The Deutsch rivet is available in diameters of 16 4, or ⅛ inch. Grip lengths for this rivet range from ¾6 to 1 inch. Some variation is allowed in grip length when installing the rivet; for example, a rivet with a grip length of ¾6 inch can be used where the total thickness of materials is between 0.198 and 0.228 inch.
When driving a Deutsch rivet, an ordinary hammer or a pneumatic rivet gun and a flathead set are used. The rivet is seated in the previously drilled hole and then the pin is driven into the sleeve. The driving action causes the pin to exert pressure against the sleeve and forces the sides of the sleeve out. This stretching forms a shop head on the end of the rivet and provides positive fastening. The ridge on the top of the rivet head locks the pin into the rivet as the last few blows are struck.
Pin Rivets Pin (Hi-shear) rivets are classified as special rivets but are not of the blind type. Access to both sides of the material is required to install this type of rivet. Pin rivets have the same shear strength as bolts of equal diameters, are about 40 percent of the weight of a bolt, and require only about one-fifth as much time for installation as a bolt, nut, and washer combination. They are approximately three times as strong as solidshank rivets.
Pin rivets are essentially threadless bolts. The pin is headed at one end and is grooved about the circumference at the other. A metal collar is swaged onto the grooved end effecting a firm, Stud Collar FIGURE 6-54. Pin (Hi-shear) rivet.
Pin rivets are fabricated in a variety of materials but should be used only in shear applications. They should never be used where the grip length is less than the shank diameter.
Part numbers for pin rivets can be interpreted to give the diameter and grip length of the individual rivets. A typical part number breakdown would be:
NAS 17-14 - -Maximum grip length in 16ths of an inch.
Nominal diameter in 32nds of an inch.
|177 = 100° countersunk head rivet.
178 = flathead rivet.
National Aircraft Standard.
PLASTICS Plastics are used in many applications throughout modern aircraft. These applications range from structural components of thermosetting plastics reinforced with fiber glass to decorative trim of thermoplastic materials.
Transparent Plastics Transparent plastic materials used in aircraft canopies, windshields, and other similar transparent enclosures may be divided into two major classes or groups. These plastics are classified according to their reaction to heat. The two classes are Thermoplastic and Thermosetting.
Thermoplastic materials will soften when heated and harden when cooled. These materials can be heated until soft, and then formed into the desired shape. When cooled, they will retain this shape.
The same piece of plastic can be reheated and reshaped any number of times without changing the chemical composition of the material.
Thermosetting plastics harden upon heating, and reheating has no softening effect. These plastics cannot be reshaped after once being fully cured by the application of heat.
In addition to the above classes, transparent plastics are manufactured in two forms, monolithic (solid) and laminated. Laminated transparent plastics are made from transparent plastic face sheets bonded by an inner-layer material, usually polyvinyl butyral. Because of its shatterresistant qualities, laminated plastic is superior 163
to solid plastics and is used in many pressurized aircraft. Most of the transparent sheet used in aviation is manufactured in accordance with various military specifications.
A new development in transparent plastics is stretched acrylic. Stretched acrylic is a type of plastic which, before being shaped, is pulled in both directions to rearrange its molecular structure. Stretched acrylic panels have a greater resistance to impact and are less subject to shatter;
its chemical resistance is greater, edging is simpler, and crazing and scratches are less detrimental.
Individual sheets of plastic are covered with a heavy masking paper to which a pressure-sensitive adhesive has been added. This paper helps to prevent accidental scratching during storage and handling. Care should be taken to avoid scratches and gouges which may be caused by sliding sheets against one another or across rough or dirty tables.
Sheets should be stored in bins which are tilted at approximately 10° from vertical, if possible. If they must be stored horizontally, piles should not be over 18 inches high, and small sheets should be stacked on the larger ones to avoid unsupported overhang. Storage should be in a cool, dry place away from solvent fumes, heating coils, radiators, and steampipes. The temperature in the storage room should not exceed 120° F.
While direct sunlight does not harm acrylic plastic, it will cause drying and hardening of the masking adhesive, making removal of the paper difficult. If the paper will not roll off easily, place the sheet in an oven at 250° F. for 1 minute, maximum. The heat will soften the masking adhesive for easy removal of the paper.
If an oven is not available, hardened masking paper may be removed by softening the adhesive with aliphatic naphtha. Rub the masking paper with a cloth saturated with naphtha. This will soften the adhesive and free the paper from the plastic. Sheets so treated must be washed immediately with clean water, taking care not to scratch the surfaces.
NOTE: Aliphatic naphtha is not to be confused with aromatic naphtha and other dry-cleaning solvents which are definitely harmful in their effects on plastic. However, aliphatic naphtha is flammable and all precautions regarding the use of flammable liquids must be observed.
Reinforced Plastic Reinforced plastic is a thermosetting material used in the manufacture of radomes, antenna covers, and wingtips, and as insulation for various pieces of electrical equipment and fuel cells. It has excellent dielectric characteristics which make it ideal for radomes; however, its high strengthweight ratio, resistance to mildew, rust, and rot, and ease of fabrication make it equally suited for other parts of the aircraft.
Reinforced plastic components of aircraft are formed of either solid laminates or sandwich-type laminates. Resins used to impregnate glass cloths are of the contact-pressure type (requiring little or no pressure during cure). These resins are supplied as a liquid which can vary in viscosity from a waterlike consistency to a thick sirup.
Cure or polymerization is effected by the use of a catalyst, usually benzoyl peroxide.
Solid laminates are constructed of three or more layers of resin-impregnated cloths "wet laminated" together to form a solid sheet facing or molded shape.
Sandwich-type laminates are constructed of two or more solid sheet facings or a molded shape enclosing a fiberglass honeycomb- or foam-type core. Honeycomb cores are made of glass cloths impregnated with a polyester or a combination of nylon and phenolic resins. The specific density and cell size of honeycomb cores varies over a considerable latitude. Honeycomb cores are normally fabricated in blocks that are later cut to the desired thickness on a bandsaw.
Foam-type cores are formulated from combinations of alkyd resins and metatoluene di-isocyanate. Sandwich-type fiberglass components filled with foam-type cores are manufactured to exceedingly close tolerances on overall thickness of the molded facing and core material. To achieve this accuracy, the resin is poured into a closetolerance, molded shape. The resin formulation immediately foams up to fill the void in the molded shape and forms a bond between the facing and the core.
RUBBER Rubber is used to prevent the entrance of dirt, water, or air, and to prevent the loss of fluids, 164
gases, or air. It is also used to absorb vibration, reduce noise, and cushion impact loads.
The term "rubber" is as all inclusive as the term "metal." It is used to include not only natural rubber, but all synthetic and silicone rubbers.
Natural Rubber Natural rubber has better processing and physical properties than synthetic or silicone rubber.
These properties include: Flexibility, elasticity, tensile strength, tear strength, and low heat buildup due to flexing (hysteresis). Natural rubber is a general-purpose product; however, its suitability for aircraft use is somewhat limited because of its inferior resistance to most influences that cause deterioration. Although it provides an excellent seal for many applications, it swells and often softens in all aircraft fuels and in many solvents (naphthas, etc.). Natural rubber deteriorates more rapidly than synthetic rubber. It is used as a sealing material for water/methanol systems.
Synthetic Rubber Synthetic rubber is available in several types, each of which is compounded of different materials to give the desired properties. The most widely used are the butyls, Bunas, and neoprene.
Butyl is a hydrocarbon rubber with superior resistance to gas permeation. It is also resistant to deterioration; however, its comparative physical properties are significantly less than those of natural rubber. Butyl will resist oxygen, vegetable oils, animal fats, alkalies, ozone, and weathering.
Like natural rubber, butyl will swell in pe.
troleum or coal tar solvents. It has a low waterabsorption rate and good resistance to heat and low temperature. Depending on the grade, it is suitable for use in temperatures ranging from
- 65° F. to 300° F. Butyl is used with phosphate ester hydraulic fluids (Skydrol), silicone fluids, gases, ketones, and acetones.
Buna-S rubber resembles natural rubber both in processing and performance characteristics.
Buna-S is as water-resistant as natural rubber, but has somewhat better aging characteristics.
It has good resistance to heat, but only in the absence of severe flexing. Generally, Buna-S has poor resistance to gasoline, oil, concentrated acids, and solvents. Buna-S is normally used for tires and tubes as a substitute for natural rubber.
Buna-N is outstanding in its resistance to hydrocarbons and other solvents; however, it has poor resilience in solvents at low temperature.
Buna-N compounds have good resistance to temperatures up to 300° F., and may be procured for low temperature applications down to - 75° F.
Buna-N has fair tear, sunlight, and ozone resistance. It has good abrasion resistance and good breakaway properties when used in contact with metal. When used as a seal on a hydraulic piston, it will not stick to the cylinder wall. Buna-N is used for oil and gasoline hose, tank linings, gaskets, and seals.
Neoprene can take more punishment than natural rubber and has better low-temperature characteristics. It possesses exceptional resistance to ozone, sunlight, heat, and aging. Neoprene looks and feels like rubber. Neoprene, however, is less like rubber in some of its characteristics than butyl or Buna. The physical characteristics of neoprene, such as tensile strength and elongation, are not equal to natural rubber but do have a definite similarity. Its tear resistance as well as its abrasion resistance is slightly less than that of natural rubber. Although its distortion recovery is complete, it is not as rapid as natural rubber.
Neoprene has superior resistance to oil. Although it is good material for use in nonaromatic gasoline systems, it has poor resistance to aromatic gasolines. Neoprene is used primarily for weather seals, window channels, bumper pads, oil-resistant hose, and carburetor diaphragms. It is also recommended for use with Freons and silicate ester lubricants.
Thiokol, known also as polysulfide rubber, has the highest resistance to deterioration but ranks the lowest in physical properties. Thiokol, in general, is not seriously affected by petroleum, hydrocarbons, esters, alcohols, gasoline, or water.
Thiokols are ranked low in such physical properties as compression set, tensile strength, elasticity, and tear abrasion resistance. Thiokol is used for oil hose, tank linings for aromatic aviation gasolines, gaskets, and seals.
Silicone rubbers are a group of plastic rubber Year marking materials made from silicon, oxygen, hydrogen, Year Threads Color and carbon. The silicons have excellent heat stability and very low temperature flexibility. They 1968_ 2 blue are suitable for gaskets, seals, or other applications 1969- 2 yellow where elevated temperatures up to 600° F. are 1970- 2 black prevalent. Silicone rubbers are also resistant to 1971- 2 green temperatures down to - 150° F. Throughout this 1972- 2 red temperature range, silicone rubber remains extremely flexible and useful with no hardness or Quarter marking gumminess. Although this material has good re- Quarter Threads sistance to oils, it reacts unfavorably to both aromatic and nonaromatic gasolines. Jan., Feb., Mar.
Apr., May, June 1 Silastic, one of the best known silicones, is used July, Aug., Sept...
Oct., Nov., Dec._-- 1 to insulate electrical and electronic equipment.
Because of its dielectric properties over a wide range of temperatures, it remains flexible and free FICURE 6-55. Shock absorber cord color coding.
from crazing and cracking. Silastic is also used for gaskets and seals in certain oil systems.
SHOCK ABSORBER CORD Shock absorber cord is made from natural rubber strands encased in a braided cover of woven cotton cords treated to resist oxidation and wear. Great tension and elongation are obtained by weaving the jacket upon the bundle of rubber strands while they are stretched about three times their original length.
There are two types of elastic shock-absorbing cord. Type I is a straight cord, and type II is a continuous ring, known as a "bungee." The advantages of the type II cord are that they are easily and quickly replaced and do not have to be secured by stretching and whipping. Shock cord is available in standard diameters from ¼ inch to 16 inch.
SEALS Seals are used to prevent fluid from passing a certain point, as well as to keep air and dirt out of the system in which they are used. The increased use of hydraulics and pneumatics in aircraft systems has created a need for packings and gaskets of varying characteristics and design to meet the many variations of operating speeds and temperatures to which they are subjected. No one style or type of seal is satisfactory for all installations. Some of the reasons for this are:
(1) Pressure at which the system operates, (2) the type fluid used in the system, (3) the metal finish and the clearance between adjacent parts, and
(4) the type motion (rotary or reciprocating), if any. Seals are divided into three main classes:
(1) Packings, (2) gaskets, and (3) wipers.
Three colored threads are braided into the outer cover for the entire length of the cord.
Two of these threads are of the same color and represent the year of manufacture; the third thread, a different color, represents the quarter of the year in which the cord was made. The code covers a 5-year period and then repeats itself. This makes it easy to figure forward or backward from the years shown in figure 6-55.
Packings Packings are made of synthetic or natural rubber. They are generally used as "running seals," that is, in units that contain moving parts, such as actuating cylinders, pumps, selector valves, etc. Packings are made in the form of O-rings, V-rings, and U-rings, each designed for a specific purpose. (See figure 6-56.)
Advances in aircraft design have necessitated new O-ring compositions to meet changed operating conditions. Hydraulic O-rings were originally established under AN specification numbers (6227, 6230, and 6290) for use in MIL-H-5606 fluid at U-ring O-ring temperatures ranging from -65° F. to +160° F.
When new designs raised operating temperatures to a possible 275° F., more compounds were developed and perfected.
Recently a compound was developed that offered improved low-temperature performance V-ring U-cup without sacrificing high-temperature performance, rendering the other series obsolete. This superior material was adopted in the MS28775 series. This series is now the standard for MIL-H-5606 systems where the temperature may vary from
- 65° F. to + 275° F.
Male V-ring adapter Female FIGURE 6-56. Packing rings.
O-Ring Packings O-ring packings are used to prevent both internal and external leakage. This type of packing ring seals effectively in both directions and is the type most commonly used. In installations subject to pressures above 1,500 p.s.i., backup rings are used with O-rings to prevent extrusion.
When an O-ring packing is to be subjected to pressure from both sides, as in actuating cylinders, two backup rings must be used (one on either side of the O-ring). When an O-ring is subject to pressure on only one side, a single backup ring is generally used. In this case the backup ring is always placed on the side of the O-ring away from the pressurc.
The materials from which O-rings are manufactured have been compounded for various operating conditions, temperatures, and fluids. An O-ring designed specifically as a static (stationary) seal will probably not do the job when installed on a moving part such as a hydraulic piston. Most O-rings are similar in appearance and texture, but their characteristics may differ widely. An O-ring will be useless if it is not compatible with the system fluid and operating temperature.
Manufacturers provide color coding on some O-rings, but this is not a reliable or complete means of identification. The color coding system does not identify sizes, but only system fluid or vapor compatibility and in some cases the manufacturer. Color codes on O-rings that are compatible with MIL-H-5606 fluid will always contain blue, but may also contain red or other colors. Packings and gaskets suitable for use with Skydrol fluid will always be coded with a green stripe, but may also have a blue, grey, red, green;
or yellow dot as a part of the color code. Color codes on O-rings that are compatible with hydrocarbon fluid will always contain red, and will never contain blue. A colored stripe around the circumference indicates that the O-ring is a boss gasket seal. The color of the stripe indicates fluid compatibility: red for fuel, blue for hydraulic fluid.
The coding on some rings is not permanent.
On others it may be omitted due to manufacturing difficulties or interference with operation. Furthermore, the color coding system provides no means to establish the age of the O-ring or its temperature limitations.
Because of the difficulties with color coding, O-rings are available in individual hermetically sealed envelopes, labeled with all pertinent data.
When selecting an O-ring for installation, the basic part number on the sealed envelope provides the most reliable compound identification.
Male V-ring Adjustment nuts adapter Female V-ring adapter V-ring packing FICURE 6-57. V-ring installation.
Although an O-ring may appear perfect at first glance, slight surface flaws may exist. These flaws are often capable of preventing satisfactory O-ring performance under the variable operating pressures of aircraft systems; therefore, O-rings should be rejected for flaws that will affect their performance. Such flaws are difficult to detect, and one aircraft manufacturer recommends using a 4-power magnifying glass with adequate lighting to inspect each ring before it is installed.
By rolling the ring on an inspection cone or dowel, the inner diameter surface can also be checked for small cracks, particles of foreign material, or other irregularities that will cause leakage or shorten the life of the O-ring. The slight stretching of the ring when it is rolled inside out will help to reveal some defects not otherwise visible.
Backup Rings Backup rings (MS28782) made of Teflon do not deteriorate with age, are unaffected by any system fluid or vapor, and can tolerate temperature extremes in excess of those encountered in highpressure hydraulic systems. Their dash numbers indicate not only their size but also relate directly to the dash number of the O-ring for which they are dimensionally suited. They are procurable under a number of basic part numbers, but they are interchangeable; that is, any Teflon backup ring may be used to replace any other Teflon backup ring if it is of proper overall dimension to support the applicable O-ring. Backup rings are not color coded or otherwise marked and must be identified from package labels.
The inspection of backup rings should include a check to ensure that surfaces are free from irregularities, that the edges are clean-cut and sharp, and that scarf cuts are parallel. When checking Teflon spiral backup rings, make sure that the coils do not separate more than ¾ inch when unrestrained.
V-Ring Packings V-ring packings (AN6225) are one-way seals and are always installed with the open end of the "V" facing the pressure. V-ring packings must have a male and female adapter to hold them in the proper position after installation. It is also necessary to torque the seal retainer to the value specified by the manufacturer of the component being serviced, or the seal may not give satisfactory service. An installation using V-rings is shown in figure 6-57.
U-Ring Packings U-ring packings (AN6226) and U-cup packings are used in brake assemblies and brake master cylinders. The U-ring and U-cup will seal pressure in only one direction; therefore, the lip of the packings must face toward the pressure. U-ring packings are primarily low-pressure packings to be used with pressures of less than 1,000 p.s.i.
GASKETS Gaskets are used as static (stationary) seals between two flat surfaces. Some of the more 168
common gasket materials are asbestos, copper, cork, and rubber. Asbestos sheeting is used wherever a heat-resistant gasket is needed. It is used extensively for exhaust system gaskets. Most asbestos exhaust gaskets have a thin sheet of copper edging to prolong their life.
A solid copper washer is used for spark plug gaskets where it is essential to have a noncompressible yet semisoft gasket.
Cork gaskets can be used as an oil seal belween the engine crankcase and accessories, and where a gasket is required that is capable of occupying an uneven or varying space caused by a rough surface or expansion and contraction.
Rubber sheeting can be used where there is a need for a compressible gasket. It should not be used in any place where it may come in contact with gasoline or oil because the rubber will deteriorate very rapidly when exposed to these Gaskets are used in fluid systems around the end caps of actuating cylinders, valves, and other units. The gasket generally used for this purpose is in the shape of an O-ring, similar to O-ring WIPERS Wipers are used to clean and lubricate the exposed portions of piston shafts. They prevent dirt from entering the system and help protect the piston shaft against scoring.
Wipers may be either the metallic or the felt type. They are sometimes used together, with the felt wiper installed behind the metallic wiper.
SEALING COMPOUNDS Certain areas of all aircraft are sealed to withstand pressurization by air, to prevent leakage of fuel, to prevent passage of fumes, or to prevent corrosion by sealing against the weather. Most sealants consist of two or more ingredients properly proportioned and compounded to obtain the best results. Some materials are ready for use as packaged, but others will require mixing before application.
One-Part Sealants One-part sealants are prepared by the manufacturer and are ready for application as packaged.
However, the consistency of some of these compounds may be altered to satisfy a particular method of application. If thinning is desired, the thinner recommended by the sealant manufacturer should be used.
Two-Part Sealants Two-part sealants are compounds requiring separate packaging to prevent cure prior to application and are identified as the base sealing compound and the accelerator. Any alteration of the prescribed ratios will reduce the quality of the material. Generally two-part sealants are mixed by combining equal portions (by weight) of base compound and accelerator.
All sealant material should be carefully weighed in accordance with the sealant manufacturer's recommendations. Sealant material is usually weighed with a balance scale equipped with weights specially prepared for various quantities of sealant and accelerator.
Before weighing the sealant materials, both the base sealant compound and the accelerator should be thoroughly stirred. Accelerator which is dried out, lumpy, or flaky should not be used. Preweighed sealant kits do not require weighing of the sealant and accelerator before mixing when the entire quantity is to be mixed.
After the proper amount of base sealant compound and accelerator has been determined, add the accelerator to the base sealant compound.
Immediately after adding the accelerator, thoroughly mix the two parts by stirring or folding, depending on the consistency of the material. The material should be mixed carefully to prevent entrapment of air in the mixture. Too rapid or prolonged stirring must be avoided as it will build up heat in the mixture and will shorten the normal application time (working life) of the mixed sealant.
To ensure a well-mixed compound, it may be tested by smearing a small portion on a clean, flat metal or glass surface. If flecks or lumps are found, continue mixing. If the flecks or lumps cannot be eliminated, the batch should be rejected.
The working life of mixed sealant is from onehalf hour to 4 hours (depending upon the class of sealant); therefore, mixed sealant should be applied as soon as possible or placed in refrigerated storage. Figure 6-58 presents general information concerning various sealants.
The curing rate of mixed sealants varies with changes in temperature and humidity. Curing of sealants will be extremely, slow if the temperature 169
Storage (shelf) Accelerator Mixing ratio Application life after Storage (shelf) Temperature Application and Sealant Base (catalyst) by weight life (work) mixing life unmixed range limitations EC-801 (black) EC-807. 12 parts of EC- 5 days at -65°F. to Faying surfaces, MIL-S- -20°F. after fillet seals, and 7502A Class parts of EC- flash freeze packing gaps.
B-2. at -65° F.
EC-800 (red) None. Use as is. 8-12 hours. Not applicable. 6-9 months. -65° F. to Coating rivets.
200°F.
EC-612 P None. Use as is. Indefinite non- Not applicable. 6-9 months. - 40°F. to Packing voids (pink) MIL-P- drying. 200° F. up to 1/4 inch.
20628.
PR-1302HT PR-1302HT-A. 10 parts of PR- 2-4 hours. 5 days at 6 months. - 65°F. to Sealing access.
(red) MII.-S- 1302HT-A to -20°F. after 200° F. door gaskets.
8784. 100 parts of flash freeze PR-1302HT. at -65°F.
PR-727 potting PR-727A. 12 parts of PR- 1½ hours mini- 5 days at 6 months. -65°F. to Potting electrical compound 727A to 100 mum. -20° F. after 200° F. connections MIL-S- parts of PR- flash freeze and bulkhead 8516B. 727. at -65°F. seals.
HT-3 (greygreen) None. Use as is. Solvent release, Not applicable. 6 to 9 months. - 60°F. to Sealing hot air sets up in 2 850° F. ducts passing to 4 hours. through bulkheads.
EC-776 (clear None. Use as is. 8-12 hours. Not applicable. Indefinite in -65°F. to Top coating.
amber) MIL- airtight con- 250°F.
S-4383B. tainers.
FIGURE 6-58. General sealant information.
is below 60° F. A temperature of 77° F. with 50 percent relative humidity is the ideal condition for curing most sealants.
Curing may be accelerated by increasing the temperature, but the temperature should never be allowed to exceed 120° F. at any time in the curing cycle. Heat may be applied by using infrared lamps or heated air. If heated air is used, it must be properly filtered to remove moisture and Heat should not be applied to any faying surface sealant installation until all work is completed.
All faying surface applications must have all attachments, permanent or temporary, completed within the application limitations of the sealant.
Sealant must be cured to a tack-free condition before applying brush top coatings. (Tack-free consistency is the point at which a sheet of cellophane pressed onto the sealant will no longer
CORROSION CONTROL Metal corrosion is the deterioration of the metal by chemical or electrochemical attack and can take place internally as well as on the surface.
As in the rotting of wood, this deterioration may change the smooth surface, weaken the interior, or damage or loosen adjacent parts.
Water or water vapor containing salt combine with oxygen in the atmosphere to produce the main source of corrosion in aircraft. Aircraft operating in a marine environment or in areas where the atmosphere contains industrial fumes which are corrosive are particularly susceptible to corrosive attacks.
Corrosion can cause eventual structural failure if left unchecked. The appearance of the corrosion varies with the metal. On aluminum alloys and magnesium it appears as surface pitting and etching, often combined with a grey or white powdery deposit. On copper and copper alloys the corrosion forms a greenish film; on steel a reddish rust. When the grey, white, green, or reddish deposits are removed, each of the surfaces may appear etched and pitted, depending upon the length of exposure and severity of attack. If these surface pits are not too deep, they may not significantly alter the strength of the metal; however, the pits may become sites for crack development. Some types of corrosion can travel beneath surface coatings and can spread until the part fails.
Types of Corrosion There are two general classifications of corrosion which cover most of the specific forms. These are direct chemical attack and electrochemical attack.
In both types of corrosion the metal is converted into a metallic compound such as an oxide, hydroxide, or sulfate. The corrosion process always involves two simultaneous changes: The metal that is attacked or oxidized suffers what may be called anodic change, and the corrosive agent is reduced and may be considered as undergoing cathodic change.
Direct Chemical Attack Direct chemical attack, or pure chemical corrosion, is an attack resulting from a direct exposure of a bare surface to caustic liquid or gaseous agents. Unlike electrochemical attack where the anodic and cathodic changes may be taking place a measurable distance apart, the changes in direct chemical attack are occurring simultaneously at the same point. The most common agents causing direct chemical attack on aircraft are: (1) Spilled battery acid or fumes from batteries; (2) residual flux deposits resulting from inadequately cleaned, welded, brazed, or soldered joints; and (3) entrapped caustic cleaning solutions. Spilled battery acid is becoming less of a problem with the advent of aircraft using nickel-cadmium batteries which are usually closed units. The use of these closed units lessens the hazards of acid spillage and battery fumes.
Many types of fluxes used in brazing, soldering, and welding are corrosive and they chemically attack the metals or alloys with which they are used. Therefore, it is important that residual flux be removed from the metal surface immediately after the joining operation. Flux residues are hygroscopic in nature; that is, they are capable of absorbing moisture, and unless carefully removed, tend to cause severe pitting.
Caustic cleaning solutions in concentrated form should be kept tightly capped and as far from aircraft as possible. Some cleaning solutions used in corrosion removal are, in themselves, potentially corrosive agents, and particular attention should be directed toward their complete removal after use on aircraft. Where entrapment of the cleaning solution is likely to occur, a noncorrosive cleaning agent should be used even though it is less efficient.
Electrochemical Attack An electrochemical attack may be likened chemically to the electrolytic reaction which takes place in electroplating, anodizing, or in a drycell battery. The reaction in this corrosive attack requires a medium, usually water, which is capable of conducting a tiny current of electricity. When a metal comes in contact with a corrosive agent and is also connected by a liquid or gaseous path through which electrons may flow, corrosion begins as the metal decays by oxidation. During the attack, the quantity of corrosive agent is reduced and, if not renewed or removed, may completely react with the metal (become neutralized). Different areas of the same metal surface have varying levels of electrical potential and if connected by a conductor, such as salt water, will set up a series of corrosion cells and corrosion will All metals and alloys are electrically active and have a specific electrical potential in a given chemical environment. The constituents in an alloy also have specific electrical potentials which are generally different from each other. Exposure of the alloy surface to a conductive, corrosive medium causes the more active metal to become anodic and the less active metal to become cathodic, thereby establishing conditions for corrosion. These are called local cells. The greater the difference in electrical potential between the two metals, the greater will be the severity of a corrosive attack, if the proper conditions are allowed As can be seen, the conditions for these corrosion reactions are a conductive fluid and metals having a difference in potential. If, by regular cleaning and surface refinishing, the medium is removed and the minute electrical circuit eliminated, corrosion cannot occur; this is the basis for effective corrosion control.
The electrochemical attack is responsible for most forms of corrosion on aircraft structure and component parts.
FORMS OF CORROSION There are many forms of corrosion. The form of corrosion depends on the metal involved, its size and shape, its specific function, atmospheric conditions, and the corrosion-producing agents present. Those described in this section are the more common forms found on airframe structures.
Surface Corrosion Surface corrosion appears as a general roughening, etching, or pitting of the surface of a metal, frequently accompanied by a powdery deposit of corrosion products. Surface corrosion may be caused by either direct chemical or electrochemical attack. Sometimes corrosion will spread under the surface coating and cannot be recognized by either the roughening of the surface or the powdery deposit. Instead, the paint or plating will be lifted off the surface in small blisters which result from the pressure of the underlying accumulation of corrosion products.
Dissimilar Metal Corrosion Extensive pitting damage may result from contact between dissimilar metal parts in the presence of a conductor. While surface corrosion may or may not be taking place, a galvanic action, not unlike electroplating, occurs at the points or areas of contact where the insulation has broken down or been omitted. This electrochemical attack can be very serious because the action is, in many instances, taking place out of sight. and the only way to detect it prior to structural failure is by disassembly and inspection.
Intergranular Corrosion This type of corrosion is an attack along the grain boundaries of an alloy and commonly results from a lack of uniformity in the alloy structure.
Aluminum alloys and some stainless steels are particularly susceptible to this form of electrochemical attack. The lack of uniformity is caused by changes that occur in the alloy during heating and cooling. intergranular corrosion may exist without visible surface evidence. Very severe intergranular corrosion may sometimes cause the surface of a metal to "exfoliate." This is a lifting or flaking of the metal at the surface due to delamination of the grain boundaries caused by the pressure of corrosion residual product buildup.
This type of corrosion is difficult to detect in its original stage. Ultrasonic and eddy current inspection methods are being used with a great deal of success.
Stress Corrosion Stress corrosion occurs as the result of the combined effect of sustained tensile stresses and a corrosive environment. Stress corrosion cracking 172
is found in most metal systems; however, it is particularly characteristic of aluminum, copper, certain stainless steels, and high-strength alloy steels (over 240,000 p.s.i.). It usually occurs along lines of cold working and may be transgranular or intergranular in nature. Aluminum alloy bellcranks with pressed-in bushings, landing gear shock struts with pipe-thread type grease fittings, clevis pin joints, shrink fits, and overstressed tubing B-nuts are examples of parts which are susceptible to stress corrosion cracking.
Fretting Corrosion Fretting corrosion is a particularly damaging form of corrosive attack which occurs when two mating surfaces, normally at rest with respect to one another, are subject to slight relative motion.
It is characterized by pitting of the surfaces, and the generation of considerable quantities of finely divided debris. Since the restricted movements of the two surfaces prevent the debris from escaping very easily, an extremely localized abrasion occurs. The presence of water vapor greatly increases this type of deterioration. If the contact areas are small and sharp, deep grooves resembling called false brinelling.
FACTORS AFFECTING CORROSION Many factors affect the type, speed, cause, and seriousness of metal corrosion. Some of these factors can be controlled and some cannot.
Climate The environmental conditions under which an aircraft is maintained and operated greatly affect corrosion characteristics. In a predominately marine environment (with exposure to sea water and salt air), moisture-laden air is considerably more detrimental to an aircraft than it would be if all operations were conducted in a dry climate.
Temperature considerations are important because the speed of electrochemical attack is increased in a hot, moist climate.
Size and Type of Metal It is a well known fact that some metals will corrode faster than others. It is a less known fact that variations in size and shape of a metal can indirectly affect its corrosion resistance.
Poor heat Good heat treatment zone treatment zone Original surface Tapering exposes zone of poor heat treatment to corrosive environment FIGURE 6-59. The effect of machining thick heattreated wrought aluminum alloys.
Thick structural sections are more susceptible to corrosive attack than thin sections because variations in physical characteristics are greater.
When large pieces are machined or chemically milled after heat treatment, the thinner areas will have different physical characteristics than the thicker areas. (See figure 6-59.)
From a corrosion control standpoint, the best approach is to recognize the critical nature of the integrity and strength of major structural parts and to maintain permanent protection over such areas at all times to prevent the onset of deterioration.
Foreign Material Among the controllable factors which affect the onset and spread of corrosive attack is foreign material which adheres to the metal surfaces.
Such foreign material includes:
(1) Soil and atmospheric dust.
(2) Oil, grease, and engine exhaust residues.
(3) Salt water and salt moisture condensation.
(4) Spilled battery acids and caustic cleaning solutions.
(5) Welding and brazing flux residues.
It is important that aircraft be kept clean.
How often and to what extent an aircraft should be cleaned depends on several factors, such as location, model of aircraft, and type of operation.
PREVENTIVE MAINTENANCE Much has been done to improve the corrosion resistance of aircraft: improvement in materials, surface treatments, insulation, and protective finishes. All of these have been aimed at reducing 173
maintenance effort as well as improving reliability.
In spite of these improvements, corrosion and its control is a very real problem that requires continuous preventive maintenance.
Corrosion-preventive maintenance includes the following specific functions:
(I) An adequate cleaning.
(2) Thorough periodic lubrication.
(3) Detailed inspection for corrosion and failure of protective systems.
(4) Prompt treatment of corrosion and touchup of damaged paint areas.
(5) Keeping drainholes free of obstruction.
(6) Daily draining of fuel cell sumps.
(7) Daily wipe-down of exposed critical areas.
(8). Sealing of aircraft against water during foul weather and proper ventilation on warm, sunny days.
(9) Making maximum use of protective covers on parked aircraft.
After any period during which regular corrosion-preventive maintenance is interrupted, the amount of maintenance required to repair accumulated corrosion damage and bring the aircraft back up to standard will usually be quite high.
INSPECTION Inspection for corrosion is a continuing problem and should be handled on a daily basis. Overemphasizing a particular corrosion problem when it is discovered and then forgetting about corrosion until the next crisis is an unsafe, costly, and troublesome practice.
Most scheduled maintenance checklists are conplete enough to cover all parts of the aircraft or engine, and no part of the aircraft should go unchecked. Use these checklists as a general guide when an area is to be inspected for corrosion.
Through experience it will be learned that most aircraft have trouble areas where corrosion will set in despite routine inspection and maintenance.
In addition to routine maintenance inspections, amphibians or seaplanes should be checked daily and critical areas cleaned or treated, as necessary.
CORROSION-PRONE AREAS Discussed briefly in this section are most of the trouble areas common to all aircraft. However, this coverage is not necessarily complete and may be amplified and expanded to cover the special characteristics of the particular aircraft model involved by referring to the applicable maintenance manual.
Exhaust Trail Areas Both jet and reciprocating engine exhaust deposits are very corrosive and give particular trouble where gaps, seams, hinges, and fairings are located down the exhaust path and where deposits may be trapped and not reached by normal cleaning methods. Special attention should be paid to areas around rivet heads and in skin crevices. Fairings and access plates in the exhaust areas should be removed for inspection. Exhaust deposit buildup in remote areas such as the empennage surfaces should not be overlooked.
Buildup in these areas will be slower and sometimes completely absent, but it has become a problem on some currently operating aircraft.
Battery Compartments and Battery Vent Openings Despite improvements in protective paint finishes and in methods of sealing and venting, battery compartments continue to be corrosion problem areas. Fumes from overheated electrolyte are difficult to contain and will spread to adjacent cavities and cause a rapid, corrosive attack on all unprotected metal surfaces. Battery vent openings on the aircraft skin should be included in the battery compartment inspection and maintenance procedure. Regular cleaning and neutralization of acid deposits will minimize corrosion from this cause.
Bilge Areas These are natural sumps for waste hydraulic fluids, water, dirt, and odds and ends of debris.
Residual oil quite often masks small quantities of water which settle to the bottom and set up a hidden chemical cell. Seaplane and amphibian aircraft bilge areas are protected by small bags of potassium dichromate inhibitor suspended near the low point in each bilge compartment. These crystals dissolve in any waste water and tend to inhibit the attack on exposed metal surfaces.
Inspection procedures should include replacement of these bags when most of the chemical has been dissolved. Particular attention must be paid to areas located under galleys and lavatories and to human waste disposal openings on the aircraft exteriors. Human waste products and the 174