SAE ARP 4118-1994 Isolation and Corrosion Protection of Dissimilar Materials.pdf

1、SAE ARP+4LL8 74 m 7943725 0523768 740 m ma Advancing Mobili -Land Sea Air and Space INTERNATIONAL RECOMMENDED TheEngl martensitic stainless steel Not compatible Data sumnary compiled by National Aerospace Standards Cannittee of the Aerospace Industries Association (AIA) of America, Inc. 3.2.2 Seal i

2、ng Procedures for Bolted Repairs: 3.2.2.1 Sealant Materials: The sealing procedures outlined below are mandatory for all bolted metal patch repairs of composi te parts. lhe effectiveness of these sealing procedures has been demonstrated by extended sea-coast exposure (Reference 2.1.3.3). lhe recomme

3、nded sealant materials for bolted repairs of carbon composites are the same as those used in metal-metal joints. For normal temperature applications (-65 to t325 OF) (-54 to +163 OC) polysulfide sealants are available in a wide variety of types. These include systems specifically formulated for fayi

4、ng surface sealing and wet fastener installation. Other types are specifically formulated for fillet and aerodynamic sealing, fuel tank sealing, low density, low adhesion sealing for removable assemblies, and other special conditions. Within each of these categories, there are long pot life, fast cu

5、re, low viscosity (spray or brush application), and thixotropic (spatula or extrusion gun application) versions. A list of sealants identified by specification and usage is shown in Table 2. Most of these specifications have several types, grades, and classes which identify the application character

6、istics discussed above. It should be noted that polysulfide sealants vary somewhat in their maximum use temperature, with maximum recommended temperatures varying from 180 OF (82 “C) (continuous) to 360 OF (182 “C) (peak). Some sealants have been formulated to withstand typical adhesive bond cycles

7、of 1 to 2 h at 250 to 350 OF (121 to 177 OC). For temperatures in excess of 200 OF (93 OC), care should be taken to select a system capable of operating under the required environmental conditions. For applications requiring temperature extremes (-80 to over 400 OF), (-62 to over 204 OC), sil icone

8、seal ants are the appropriate choice. -5- SAE ARP*:4LL8 94 m 7943725 0523973 008 m SAE ARP4 1 18 TABLE 2 - Faying Surface Sealant Categorles Material Type Use Specification Polysulfide General purpose, corrosion inhibiting sealant MIL-S-81733 Fuel tank sealant (also suited for general purpose, MIL-S

9、-8802 nonfuel areas) General purpose, low adhesion sealant MIL-S-8784 Low adhesion sealant for fuel tank areas AHS 3267 High temperature sealant (to 360 “F (182 c) peak) MIL-S-83430 Silicone High temperature sealing applications AUS 3373 (-80 to +400 9) (-62 to +204 “c) I Some of these specification

10、s have several types, grades, and classes. A careful revlew of these categories and the recunnended use for each must be made prior to sealant selection. Two-part silicone sealant, with catalytic curing agents, must be used. One-part silicones require moisture (from exposure to air) for cure, and th

11、us cannot be used in a faylng surface greater than 1 in (25 m) width. 3.2.2.1 (Continued): Table 3 summarizes some of the sealant types covered by military and industry specifications and the ranges of service temperatures, application methods, application times, and cure times that are available. T

12、he applicable specifications and/or suppliers should be consulted for specific selections . The polysulfide sealants 1 isted in Tables 2 and 3 are generally usable either as faying surface sealants or fillet/brush sealants. There are some instances where the curing system is anaerobic and cure will

13、take place only in the absence of air. These sealants would be suited only for faying surface sealing and wet fastener installation. One major precaution for use of sealants in faying surfaces and fastener installation is that some types, especially low viscosity sprayable and brushable sealants, ha

14、ve significant amounts of solvents. Sufficient time after application (typically 15 min) must be allowed for the solvent to flash off before assembling the joint or installing the fasteners. 3.2.2.2 Faying Surface Seal ing Procedures for Bolted Repairs: These procedures are identical to those used i

15、n the bolted assembly of metal components and consist of application of the selected sealant to one or both faying surfaces by brush, spray, or spatula depending on the sealant type and viscosity. As mentioned previously, most sealants come in varying types and grades suited for each application: i

16、.e., (1) minimum viscosity, sprayable; (2) low viscosity, brushable; or (3) thixotropic, spatula or extrusion. Sealants applied by extrusion gun are typically supplied in the form of frozen cartridges (or cartridges containing both sealant and catalyst separated by a breakable membrane) which can be

17、 used in the extrusion gun. The sealant may be applied to both faying surfaces, but -6- SAE ARPx4LLB 94 m 7943725 0523974 T44 m SAE ARP4 1 18 TABLE 3 - Characteristics of Selected Sealant Materials Continuous Servi ce Application Time Time at 77 “F (25 “c) Temperature or Assembly Time to Tack-Free S

18、pecification ”F (“cl Application Method After Mi xi ng Condition (h) MrL-s-aaoz -65 to +250 Class A - Brush for Class A and B, dash (Polysulfide)* (-54 to +121) Class B - Extrusion gun or numbers refer to minimum spatula application time in (for faying numbers are: -1/2, -1, surface sealing) -2, and

19、 -4 (Class B Class C - Brush or spatula hours. Available dash only). For Class C, dash numbers refer to assembly time in hours, all with 8 h application time. Avai 1 ab1 e dash numbers are: -20 and -80. -65 to +250 Same Classes as MIL-S-8802 Oash numbers in most MIL-S-83430 (Pol ysul fi de ) (-54 to

20、 +121) Short-tem to 360 (182) cases refer to minimum appl ication time i n hours. Available dash numbers are: Class A: -1/2, -2, -4 Class B: -1/4, -1/2, -2, Class C: -I/,?, -2, -6, -20 -4, -6 (Class C -6 and -20 refer to assembly time in Classes A and B: -1/2: 10 -1: 20 -2: 40 -4: Class 6-48 Class C

21、: -20: 96 -ao: 120 Classes A, B, C as appl i cable : -114: 6 -1/2: 10 -2: 24 -4: Class A-TBU Class B-36 -6: 48 -20: 96 -7- SAE ARPa4LL8 94 m 7943725 0523975 980 m SAE ARP4 I 18 TABLE 3 (Continued) Continuous Temperature or Assembly Time to Tack-free Specification 9 (“c) Appl icat i on Method After M

22、i xi ng Condition (h) Servi ce Application Time Time at 77 9 (25 “c) MIL-S-81733 -65 to +ZOO Type I - Brush or dip (Polysulfide) (-54 to +93) Type II - Extruslon or spatula Type III - Spray gun Type IV - Extrusion gun or spatula (for faying surface seal i ng) Dash number for all types refer to minim

23、um application time in hours. Avai lab1 e dash numbers are: Type I : -1/2. -2 Type II : -1/6. -1/2, -1/2, -4 Type III: -1 Type IV : -12, -24, -40, -48 AUS 3373 -80 to +400 Primrily for potting or 1-3 h but may be ordered (Si 1 i cone) (-62 to +204) encapsulation of electrical with longer or shorter

24、canponents; viscosity range pot life suited for brush application for seal i ng Available in two types: Type I, dichromate cured and Type II, manganese cured. Types I, II, III as applicable: -1/6: 4 -1/2: 16 -1: 8 -2: 24 -4: 32 Type IV: Not specified Not specified TB0 - To be determined. -8- SAE ARP

25、*ll118 94 m 7943725 0523976 817 m SAE ARP4 1 18 3.2.2.2 (Continued): application to only one surface is recommended by some fabricators to avoid excessive surface separation which may prevent proper fastener seating. A lightweight scrim cloth can be laid down onto one of the surfaces if desired for

26、thickness control, but may result in a leak path along the fibers in the scrim. 3.2.2.3 Wet Fastener Installation: Wet fastener installation is equally important to faying surface sealing as a means of galvanically isolating a bolted metal patch from the carbon composite part. Figure 2 gives a typic

27、al configuration for a carbon composite joint, showing both permanent and removable installations. The sealant materials used are the same as given for faying surface sealing in Tables 2 and 3, except that only the higher viscosity, thixotropic types are used when application is by spatula or extrus

28、ion gun. CORROSION INHIBITING SEALANT CARBONIEPOXY STANDARD CORROSION INHIBITING CRES SEALANT WASHER CARBON/ EPOXY LOW ADHESION CORROSION INHIBITING SEALANT TI OR CRES FASTENER TI OR CRES FASTENER PERMANENT INSTALLATION OR CARBON/ EPOXY REMOVABLE INSTALLATION FIGURE 2 - Sea ,1 ing Arrangement for Ca

29、rbon Composite Assembl ies 3.2.3 Surface Preparation Procedures for Bolted Repairs: The following additional precautions are strongly recommended for al uminurn patch repairs of carbon composite parts, the worst case galvanically. The aluminum should be surface treated by a suitable corrosion protec

30、tion procedure such as chromic or sulfuric acid anodizing treatment. A protective conversion coating (chemical film treatment) can be used but it is less effective than the anodize, and it is conductive, which could be a problem. Application of a finish system to the aluminum such as epoxy primer an

31、d aliphatic polyurethane topcoat is a further protective measure which can be used if a Darticularlv corrosive environment is anticioated and which can be readily SAE ARP*4LL8 94 m 7943725 0523977 753 SAE ARP4 118 3.2.3 (Continued): accomplished in an assembly operation where that finish system is b

32、eing applied. Application of an aliphatic polyurethane topcoat to the carbon composite is an additional protective measure that is strongly recommended. Any surface treatments and finishes on the part should extend at least 1 in (25 mm) beyond the edges of the joint. Another protective measure is ap

33、plication of a surface ply of electrochemically inert fiberglass or aramid to the carbon composite part. This is equally appl icable to bonded repairs, and is described in 3.3. Surface treatment and finishing as described above are supplemental procedures only, and proper faying surface sealant and

34、wet fastener installation as described in 3.2.2.2 and 3.2.2.3 are the essential procedures required to protect against galvanic corrosion. Comparable surface treatment procedures can be used for carbon composite repairs with titanium and steel patches, but in most cases are not essential. 3.2.4 Insp

35、ection of Assemblies: The most critical concern in sealant application for faying surfaces and fastener installation is that full coverage has been achieved. After assembly, the joint edges should be inspected for a continuous bead of sealant around the edges, and each fastener should likewise be in

36、spected for a continuous sealant bead around the fastener head. Absence of such continuous beads indicates incomplete sealing and gaps, and this will be a serious potential corrosion hazard as well as a potential leak path in carbon composite assemblies. Application of a fillet of sealant around exp

37、osed joint edges is highly recommended for bolted and bonded patch repairs of composite parts as an added seal. The fillet seal in fact constitutes the primary seal (backed up by the faying surface sealant) in many joint designs. 3.3 Bonded Repairs: In most cases, the patch material used for a bonde

38、d repair of carbon composite structure will be the same (or similar) carbon composite. However, the use of bonded metal patches (aluminum or titanium) is feasible, and requires consideration of galvanic isolation. The primary protection against galvanic corrosion in a bonded metal repair of composit

39、e structure is the adhesive itself, which provides the same protection as a faying surface sealant functioning as a moisture impermeable barrier. In some cases, for example, an epoxy film adhesive with a nylon scrim fabric support, the tendencies of the epoxy to absorb moisture and the nylon filamen

40、ts to wick moisture may result in a less effective barrier than that provided by the sealant over a long-term service exposure. Selection of film adhesives with less tendency to absorb moisture is advisable. Service data show varying degrees of effectiveness for various commercially available epoxy

41、film adhesives in preventing galvanic corrosion (Reference 2.1.3.4). Selection of a specific adhesive for a particularly severe corrosive environment might thus require an evaluation of galvanic protection in addition to other structural/environmental test data. A fillet seal around joint edges (see

42、 3.2.4) is also recommended for bond repairs. - 10 - SAE ARP4 1 18 3.3 (Continued): Surface preparation of the metal patches and carbon composite part is based on bond strength requirements, but an additional protective measure can be used for improved galvanic isolation. This is the application of

43、a surface ply of fiberglass or aramid fibers both of which are electrochemically inert. The use of these fibers on the surface provides additional barrier material (primarily the additional resin which encapsulates this outer ply) between the carbon fibers and the metal. This layer is most effective

44、ly applied as a Co-cured outer ply in the original part fabrication, but could also be secondarily bonded either in the form of a prepreg or dry fabric impregnated with a wet lay-up resin. This approach, particularly, with Co-cured surface plies, provides a bonded surface essentially the same as the

45、 basic carbon composite, and can be prepared for bonding in the same manner. The effectiveness of this approach has been verified by test programs (Reference 2.1.3.5), in which specimens were exposed to 7 months of sea coast exposure with no corrosion of aluminum wire mesh Co-cured to carbon composi

46、te surfaces with fiberglass and aramid interlayers. A repair approach that should be avoided is a bolted/bonded repair in which the fasteners provide back-up loading carrying capability for the adhesive. Failure of the adhesive eliminates any galvanic protection, and the remaining unprotected bolted

47、 joint may be a serious potential corrosion hazard. 3.4 Bonding of Metallic Inserts into a Repaired Composite Part: A commonly encountered repair involves reinstallation of metallic inserts, such as the threaded collar being installed into a damaged edge replacement patch as shown in Figure 3. These

48、 inserts must conform to the criteria for material selection given in 3.2.1 for fastener selection (no use of aluminum or cadmium plating). Surface preparation of the insert for bonding is governed by bond strength requirements as discussed in 3.3. Thus, the adhesive will constitute the principal ba

49、rrier against galvanic corrosion and moisture entry. This makes selection of one of the less galvanically active materials (titanium or stainless steel) a critical factor in preventing galvanic corrosion. PREPARED BY AMS COMMITTEE “P“ - 11 - SAE ARP*4LLB 94 m 7943725 0523979 526 SAE ARP4 1 18 0 0 (b MATERIAL FOR REPLACEMENT I DAMAGED EDGE ORIGINAL PART h ADHESIVE 1 THREADED METAL INSERT BONDED A- A FIGURE 3 - Repair Procedure With Reinstallation of Metal Inserts - 12 -