Nickel Base Alloys, Cobalt Base Alloys, Carbon Graphite, Stainless Steels, Titanium and Titanium Alloys.
NOTE: 1. Metals listed in the same group are considered similar to one another.
2. Metals listed in different groups are considered dissimilar to one another.
C. Protective Finishes. Protective finishes provide protection for the base material from corrosion and other forms of deterioration. Protective finishes are divided into 2 separate categories, sacriticial and nonsacrificial. Sacrificial coatings include cadmium, zinc, and aluminum.
Non-sacrificial coatings include hard plating (chromium and nickel), chemical conversion coatings, sealant, primers, and top coat.
d. Geographical Location and Environment. This factor concerns systems exposed to marine atmospheres, moisture, acid rain, tropical temperature conditions, industrial chemicals, and soils and dust in the atmosphere. Limit, whenever possible, the requirement for operation of aircraft in adverse environments.
(1) Moisture is present in the air as a gas (water vapor) or as finely divided droplets of liquid (mist or fog) and often contains contaminants such as chlorides, sulfates, and nitrates, which increase its corrosive effects.
Condensed moisture and its contaminants can also be trapped in close fitting, wettable joints, such as faying surfaces and be drawn along poor bond lines by capillary action.
electrolytes. Normal sea winds carry dissolved salt which makes coastal Salt particles, when dissolved in water, form strong environments highly corrosive.
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3) Industrial pollutants (such as carbon, nitrates, ozone, sultur dioxide, and sulfates) contribute to the deterioration of nonmetallic materials and can cause severe corrosion of metals.
(4) Warm, moist air, normally found in tropical climates accelerates corrosion while cold, dry air normally found in arctic climates reduces corrosion.
e. Heat Treatment. Proper heat treatment of materials is a vital factor in maximizing resistance to corrosion.
412. PREVENTATIVE MAINTENANCE.
a. Prevention. Corrosion prevention of aircraft structure depends on a comprehensive corrosion prevention and control plan, implemented from the start of operation of an aircraft, which includes:
(1) Adequately trained personnel in:
(ї) Recognition of corrosion inducing conditions;
(її) Corrosion identification techniques;
(iii) Corrosion detection, cleaning, and treating; and
(iv) Lubrication and preservation of aircraft structure and components.
(2) Inspection for corrosion on a scheduled basis.
(3) Thorough cleaning, inspection, lubrication, and preservation at prescribed intervals. Suggested intervals based on operating environment (see Figures 4-15 through 4-20) are:
(ї) Mild zones -- Every 90 days;
(ii) Moderate zones -- Every 45 days; and
(iii) Severe zones -- Every 15 days.
(4) Prompt corrosion treatment after detection.
(5) Accurate record keeping and reporting of material or design deficiencies to the manufacturer and the FAA.
(6) Use of appropriate materials, equipment, and technical publications.
(7) Maintenance of the basic finish systems.
Chap 4 70 Par 411
7/25/91 AC 43-4A leveling compounds, miscellaneous debris, or corrosion inhibitors should not block drain paths see Figure 4-21). (8) Keeping drain holes and passages open and functional. Sealants,
(9) Replacing deteriorated or damaged gaskets and sealants (using noncorrosive type sealants) to avoid water intrusion and entrapment which leads to corrosion.
such as hangaring away from salt spray. (10) Minimizing the exposure of aircraft to adverse environments, Chap 4 Par 412
7/25/91 AC 43-4A
FAIRBANKS S* ANCHORAGE SEATT MONTHE.
NEW YORK
CHICAGO
• DENVER LOS ANGELE! ANTA
• MIAMI
HAITI EI RICO PUERTO CORROSION SEVERITY ZONE ST.
OMILO MEXICO: JAMAICA DOMINGO
MODERATE
• SEVERE FIGURE 4-15. NORTH AMERICA CORROSION SEVERITY MAP Chap 4 Par 412 72
T7529TRINIDAO
CARACAS
• BOGOTA BELEM
PGUAYAGUIL
FORTALEZA RECIFE LIMA
BRASILIA SALVADOR * LA PAZ ASUNCION SAO PAULON * RIO DE JANIERO
SANTIAGO
BUENOS MONTEVIDEO AIRES
CORROSION SEVERITY ZONE MILD
(1) MODERATE
• SEVERE FIGURE 4-16. SOUTH AMERICA CORROSION SEVERITY MAP Chap 4 Par 412 73
ALGIER
TEHERAN
DAMASCUS JUDAH
KHARTOUM SANO
NAIROSI
LEOPOLDVILLE
CORROSION SEVERITY ZONE
OMILO JONANNESBURG @ MODERATE HI| FLOURENCO MAROVES
• SEVERE Y DURBAN
CAPE TOWN FIGURE 4-17. AFRICA CORROSION SEVERITY MAP 74
7/25/91 AC 43-4A
• MILD I MODERATE SEVERE
CORROSION SEVERITY ZONE SOUTH PACIFIC FIJI (SEVERE * VELLINGTON WAKE • (SEVERE) • SEVERE SOLOMONS CORAL SEA SIDNEY TASMAN SEA CHRIST CHURCI TASMANIA
PHILIPPINES NEW GUINEA A AUSTRALIA MELBOUR
INDO PERTH OCEAN FIGURE 4-18. SOUTH PACIFIC CORROSION SEVERITY MAP Chap 4 Par 412 75
SEVERITY ZONE CORROSION
I MODERATE MILO
SEA ARABIAN
OCEAN INDIAN BOMBAY TASHKENT
KUALA BENGAL BAY OF DRAS RANGOON CALCYTI INDIA SOVIET UNION IMPORE IMPUR SAIGON SEA CHINA SOUTH O HANNAN E, MANILA AND HONG KONG CAN @ TAIWAN CHUNKING: NANKING! SHANGHAI PEL VLADIV (SEVERE) OKINAWA FIGURE 4-19. ASIA CORROSION SEVERITY MAP Chap 4 Par 412 76
7/25/91 AC 43-4A REYKJAVIK tY
CORMOSION SEVERITY ZONE OMILO @MODERATE THENS
• SEVERE FIGURE 4-20. EUROPE AND ASIA MINOR CORROSION SEVERITY MAP Chap 4 Par 412 77