C 43.13-1A Page 115 972 Chapter 5. AIRCRAFT HARDWARE Section 1. IDENTIFICATION AND USE OF AIRCRAFT HARDWARE
27. BOLTS. Most bolts used in aircraft strucures are either general purpose AN bolts, or IAS (National Aircraft Standard) internal frenching or close-tolerance bolts. In certain ases, aircraft manufacturers make up special olts for a particular application and it is necesary to use them or their equivalent in relacement.
a. Identification. AN-type aircraft bolts can be dentified by the code markings on the bolteads. The markings generally denote the bolt nanufacturer, the material of which the bolt is nade, and whether the bolt is a standard AN- ype or a special purpose bolt. AN standard teel bolts are marked with either a raised ash or asterisk, corrosion-resistant steel is inlicated by a single raised dash, and AN alumium alloy bolts are marked with two raised ashes. The strength and dimensional details f AN bolts are specified on the Army/Navy Aeronautical Standard Drawings.
Special purpose bolts include the hightrength and low-strength types, close-tolerance ypes; such bolts are normally inspected y magnetic, fluorescent or equivalent inspecmethods, Typical markings include 'SPEC (usually highly heat treated), an ircraft manufacturer's part number stamped n the head, or plain heads (low strength).
Close-tolerance National Aircraft Standards (NAS) bolts are marked with either a raised r recessed triangle. The material markings or NAS bolts are the same as for AN bolts, xcept that they may be either raised or reessed. Bolts inspected magnetically (Magnalux) or by fluorescent means (Zyglo) are idenified by means of colored lacquer, or a head narking of a distinctive type. Figure 5.1 shows he typical coding used on aircraft boltheads.
b. Grip Length. In general, bolt-grip lengths should equal the material thickness. However, bolts of slightly greater grip length may be used provided washers are placed under the nut or the bolthead. In the case of plate nuts, add shims under the plate. For proper washers, refer to paragraph 231.
c. Locking or Safetying of Bolts. Lock or safety all bolts and/or nuts, except self-locking nuts.
Do not reuse cotter pins and safety wire.
d. Bolt Fit. Many boltholes, particularly those in primary connecting elements, have close tolerances. Generally, it is permissible to use the frst lettered drill size larger than the normal bolt diameter, except where the AN hexagon bolts are used in light-drive fit (reamed) applications and where NAS close-tolerance bolts or AN clevis bolts are used. Boltholes are to be normal to the surface involved to provide full bearing surface for the bolthead and nut, and not be oversized or elongated. In case of oversized or elongated holes in critical members, obtain advice from an FAA inspector or engineer, or the aircraft manufacturer before drilling or reaming the hole to take the next larger bolt, as usually, items such as edge distance, clearance, etc., must be considered.
e. Torques. The importance of correct applicacation can not be overemphasized. Undertorque can result in unnecessary wear of nuts and bolts as well as the parts they are holding together. When insufficient pressures are applied, uneven loads will be transmitted throughout the assembly which may result in excessive wear or premature failure due to fatigue. Overtorque can be equally damaging because of failure of a bolt or nut from overstressing the threaded areas. There are a few simple, but very important, procedures that should be followed to assure that correct torque is applied:
hap S Par 227
Page 116 AC 43.13- AIR E дО}> ~ M
•cIt
AN STANDARD AN STANDARD AN STANDARD AN STANDARD AN STANDARD
STEEL BOLT STEEL BOLT STEEL BOLT STEEL BOLT STEEL BOLT (CORROSION RESISTANT) SPEC
AN STANDARD AN STANDARD AN STANDARD AN STANDARD SPECIAL BOLT
STEEL BOLT STEEL BOLT STEEL BOLT STEEL BOLT 63-59131 SPECIAL BOLT DRILLED HEAD BOLT SPECIAL BOLT TOLERANCE NA S CLOSEOLT (2024) BOLT ALUMINUM ALLOY
MAGNETICALLY ORANGE- DYED CLEVIS BOLT REWORKED BOLT LOW STRENGTH INSPECTED MAGNETIC INS. MATERIAL BOLT FIGURE 5.1.-Bolt identification.
(1) Calibrate the torque wrench periodically to assure accuracy; and re-check frequently.
(2) Be sure that bolt and nut threads are clean and dry (unless otherwise specified by the manufacturer).
(8) Run nut down to near contact with the washer or bearing surface and check &friction drag torque" required to turn the nut.
(4) Add the friction drag torque to the desired torque recommended by the manufacturer, or obtain desired torque as shown in figure 5.2. This is referred to as final torque Par 227 which should register on the indicator or the setting for a snapover type wrench.
(b) Apply a smooth even pull when applying torque pressure. If chattering or a jerking motion occurs during final torque, back off and re-torque.
(6) When installing a castle nut, start alignment with the cotter pin hole at minimum recommended torque, plus friction drag torque, and do not exceed maximum plus friction drag.
If the hole and nut castellation do not align, change washers and try again. Exceeding the maximum recommended torque is not recommended.
Chap 5
- 43.13~1A 172
(7) If torque is applied to capscrews or theads, apply recommended torque plus fricon drag torque as determined in step (8).
(8) If special adapters are used which ill change the effective length of the torque rench, the final torque indication or wrench etting must be adjusted accordingly. Deterine the torque wrench indication or setting ith adapter installed as shown in figure 5.8.
igure 5.2 is a composite chart of recomended torque to be used when specific orque is not recommended by the manufacurer. The chart includes standard nut and bolt ombinations currently used in aviation main-
ex-head aircraft bolt is an all-purpose strucural bolt used for general applications involvug tension or shear loads. Alloy steel bolts maller than No. 10-32 and aluminum alloy olts smaller than 1/4-inch diameter are not to e used in primary structure. Do not use aluinum alloy bolts and nuts where they will be epeatedly removed for purposes of mainteance and inspection. Aluminum alloy nuts nay be used with cadmiumplated steel bolts oaded in shear on land airplanes, but are not o be used on seaplanes due to the possibility of lissimilar metals corrosion.
g. Close-Tolerance Bolts (AN-173 through AN- 86 (Hex-Head), NAS-80 through NAS-86 (100° countersunk)). Close-tolerance bolts are used in ugh-performance aircraft in applications here the bolted joint is subject to severe load eversals and vibration. The standard AN ex-head bolts may be used for the same appliations provided a light-drive fit is accomlished.
h. Internal Wrenching Bolts (MS-20004 through S-20024 or NAS-495). These bolts are suitable or use both in tension and shear applications.
n steel parts, countersink the bolthole to seat he large radius of the shank at the head or, as n aluminum alloys, use a special heat-treated vasher (NAS-143C) that fits the head to rovide adequate bearing area. A special heatreated plain washer (NAS-143) is used under he nut. Use special high-strength nuts on hap 5 Page 117 these bolts. (Refer to paragraph 280c(7).) Replace all internal wrenching bolts by another internal wrenching bolt. Standard AN hexhead bolts and washers cannot be substituted for them, as they do not have the required strength.
I. Drilled-Head Bolts (AN-73). The AN drilledhead bolt is similar to the standard hex-bolt, but has a deeper head which is drilled to receive wire for safetying. The AN-8 and the AN-78 series of bolts are interchangeable for all practical purposes from the standpoint of tension and shear strengths.
228. SCREWS. In general, screws differ from bolts by the following characteristics: Usually lower material strength, a looser thread fit (No. 2), head shapes formed to engage a screwdriver, and the shank threaded along its entire length without a clearly defined grip.
However, several types of structural screws are available that differ from the standard structural bolts only in the type of head.
The material is equivalent and a definite grip is provided. The AN-525 washerhead screws, the AN 509-100° countersunk structural screws, and the NAS-204 through NAS-235 are such parts. The material markings are the same as those used on AN standard bolts.
a. Structural Screws (NAS-204 through NAS- 235, AN-509 and AN-525). This type of screw, when made of alloy steel such as SAB-4180, ND-8680, or equivalent, and heat-treated from 125,000 p.s.i., may be used for structural assembly in shear applications similar to structural bolts.
b. Self-Tapping Screws. The AN-504 and AN- 506 screws are used for attaching minor removable parts such as nameplates and the like.
AN-580 and AN-581 are used in blind applications for the temporary attachment of sheet metal for riveting and the permanent assembly of nonstructural assemblies. AN-585 is a plain head self-tapping screw used in the attachment of nameplates or in sealing drainholes in corrosion-proofing tubular structures, and is not intended to be removed after installation. Never use self-tapping screws to replace standard Par 227
Page 118 AC 43.1 QuI Ble sion BOLT cel Tonsk tamina AN 3 thru AN 20 MB 20004 thru MB 20024 AN 3DD thru AN 20DD AN 42 thru AN 49 NAB 144 thru NAB 158 AN 173DD thru AN 186DD AN 78 thru AN 81 NAS 333 thru NAB 340 AN 509DD AN 173 thru AN 186 NAS 583 thru NAB 590 AN 525D M8 20033 thru M8 20046 NA8 624 thru NA8 644 MB 27039D MS 20073 NAB 1803 thru NAB 1320 M8 24694DD M8 20074 NAS 172 AN 500 NK9 NAS 174 M8 24694 NAS 517 AN 523 NK525 Steel shear boll M8 27030 NAH 106
NUTS NUTE NUTE Bleel Tension Steel Shear Steel Tenalon Steel Shear Aluminam Tension Aluminum Shear AN 310 AN 820 AN 810 AN 320 AN 365D AN 320D AN 315 AN 364 AN 315 AN 364 AN 310D AN 364D AN 363 NAS 1022 AN 383 NAS 1022 NA8 1021D NAS 1022D AN 365 MB 17826 AN 365 M8 17826 NAS 1021 MS 20384 M8 17825 M8 20364 MS 17825 M8 20365 MB 21045 M8 21045 MS 20365 NAS 1021 MB 20500 NA8 679 NAS 079 NAS 1291
FINE THREAD SERIES lorque Limit Torque Limit Torque Limita Torque Limits Forgus Limit Nat-bolt n.-lb in.-lbe fn.-Ibe in.-ibs sizo Min. Max. MiD. Mas. Min. Max. Min. MaI. Min. Max. Min. MAI.
8 -36 9 10-32 25 30 15 20 10 6-28 80 100 50 60 80 120 145 70 40 36-21 160 200 250 150 70 3r20 520 630 400 1-20 690 770 950 550 Kr-18 1, 000 1, 100 1, 300 800 % -18 1, 300 1, 250 1, 550 050 % -16 500 3, 650 3, 200 1, 900 76-14 3, 000 8, 550 4, 350 2, 600 1 -14 4, 500 4, 500 6, 500 3, 300 1% -12 7, 000 6, 000 7, 300 4, 400 1% -12 11, 000 11, 000 13, 400 8, 000
COARSE THREAD SERIES 8 -82 12 15 7 10 -24 20 12 15 * -20 40 25 80 ·-18 80 48 55 % -18 160 185 95 110 Kr14 285 255 140 155 ½ - 13 400 480 240 290 Kг12 500 700 300 420 % -11 700 900 420 540 4-10 1, 150 1, 600 700 950 36 - 9 2, 200 3, 000 1, 300 1, 800 1 - 8 8, 700 5, 000 2, 200 3, 000 1% - 8 5, 500 6, 500 8, 300 4,000 1¼ - 8 6, 500 8, 000 4, 000 5, 000 FIGURE 5.2.-Recommended torque values for nut-bolt combinations-- (without lubrication).
Par 228 Chay
43.13-1A Page 119
NOTE - LEGEND-
WHEN USING A TORQUE WRENCH ADAPTER WHICH T - ACTUAL (DESIREDI TORQUE CHANGES THE DISTANCE FROM THE TORQUE WRENCH Y= APPARENT (INDICATED) TORQUE DRIVE TO THE ADAPTER DRIVE, APPLY THE L. - EFFECTIVE LENOTH LEVER FOLLOWING FORMULAS TO OBTAIN THE CORRECTED = ENFECTIVE LENGTH OF EXTENSION TORQUE READING:
Hort Open End Open-End Wrench Adapter CENTERLINE ADAPTER DRIVE WRENCH TORQUE WRENCH DRIVE TORQUE WRENCH HANDGRIP CENTERLINE Adapter
WRENCH CENTERLINE ADAPTER (PREDETE MINED) DRIVE
CENTERLINE CENTERINE Set Screw Flare Nut Wrench Adapter Adapter EXAMPLE: (WITH "E" AS PLUS DIMENSIONI EXAMPLE: (ATTH "E" AS HINUS DUHENSION) T - 135 LE. IN. 8= 10715-18-11790 T - 135 LD. 04. r- 123-18 - 125-150.82 Hose Clamp Y - UNKNOWN Y - UNKNOWN Spanner Wrench Adapter L - 10.0 IN. 1 - 10.0 MN. Adapter € = 1.5 IN. Y - 117 LB. AN. 1 - 1.5 HN. Y - 159 18. 04.
FIGURE 5.3.-Torque wrench with various adapters.
crews, nuts, bolts, or rivets in the original pructure.
129. TAPER PINS (AN-385 AND AN-386), Plain und threaded taper pins are used in joints which carry shear loads and where absence of ›lay is essential. The plain taper pin is drilled and usually safetied with wire. The threaded aper pin is used with a taper-pin washer (AN-975) and shear nut (safetied with cotter ›in) or self-locking nut.
a. The Flathead Pin (MS-20392), Commonly 'alled a clevis pin, the flathead pin is used in conjunction with tie rod terminals and in sec- ›ndary controls which are not subject to coninuous operation. The pin is customarily intalled with the head up so that if the cotter ›in fails or works out, the pin will remain in lace.
b. The AN-380 Cotter Pin. This is used for safetying bolts, screws, nuts, other pins, and in arious applications where such safetying is recessary. Use AN-381 cotter pins in locations where nonmagnetic material is required or in ocations where resistance to corrosion is deired.
hap 3
230. NUTS.
a. Self-Locking Nuts. Self-locking nuts are acceptable for use on certificated aircraft subject to the restrictions on the pertinent manufacturer's recommended practice sheets. Selflocking nuts are used on aircraft to provide tight connections which will not shake loose under severe vibration. Two types of selflocking nuts are currently in use, the all-metal type and the fiber or nylon lock type. Do not use self-locking nuts at joints which subject either the nut or bolt to rotation. They may be used with antifriction bearings and control pulleys, provided the inner race of the bearing is clamped to the supporting structure by the nut and bolt. Attach nuts to the structure in a positive manner to eliminate rotation or misalignment when tightening the bolts or screws.
(1) All-metal locknuts are constructed with either the threads in the locking insert out-ofphase with the load-carrying section, or with a saw-cut insert with a pinched-in thread in the locking section. The locking action of the allmetal nut depends upon the resiliency of the metal when the locking section and loadcarrying section are engaged by screw threads.
Par 228
(2) Fiber or nylon locknuts are constructed with an unthreaded fiber-locking insert held securely in place. The fiber or nylon has a smaller diameter than the nut, and when a bolt or screw is entered, it taps into the insert, producing a locking action. After the nut has been tightened, make sure the rounded or chamfered end bolts, studs, or screws extend at least the full round or chamfer through the nut. Flat end bolts, studs, or screws should extend at least 1/32 inch through the nut. When fiber-type self-locking nuts are reused, check the fiber carefully to make sure it has not lost its locking friction or become brittle. Do not reuse locknuts if they can be run up fingertight. Bolts 5/16-inch diameter and over with cotter pinholes may be used with self-locking nuts but only if free from burrs around the holes. Bolts with damaged threads and rough ends are not acceptable. Do not tap the fiberlocking insert.
(3) Self-locking nut bases are made in a number of forms and materials for riveting and welding to aircraft structure or parts. Certain applications require the installation of self-locking nuts in channels, an arrangement which permits the attachment of many nuts with only a few rivets. These channels are track-like bases with regularly spaced nuts which are either removable or nonremovable.
The removable type carries a floating nut, which can be snapped in or out of the channel, thus making possible the ready removal of damaged nuts. Nuts such as the clinch-type and spline-type which depend on friction for their anchorage are not acceptable for use in aircraft structures.
(4) Self-locking nuts may be used on aircraft engines and accessories when their use is specified by the engine manufacturer in his bulletins or manuals.
b. Aircraft Castle Nut (AN-310). The castle nut is used with drilled-shank AN hex-head bolts, clevis bolts, eye bolts, drilled-head bolts or studs, and is designed to accommodate a cotter pin or lockwire as a means of safetying.
c. Miscellaneous Aircraft Nuts.
(1) The plain nut (AN-315 and AN-385) has limited use on aircraft structures and re- Par 230 AC 43.13 quires an auxiliary locking device such as checknut or lockwasher.
(2) Light hex-nuts (AN-840 and AN-345 are used in miscellaneous applications an must be locked by an auxiliary device.
(3) The checknut (AN-316) is used as locking device for plain nuts, screws, threade rod ends, and other devices.
(4) The castellated shear nut (AN-320) i designed for use with clevis bolts and threade taper pins, which are normally subjected t shearing stress only.
(5) Wing nuts (AN-350) are intended fo we were the desiren i battes is onditiont obtained by the use of the fingers or han tools.
(6) Sheet spring nuts, such as speed nuts are used with standard and sheet metal self tapping screws in nonstructural locations They find various uses in supporting lin clamps, conduit clamps, electrical equipment access doors, and the like, and are available i several types.
(7) Two commercial types of high-strengt internal or external wrenching nuts are availa ble, the internal and external wrenching elastic stop nut and the Unbrako internal and externa wrenching nut. Both are of the self-lockin type, are heat-treated, and are capable of carry ing the high-strength bolt-tension load.
231. WASHERS. The types of washers used i aircraft structure are: plain washers, lock washers, and special washers.
a. Plain washers (AN-960 and AN-970) ar widely used under hex nuts to provide smoothbearing surface, to act as a shim and t adjust holes in bolts. Use plain washers unde lockwashers to prevent damage to surfaces Cadmiumplated steel washers are recom mended for use under boltheads or nuts on alu minum alloy or magnesium structures wher corrosion, if it occurs, will then be between the washer and the steel. The AN-970 stee washer provides a greater bearing area than th plain type, and is used in wooden structure:
under both boltheads and nuts to prevent loca crushing of the surface.
Chap !
|43.13-1A
b. Lock washers (AN-935 and AN-936) may be ed with machine screws or bolts whenever e self-locking or castellated type of nut is t applicable. They are not to be used as fasnings to primary or secondary structures, or here subject to frequent removal or corrosive nditions.
c. Ball-socket and seat-washers (AN-950 and V-955) are used in special applications where e bolt is installed at an angle to the surface, ' when perfect alinement with the surface is quired at all times. These washers are used gether.
d. Taper-pin washers (AN-975) are used with le threaded taper pin.
e. NAS-143 washers are used with NAS inernal wrenching bolts and internal wrenching uts. Type "C" is countersunk to seat the boltead shank radius and a plain-type washer is sed under the nut. Both of these washers are eat treated from 125,000 to 145,000 p.s.i.
32. TYPES OF RIVETS.
a. Standard solid-shank rivets and the univeral head rivets (AN-470) are used in aircraft onstruction in both interior and exterior locaions.
b. Roundhead rivets (AN-430) are used in the nterior of aircraft except where clearance is equired for adjacent members.
c. Flathead rivets (AN-442) are used in the inerior of the aircraft where interference of adacent members does not permit the use of 'oundhead rivets.
d. Brazierhead rivets (AN-455 and AN-456) tre used on the exterior surfaces of aircraft where flush riveting is not essential.
e. All protruding head rivets may be replaced y MS-20470 (supersedes AN-470) rivets.
This has been adopted as the standard for proruding head rivets in this country.
f. Countersunk head rivets MS-20426 (supersedes AN-426 100° ) are used on the exterior surfaces of aircraft to provide a smooth aerodynamic surface, and in other applications where a smooth finish is desired. The 100° Chap S Page 121 countersunk head has been adopted as the standard in this country.
233, MATERIAL APPLICATIONS.
a, 2117-13 is the most commonly used rivet material utilized in aluminum alloy structures.
Its main advantage lies in the fact that it may be used in the condition received without further treatment.
b. The 2017-T3, 2017-731, and 2024-74 rivets are used in aluminum alloy structures where strength higher than that of the 2117-T8 rivet is needed. See Handbook MIL-HDBK-5 for differences between the two types of 17ST rivets specified here.
c. The 1100 rivets of pure aluminum are used for riveting nonstructural parts fabricated from the softer aluminum alloys, such as 1100, 3003, and 5052.
d. When riveting magnesium alloy structures, 5056 rivets are used exclusively due to their corrosion-resistant qualities in combination with the magnesium alloys.
•. Mild steel rivets are used primarily in riveting steel parts. Do not use galvanized rivets on steel parts subjected to high heat.
f. Corrosion-resistant steel rivets are used primarily in riveting corrosion-resistant steel parts such as firewalls, exhaust stack bracket attachments, and similar structures.
g. Monel rivets are used in special cases for riveting high-nickel steel alloys and nickel alloys. They may be used interchangeably with stainless steel rivets as they are more easily driven. However, it is preferable to use stainless steel rivets in stainless steel parts.
h. Copper rivets are used for riveting copper alloys, leather, and other nonmetallic materials. This rivet has only limited usage in aircraft.
I. Hi-shear rivets are sometimes used in connections where the shearing loads are the primary design consideration. Its use is restricted to such connections. It should be noted that hishear rivet patterns are not to be used for the installation of control surface hinges and hinge Par 231
brackets. Do not paint the rivets prior to assembly, even where dissimilar metals are being joined. However, it is advisable to touch up each end of the driven rivet with zinc chromate primer to allow the later application of the general airplane finish.
i. Blind rivets in the MS-20600 through MS- 20603 series rivets and the mechanically-locked stem NAS 1898, 1399, 1738, and 1739 rivets may be substituted for solid rivets in accordance with the blind rivet manufacturer's recommendations. They should not be used where the looseness or failure of a few rivets will impair the airworthiness of the aircraft. Design allowables for blind rivets are specified in MIL- HDBK-5, Metallic Materials and Elements for Flight Vehicle Structures. Specific structural applications are outlined in MS-33522. Non- Par 233 AC 43.13- structural applications for such blind rivets MS-20604 and MS-20605 are contained in MS 33557.
234. FASTENERS (COWL AND FAIRING). A nun ber of patented fasteners are in use on aircraf A variety of these fasteners are commel cially available and the manufacturer's recon mendations concerning the proper use of thes types of fasteners should always be considere in other than replacement application.
235. UNCONVENTIONAL ATTACHMENTS. Do no use unconventional or new attachment device in the primary structure unless approved by representative of the Federal Aviation Admin istration.
236.-246. RESERVED.
Chap s
43.13-1A Page 133 Chapter 6. CORROSION PROTECTION
7. TYPES OF CORROSION. Almost all metals ed in aircraft are subject to corrosion. The tack may take place over an entire metal surce, or it may be penetrating in nature, formg deep pits. It may follow grain boundaries its attack on metallic surfaces or it may penrate a surface at random. It may be accenated by stresses from external loads or existg in the metallic structure from lack of mogeneity or improper heat treatment. It is romoted by contact of the metals with marials that absorb water, such as wood, sponge abber, felt, dirt, surface film, etc. Corrosion is ferred to as the following types:
a. Direct Surface Atfack, The most common pe of general surface corrosion results from rect reaction of metal surface with oxygen in e air. Unless properly protected, steel will ist and aluminum and magnesium will form rrosion products. The attack may be accelerted by salt spray or salt-bearing air, by inustrial gases, or by the aircraft engine exaust gases.
b. Dissimilar Metals Corrosion. When two dismilar metals are in contact and are connected y an electrolyte (continuous liquid or gas ath-salt spray, exhaust gas, condensate) acelerated corrosion of one of the metals may ccur. The most easily oxidized surface beomes the anode and corrodes. The less active ember of the couple becomes the cathode of he galvanic cell. The degree of attack depends n the relative activity of the two surfaces; the reater the difference in activity, the more seere the attack. The materials listed in Group I re quite active and corrode easily. They reuire maximum protection. Group IV materials re the least active and therefore require minium protection. Except as noted below, whenver metals from two different groups are in ontact with each other, special protection is equired to assure that dissimilar metal corrosion does not occur. Although aluminum alloys and tin are in different groups than magnesium, tin and the 5000 and 6000 series aluminum alloys may each be used in contact with magnesium without such protection. Tin may also be used with all aluminum alloys without special protection.
(1) Group I. Magnesium and its alloys.
(2) Group II. All aluminum alloys, cadmium, zine.
(a) Subgroup A. 1100, 3003, 5052, 6061, 220, 855, 856. All clad alloys.
(b) Subgroup B. 2014, 2017, 2023, 7075,
195. Under severe corrosive conditions, the subgroups should be considered as dissimilar metals insofar as corrosion protection is concerned.
This is particularly true when a large area of an alloy of Subgroup B is in contact with a small area of Subgroup A. Severe corrosion of the alloy from Subgroup A may be expected.
(3) Group III. Iron, lead, and tin and their alloys (except stainless steels).
(4) Group IV. Stainless steels, titanium, chromium, nickel, and copper and their alloys, graphite (including dry film lubricants containing graphite).
c. Pitting. While pitting may occur in any metal, it is particularly characteristic of passive materials such as the alloys of aluminum, nickel, and chromium. It is usually a localized breakdown of protection and may be due to a lack of homogeneity in the alloy surface, either from mechanical working or faulty heat treatment. It may also be due to an inclusion or rough spot in the metal surface or from localized contamination that breaks down the surface protection. Pitting takes place at random with no selective attack along grain boundaries. Isolated areas become anodic to the rest of the surface. Corrosion products formed accentuate the anodic characteristics in the pit hap 6
area, and deep penetrating attack develops rather than a general surface attack.
d. Intergranular Corrosion. Selective attack along the grain boundaries of metal alloys is referred to as intergranular corrosion. It results from lack of uniformity in the alloy structure. It ia particularly characteristic of precipitation hardened alloys of aluminum and some stainless steels. Aluminum alloys 2024 and 7075 which contain appreciable amounts of copper and zinc respectively are highly vulnerable to this type of attack if not quenched rapidly during heat treatment or given other special treatment such as the T78 temper condition for the 7075 alloys. Aluminum extrusions and forgings in general may contain nonuniform areas, which in turn may result in galvanic attack along the grain boundaries.
This type of corrosion is difficult to detect in its original stage although ultrasonic and eddy current inspection methods are being used.
When attack is well advanced, the metal may blister or delaminate. This is referred to as "exfoliation."
e. Stress Corrosion. This results from the combined effect of static tensile stresses applied to a surface over a period of time under corrosive conditions. In general, cracking susceptibility increases with stress, particularly at stresses approaching the yield point, and with increasing temperature, exposure time, and concentration oi corrosive ingredients in the surrounding environment. Aluminum alloy bellcranks employing pressed-in taper pins, landing gear shock struts with pipe thread-type grease fittings, clevis joints, and shrink fits are examples of parts which are susceptible to stress corrosion cracking.
f. Corrosion Fatigue. Corrosion fatigue is a type of stress corrosion resulting from cyclic stresses on a metal in corrosive surroundings.
Corrosion may start at the bottom of a shallow pit in the stressed area. Once attack begins, the continuous flexing prevents the repair of protective surface coating or oxide films and additional corrosion takes place in the area of stress. It is difficult to detect this type of attack in advance except as cracking develops.
Par 247 AC 43.1
g. Fretting, Fretting corrosion is a lim type of attack that develops when relative tion of small amplitude takes place betw close fitting components. The rubbing con destroys any protective film that may be p ent on the metallic surface and addition removes small particles of virgin metal f the surface. These particles act as an abra:
and prevent the formation of any protec oxide film and exposes fresh active metal to atmosphere. If the contact areas are small.
sharp, deep grooves resembling brinnell me ings or pressure indentations may be worr the rubbing surface. As a result, this type corrosion has also been called false brinnell when developed on bearing surfaces.
248. CORROSION PROTECTION MEASURES I BASIC MATERIALS. In the repair or alteratior aircraft, apply corrosion proofing of the st type or equivalent to that originally appl unless the repair or alteration would resul increased susceptibility to corrosion, in wh case use additional corrosion protection me ures. Aluminum and magnesium alloys n be protected by a variety of surface tre ments. Most aluminum alloy structural surfa have been electrically anodized in chromicfuric acid tanks before fabrication. Magnesi sheet has received cold nitric-chromic a treatment; magnesium castings have usua received hot tank processing in fluoride dich mate mixtures. Steel surfaces may have b oxidized or plated during manufacture.
a. Anodizing and Related Processes. In a dizing, aluminum alloys are placed in an el trolytic bath causing a thin film of alumin oxide to form on the surface of the alumin:
This is resistant to corrosion and affords a g‹ paint base. Other processes which do not p vide as good a corrosive protection as anodiz are, however, good paint bases. These p cesses are:
(1) Alkaline cleaning followed by chror acid dip.
(2) Alcoholic phosphoric acid cleaner.
(3) Alkaline dichromate treatment.
b. Plating. Steels are commonly plated w other metale to prevent corrosion. Plating Chal
3.13-1A mplished by placing the article in an elecytic bath and metal from the plating solu- I is deposited on it. Various metals used in ting vary in the corrosion protection they ord steel. For instance, cadmium and zinc rode before the steel; hence, slight breaks cracks through the plating of these metals I not result in rusting of the exposed steel, ce the surface metal is corroded and prots the steel. Chromium does not protect steel this method, as steel will corrode before the omium and thus depends on the tightness of plating for its protection. The process of tplate bake treatment to relieve hydrogen brittlement is a necessary part of replating ›cedures for high-strength steel parts.
gh-strength nuts and bolts are highly sus- ›tible to failure from hydrogen embrittlent, at the many stress risers and notches in ir design, through normal usage of susned and dynamic stresses. Because of the poitial failures of embrittled parts, careful trol over the heat treatment, grinding, preite cleaning, plating, and postplate baking of rh-strength parts is necessary.
c. Sherardizing. Steel parts may be sherardd by heating them in an atmosphere of zine wder; however, this is not considered to be effective against corrosion as zinc or cadum plating.
d. Phosphate Rust-Proofing, This process is mmercially known as Parkerizing, Bonderiz- 3, Granodizing, etc. The coating placed on e part is used to protect steel parts after chining and before painting.
e. Chrome-Pickle Treatment. Magnesium parts lich have been immersed or brushed with a lution of nitric acid and sodium dichromate Il be protected for temporary storage. The ating also will serve as a bond for subseent organic finishes. Sealed chrome-pickle eatment is used on magnesium parts for long rm protection. Diluted chromic acid is a uch-up treatment. It is less critical to apply d can be applied over previously applied thin romate films.
f. Dichromate Treatment. The dichromate treatent consists of boiling magnesium parts in a ap 6 Page 125 solution of sodium dichromate. This treatment provides good paint base and protective qualities on all standard wrought magnesium alloys except the magnesium-thorium alloys HK 31A, HM 21A, and HM 31A. No coating forms on these alloys. Acid pickling of the magnesium surface prior to application of the dichromate treatment is required if maximum corrosion resistance of the finish is expected.
g. Stannate Immersion Treatment. This treatment deposits a layer of tin. It is a protective paint base for magnesium alloy parts which contain inserts and fasteners of a dissimilar metal such as brass, copper, or steel. This treatment cannot be used with parts containing aluminum inserts or fasteners because the high alkalinity of the bath attacks the aluminum.
h. Galvanic Anodizing Treatment. This is an electrolytic process used to provide a paint base and corrosion preventive film on magnesium alloys containing manganese.
I. Cladding, Aluminum alloys which are susceptible to corrosion are frequently clad with pure aluminum. Slight pits, scratches, or other detects through the cladding material will not result in corrosion of the core, since the pure aluminum on the edges of the defect will be preferentially corroded, protecting the core.
I. Metal Spraying. Metal is melted and sprayed on the surface to be protected. The surface must be properly prepared and thoroughly cleaned to prevent peeling of the sprayed coat.
k. Shot-Peening. Shot-peening and other treatments by which the surface can be placed in compression are effective in preventing stress corrosion.
1. Organic Coatings. Zinc chromate primer, enamels, chlorinated rubber compounds, etc., are organic coatings commonly used to protect metals.
m. Dopeproofing. When doped fabrics are applied over an organic finished metal structure, the dope will have a tendency to loosen the finish on the metal. For this reason, organic coatings on the metal are usually covered Par 248
with a dopeproof paint, with metal foil, or with cellulose tape to prevent the dope from striking through.
n. Tube Interiors. Protect the interiors of structural steel and aluminum tubing against corrosion. A small amount of water entrapped in a tube can corrode entirely through the tube thickness in a short period of time. Coat the tube interior by flushing with hot linseed oil, paralketone, or other corrosion inhibitor.
The flushing liquid is usually introduced through small holes drilled in the tubing.
Allow the flushing liquid to drain and plug the holes with a screw or by other means to prevent entry of moisture. Air and watertight sealing of the tubing will also give adequate protection against corrosion if the tubing is internally dry before being sealed.
249. CONSTRUCTION OF CORROSION RESISTANT STRUCTURE. During fabrication, the use of such practices as listed below will reduce the sources of corrosion initiation.
o. Use combinations of metals which are as close together as possible in the galvanic series -do not use small aluminum clips on large stainless steel webs; use stainless steel fasteners in stainless steel assemblies.
b. Insulate dissimilar metals but do not use materials which absorb moisture or conduct it in "wick" fashion.
c. Paint cut edges.
d. Seal edges of faying surfaces on butt joints.
e. Provide adequate drainholes.
250. CORROSION REMOVAL. When corrosion is detected, remove it as quickly as possible and protect the surface from further corrosion. In those cases where the corrosion has progressed extensively, the strength of the part may be in jeopardy and the part may need to be replaced.
a. For steel parts, except for highly stressed steel or stainless steel surfaces, the use of abrasive papers, buffers, hand wire brushing, and steel wool are acceptable cleanup procedures. However, it should be recognized that it Par 248 AC 43 18 practically impossible to remove a.. c sion products from the bottom of small and crevices. As a result, a part once rusty ally corrodes again, more easily than it di first time. Removing corrosion products highly stressed steel parta requires ca handling.
b. For aluminum parts, treatment include mechanical removal of as much of the c sion products as practicable, the inhibitic residual material by chemical means, folk by the restoration of permanent surface ings. Do not use steel wool, emery cloth, (except stainless steel) wire brushes, or st abrasive materials because particles of steel wool or emery cloth will become en ded in the soft material and cause corro Hand polishing corroded areas with house abrasives or with metal polish available u Specification MIL-P-6888 is acceptable fo:
on clad aluminum but must not be used or odized aluminum since it is severe enoug actually remove the protective anodized If the surface is particularly difficult clean. Type II material under Specifica MIL-C-5410 mixed 50-50 percent with sol or mineral spirits may be used. Treat any perficial corrosion with a 10 percent solutic sodium dichromate to which one percen chromium trioxide has been added or material available under Specification N C-5541. Allow these solutions to remair the corroded area for 5 to 20 minutes then wipe the surface dry. A more se cleaning procedure consists of using a 10 cent solution of chromic acid to which has added approximately 20 drops of battery trolyte per gallon. Thorough brushing wi stiff fiber brush should loosen or remove !
existing corrosion and assure complete I tration of the inhibitor into crevices and Allow the chromic acid to remain in place at least 5 minutes, then remove the exces.
flushing with water or wiping with a cloth. Apply a protective coating the same that the corrosion treatment is accomplis
c. When an anodized surface coating is d aged, it can only be partially restored by chi cal surface treatment. Therefore, exercise CH