Effects of Service Environment on Aluminium-Brazed ABTi

8/8/2019 Effects of Service Environment on Aluminium-Brazed ABTi

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NASA Contractor Report 2943

Effects of ServiceEnvironments on Aluminum-Brazed Titanium (ABTi)

W. L. Cotton

CONTRACT NASI-13681JANUARY 1978

KIRTLAND AFS, N.


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TECH LIBRARY KAFB,NY

NASA Contractor Report 2943

Effects of ServiceEnvironments on Aluminum-Brazed Titanium (ABTi)W . L. CottonBoeing Commercial Airplane CompanySeattle, Washington

Prepared forLangleyResearchCenterunder Contract NAS1-13681

National Aeronauticsand Space AdministrationScientific and TechnicalInformation Office1978


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CONTENTSPage

SUMMARY ....................................................................... 1INTRODUCTION ................................................................. 2SHORT-TERM INTERACTION EFFECTS. ......................................... 5TestSpecimens ............................................................... 5TestProceduresandSolutions ................................................ 8

Test Results and Discussion (Thermal Exposure) .............................. 10General Observations ..................................................... 10Air Exposure ............................................................. 16Synthetic Airline Service Fluid Exposure.................................. 16SeawaterExposure ....................................................... 16Excessive Thermal Exposure ............................................. 19

Test Results and Discussion (Subsequent Environmental Testing) . . . . . . . . . . . . . . 19Specimen Sensitivi ty ..................................................... 19General Observations .................................................... 23Effects of Thermal Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Corrosiveness of Test Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23AcousticHoneycomb . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . 24

SHORT-TERM EFFECTS OF COATINGS .......................................... 25Test Specimens and Procedures ............................................... 25

Coating Selection ........................................................ 25Coating Properties . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . 25Extreme Service Evaluation . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . 27

Test Results and Discussion (Coatings) ........................................ 27Dripping Phosphate Ester Fluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Salt Spray Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30WaterLeaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Acoustic Honeycomb Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

CONCLUSIONS ................................................................... 36

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TABLES

No. Page12345678910

No.123456789101112

Corrosion Test Summary and Schedule ......................................... 4Stress CorrosionSpecimen Parameters ........................................ 5Synthetic Fluid Commercial Airline Service ................................... 9Effect of Exposure Strength and Hydrogen Content ............................ 14Effect of Service Exposure and Environmental Testing n Streng th ............. 22Resistance of Coatings to Dripping Phosphate Ester Hydraulic Fluid........... 28Salt Spray Test ing of Aluminum Panels ....................................... 31pH After Immersion in Distilled Water ....................................... 31Effect of Geometry on the Number of Open Holes per Cell ...................... 33Salt Spray Exposure on Acoustic Honeycomb Sandwich Specimen . . . . . . . . . . . . . . 33

FIGURES

PageSchematic Diagram of Test Sequence .......................................... 6Structura l Honeycomb Stress CorrosionSpecimens ............................ 7Flatwise Tensile Test ......................................................... 105000 Hours Thermal Exposure in Air ......................................... 115000 Hours Exposure. Synthetic Airline Service Fluid ......................... 125000 Hours Exposure. Synthetic Seawater ..................................... 13Effect of 5000 Hours Aging on Hardness of Aluminum Braze Fillets . . . . . . . . . . . . 15Hot Salt Corrosion of Titanium Core .......................................... 17Hot Salt Corrosion Penetration Through Aluminide Layer ..................... 18Growth of Aluminide Layer ................................................... 20Metallurgical Stability of ABTi ............................................... 21Outline of Coating Tests ...................................................... 26

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EFFECTS OF SERVICE ENVIRONMENTSON ALUMINUM-BRAZED TITANIUM (ABTi)

W. L.CottonBoeing Commercial Airplane Company

SUMMARYPreviousworkon the DOT/SST follow-on program showed that heenvironmentalcorrosion resistance of aluminum-brazed titanium honeycomb sandwich was basicallysatisfactory.Short-term nvestigations ndicated hat itaniumbrazedwith 3003aluminum alloy was inherently resistant to corrosion under anticipated aircraft serviceconditions. Passivation films on both the aluminum and titanium surfaces effectivelypreventedgalvaniccouplingunder he erviceenvironmentsevaluated.Chromate-inhibited primers provided additional corrosion protection for exposed panel edges.Thepresentprogramwas designed tocontinue he ong-termflightserviceand je tengine exhaust tests initiated under the original program and to perform additionaltests to evaluate degradation of ABTi structu re during prolonged exposure to extremeenvironments.Theextremeservice tests includedelevated emperatureexposure ofuels, ubricants, deicing-cleaning-and-stripping hemicals, and seawater , followed byaccelerated laboratory corrosion tests. The evaluations were performed using solid-faceand perforated-face honeycomb sandwich panel specimens, stressed panel assemblies,and faying-surface brazed joints.The resu lts of the investigation confirm the f act t hat t he corrosion resistance of ABTistructure is satisfactory for commercial airline service. The unprotected ABTi systemproved inherently resistant to attackby all of the extreme service aircraft environmentsexcept the following: seawater at 700 K (800O F) and above, dripping phosphate esterhydraulic fluid at 505 K (450O F), and a marine environment at ambient temperatures.The natu ral oxides and deposits present on titanium surfaces in normal aircraft serviceprovide adequate protection against hot salt corrosion pitting. Coatings are required toprotect itaniumagainstdrippingphosphate ester hydraulic luid at elevatedtemperatures. Coatings are also required to protect acoustichoneycomb sandwich partsagainst corrosion when exposed to the weather in marine environment.


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INTRODUCTION

Aluminum-brazed itanium (ABTi)honeycomb andwich is attractive foraircraftstructural and acoustic applications, especiallyt service temperatures between 400 and700 K (300O an d BOOo F). The corrosion resistance of the ABTi system during short andintermediate time exposure toa broad range of service environments was established ya program under the sponsorship of the Department of Transportation (DOT, reportFAA-SS-73-5-6).The purpose of the curren t NASA sponsored program was to perform additional tests toevaluatedegra dation of ABTiduringextendedexposure oextreme erviceenvironments.Flightservice, etengineexhaustexposure,andcreep upture estsiniti ated under the DOT contract were also continued, in order to provide 4- to 8-yearenvironmental ervicedataunder heconditionsencounteredduringactualusage.Extreme environment tests wereconducted to determine the effects of flight serviceenvironmental fluids, temperatures, and stresses on ABTi structure during exposures ofup to 7 months.The overall scope of the DOT and NASA corrosion programs are shown in table .All brazed honeycomb sandwich test parts were fabr icated from i-6A1-4V titanium facesheets, Ti-3A1-2.5V titanium core, and3003aluminumbrazealloy.Specimenswerevacuum etortbrazedperBoei ng pecif icatio n XBAC 5967 seeDOT eportFAA-SS-73-5-8) by shop personnel under production onditions.Because of significant differences in processing and corrosion parameters, four differenttypes of par ts are described in this report:1. Structural honeycomb panels are all honeycomb sandwich panels which have solid

face sheets.2. Acoustic honeycomb pane ls are honeycombsandwich panels which have onesolid

face sheetand one perforated face sheet.Thesepanelsaredesigned or noiseattenuation applications.

3. Faying-surface pane ls have two solid face sheets directly brazed together.4. Open-face specimens reitanium heet,braze-coatedonone urfacewith

aluminum to obtain various aluminum to titaniumxposed area'ratios.This report covers the results of the extreme service tests; phases 11, 111, and IV of theNASA program. The extended service evaluations, phase I, ar e still underway and arenot treated in this report.The use of commercialproducts ornames of manufactu rers n his report doesnotconstitute official endorsement of such products or manufacturers, either expressed orimplied, by the National Aeronautics and Space Administration.

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Core configuration code:

M = Machined both surfacesR = Rough, as fabricatedM/R = Machined one side onlyN = Cell walls nonperforatedP = Cell walls perforatedCell wall thickness in 0.0001of an inch, e.g.; 20 = 0.0020 inch(0.051 mm)Cell size in 1/16 of an inch, e.g.;4 = 4/16 or 114 inch (6.4 mm)C = Corrugated cell wallS = Smooth cell wallS = Square cell shapeH = Hexagonal cell shape

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Table 1. - Corrosion Test Summ ary and Schedule

I I DaysExtreme service tests

~ Thermal exposureService fluidsCoatingsPhosphate lubricantsSalt sprayHumidityAlternate immersionStress corrosion

4 7 30 6000 0000030

90

. ~. -Solid figure = test completed during DOT contractopen figure = t es t completed during NASA contractx = test in progress

Structural honeycomb;Acoustic honeycomb;

A Brazed flaying-surface ointOpen-faced brazed specimen

000000I

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SHORT-TERM INTERACTION EFFECTS

The effecta of a combination of high emperaturesandsimualtedextremeserviceenvironments were evaluated to determine whether there was interaction that couldseriously degrade the structural integ rity or the corrosion resistance of ABTi. Figure 1shows the test sequence;5000-hour hermalexposurewith periodic contamination,followed by corrosion test ing and/or flatwise tens ile test ing.

TEST SPECIMENS .Acoustichoneycombsandwich panels were fabricated using 2.54-cm (1.000-in.) thickSC4-20NM core and 0.51-mm (0.020-in.) thick face sheets, one of which was staggerperforated with 1.3-mm (0.050-in.) holes to produce 35% open area. After brazing, thepanels were carefully bandsawed, without coolant, toproduce 5.08- by 5.08- by 2.64-cm(2.0- by 2.0- by 1.04-in.) specimens suitable for environmental exposure and flatwisetensile testing.Structural honeycomb sandwich panels were fabricated using 2.54-cm (1.000-in.) thickSC4-20NM core and 1.52-mm 0.060-in.) thick acesheets.Thebrazedpanelswerecarefullybandsawed,withoutcoolant, o produce 5.08- by 5.08- by2.84-cm (2.0- by2.0- by 1.12-in.) nonstressed specimens and 5.08- by 30.5- by 2.84-cm (2.0- by 12.0- by1.12-in.)tress-corrosionpecimens. .The stress-corrosion structural honeycomb specimens were assembled in pa irs as shownin figure 2, using Ti-6A1-4V titanium bolts and silver-plated A286 corrosion-resistantsteel nuts. Centered, double-taperedTi-6A1-4V titanium shims were used to roduce thefiber stresses shown in tab le 2 in the .extreme' titanium face sheets. The 5000-hoursexposure at temperaturewas expected to result napproximately 15 MPa (2.4 ksi)relaxation n heouterfiber stress. Stresses in the brazealloyand core werenotdetermined.

Table 2. - tress Corrosion Specimen Parameters'Temperature (T I

___~~~Young'sodulus (E)a t temperatureShimhickness ( 2 y )

Applied stress (0)a t temperatureCalculated residualstress after 5000hours a t temperature*

~~ ~~ ~ ".

5050K(450 F )

9.72 x 1010 Pa(14.1 x 106 psi)4.50 m m(0.177 in.)425 MPa(61.6 ksi)415 MPa(60 ksi)

589 K(600' F)

9.24 x 1010 Pa(13.4 x 106 psi)2.51 mm(0.099 in.)226 MPa(32.8 ksi)205 MPa(29 ksi)

700 K(800' F)

8.27 x 1010 Pa(12.0 x 106 psi)0.48 mm(0.019 in.)39 MPa(5.6 ksi)30 MPa(4.2 ksi)

-

755 K(900 F)

7.52 x 1010 Pa(10.9 x 106 psi)0.30 mm(0.012 in.)22 MPa(3.2 ksi)2.3MPa(0.3 ksi)

*Assuming no yielding of he core or braze alloy

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I

AcoustichoneycombsandwichI

Structural honeycomb sandwich~ II I

Unstressedtressed I I I5000-hour thermal exposure withperiodic cooling andcontamination

. .~I * 1180 days

corrosion corrosioncontrol) humiditymmersionxposurecondensinglternateo180 days

test testI I I I

Visual examination

Flatwise tensile test

Metallographicexamination

Figure I . - chematic Diagram o f Test Sequence

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L = 29.63 crn ( 1~

I . -h = 2.84 Cm

(2.23 n.)-- .. . . .

Figure 2.-Structura l Hon eycom b Stress Corrosion Specimens


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TEST PROCEDURES AND SOLUTIONSThe test procedure (fig.1)consisted of 5000-hour the rmal exposure. The specimens werethen separated into three equal groupsor:1. No further exposure2. 180-day alternate immersion corrosionexposure3. 180-daycondensinghumidity corrosion exposureSpecimen evaluation consisted of visual and metallurgical examination and flatwisetensile testing.The thermal exposure was interrupted 30 imes and a heavy, nonflowing coating of testfluid was sprayed onto all exposed surfaces of th e specimens during cooldown. The testfluids, described in detail in tab le,were as follows:1. Air no test fluid)2. A mixtureconsisting of 68 v/o engine ueland ubricants, 23 v/o cleaning,stripping, and deicing materials, and v/o synthetic seawater3. Synthetic eawater loneAll hermalexposureswereaccomplishedusingelectricallyheated, orced aircirculation urnaces.Furnaceswerecontinuouslycontrolledandmonitoredusingindependent hermocouples.Furnaceswerecurrentlycertifiedfor & 1.1K (+ 2O F)instrumentation accuracy, and for & 5.5 K (r t lo o F) maximum difference between th econtrol set temperature and any point withinhe furnace work zone.

Alternate immersion testing was accomplished per Federal Test Method Standard 151,method 823,by totally immersing the specimens for 10 minutes out of each hour in aneutral solution of 3.5% sodium chloride at 295 K (72OF). Test specimens were oriented50 tha t the solut ion would have free access during immersion and, wherever possible,would concentrate at the estbrazesurfaceduringdrying.The estdurationwas180 days.Condensing humidity testing was accomplished per ASTM D-2247 at a temperature of311 K ( l oooF). Specimens were oriented with th e faces horizontal, perforated face up.The test duration was180days.

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Table 3..ynthetic Fluid Comm ercial Airline ServiceSolution A. ircraft fuel and lubricants

Hydraulic fluid MIL-H-5606) . . . . . . . . . . . . . . 33.3 V I0Jet engine lubricant (MIL-L-7808) . . . . . . . . . . . . . 33.3 VI0Jetengine fuel (ASTM-D1655) . . . . . . . . . . . . . . 3.3 VI0

Solution B. leaning. stripping. and deicing materialsAirplane deicing fluid MIL-A-8243) . . . . . . . . . . . . 62 v10Runway deicing urea (MIL.U.1086 6. . . . . . . . . . . . . 16/lButyl Cellosolve (TT-T-776) . . . . . . . . . . . . . . . 20 v10Methylene Chloride (Mlt M- 69 98 ) . . . . . . . . . . . . . 10 v10Glacial Acetic Acid (0-A-76) . . . . . . . . . . . . . . . 4 v10Wetting Agent (Tri ton X-100) . . . . . . . . . . . . . . 4 v10

Solution C . ynthetic Seawater Der ASTM D-1141NaCl . .MgC12 .Na2S04CaC12 .KC1 . .NaHC03KBr . .H3BO3 .SrC12 .NaF . .

Procedure

.. . . . . . . . . . . . . . . . . . . . . 24.54. . . . . . . . . . . . . . . . . . . . . 5.20. . . . . . . . . . . . . . . . . . . . . 4.09. . . . . . . . . . . . . . . . . . . . . 1.16. . . . . . . . . . . . . . . . . . . . . 0.69. . . . . . . . . . . . . . . . . . . . . 0.20. . . . . . . . . . . . . . . . . . . . . 0.10. . . . . . . . . . . . . . . . . . . . . 0.30. . . . . . . . . . . . . . . . . . . . . 0.02. . . . . . . . . . . . . . . . . . . . . 0.003

Add 100ml of solution B and 40 ml of solution C to 300 ml of solution A.Shake orstir vigorously to provide maximum'dispersionand agitate during application o minimizeseparation Spray apply to deposit a continuous fi lm and allow 10 minutes for vaporationof the volatile solvents prior t o high . emperature burnoff.

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Flatwise tensile testingwas accomplished per MIL-STD-401 using 5.1- by 5.1-cm (2.0- by2.0-in.) load blocks (fig. 3). In order to permit flatwise tensile testing, stress-corrosionbeam specimens were carefully sawed, without coolant, into 5.08- by 5.08-cm (2.00- by2.00-in.) segmentshaving two environmentally exposedhoneycombedges; allotherspecimens had four environmentally exposed edges. Specimens were abrasively blastcleaned and bonded between the load blocks using Hysol EC-9628 adhesive at 395 K(250O F). Flatwise ensile esta were performed at room temperature at a crossheadspeed of 1.3m d m i n (0.05 in/min).

P""""" 0.635 cm (0.250 in.) diameter""""" steel loading pin (typical)Hysol EC-9628 adhesive (typical)Brazed test specimen5.08 x 5.08 cm (2.00 x 2.00 n.)

0 t- Steel loading block (typical)fP

Figure 3. - Flatwise Tensile TestTEST RESULTS AND DISCUSSION (THERMAL EXPOSURE)

Figures 4, 5, and 6 show the appearance of ABTi honeycomb after 5000-hours exposureto various emperaturesandfluids.Table4shows he effect of theseexposures onflatwise tensile strength and on th e hydrogen content of the exposed (outermost cellwall of acoustic) honeycomb core.GENERAL OBSERVATIONSThermal exposure at 589 and 700 K (600O and 800 F) resulted in a slight increase inflatwise tensile st rength. This phenomenon has been confirmed during other programs.The strengthening with prolonged artificialaging couldbe due o a combination ofseveral phenomena: homogenization of th e cast aluminum braze alloy, age hardening(fig. 7) of the aluminum by precipitation of finely dispersed Mn3SiAI12 particles, andatomic diffusion t o produce a Tim3 layer that becomes less brittle as the compositionapproaches stoichiometric perfection. Optical microscopy was not capable of resolvingany discernible change inmorphology or microstructure.

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Figure 4. - 5000 Hours Thermal Exposure in A i r

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Note: After 30 applications

505 K (4500 )

589 K (6OO O

700 K (800' F)

755 K (900' F)

Figure 5. - 5000 Hours Exposure, Syn thetic Airlin e Service Fluid

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Note: After 30 pplications

505 K (450F )

700 K (800 )

Figure 6. - 5000 Hours Exposure, Syn thetic Seawater

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Table 4. - Effect of Exposure on Strengthand Hydrogen Content

Thermalexposuremedia

Air

Syntheticairlineservice

Syntheticseawater

Type ofspecimen

StructuralAcousticStructuralAcoustic

StructuralAcoustic

Condition

StressedUnstressedUnstressedStressedUnstressedUnstressed

StressedUnstressedUnstressed

r505 K (45OoF)

FWTratioa

0.921 oo1.010.991.011.08

1.030.990.94

132

155

136

Exposure temperature589 K (60OoF)

FWTratioa

1.061.051.081.1 11.171.37

1.081.081.29

109

85

132

700 K (8OOOF)H2 incoreb

1.12 I1.101.20 159

1.03c0.76c0.76 760

aRoom-temperature flatwise tensile strength ratio, verage values: after exposure+ before exposure. Average FWT valuesbefore exposure: Structural specimens = 12.64 MPa (1834 psi); Acoustic specimens= 8.62 MPa (1250 psi).bHydrogen content of outer row of honeycomb core after exposure.CFailure occurred n the core, al l other failures occurred n the braze.

755 K (900K)~~

FWTratioa

0.1 100.060.080.040.1 5

000.1 2

161

284

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6CKnoophardness(50 ).KHN

40

2c

300 400 500 600 700 800 K100 300 500 700 900 OF

Temperature

Figure 7.- Effect o f 5000 Hours Agingon Hardness of Aluminum Braze Fillets

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There was no evidence of stress corrosion or of cracking other than the hot-salt pittingthat will be discussed in the section dealing with seawater exposure. The titanium facesheeta th at contacted silver-plated nuts did not show the liquid metal embrittlementthat hasbeen eported as sometimesoccurring in jet engines.Exposure at 755 K(900F) aused a reduction in the braze fillet s and wille discussed separately.AIR EXPOSUREThe 5000-hour thermal exposure in air resulted in an increasing oxide h eat tint withtemperature, as shown in igure 4. Therewere no signs of loose scale,structuraloxidation damage, or hydrogen pickup.SYNTHETIC AIRLINE SERVICE FLUID EXPOSUREThermalexposure coupled withperiodicapplication of thesyntheticservice luidresulted n hebuildup of a heavy, eddish-browndepositmainlyconsisting ofnonvolatile esidues rom the ubr icantsand ron dissolved rom thecontainer forsolution B . There was no loss of flatwise tensi le strength at 700K (800O F) and lower,and no evidence of pitting or galvanic attack of e ither the aluminum or the titanium.The increased hydrogen content after 5000-hours exposure at 755 K (900F) resultedfrom the thermal dissociation of the seawater that contamina ted the synth etic servicefluid. Total hydrogen concentrations were much less than those caused by exposure tostraight seawater because the reaction was partially suppressedy the lubricants in thesynthetic service fluid. The hydrogen increase was not sufficient to seriously embrittlethe hin itanium core and would have no effect whatever on the hicker itaniumface sheets.

SEAWATER EXPOSUREThe5000-hour hermalexposurecoupledwithperiodicapplications of syntheticseawater resulted n he buildup of a heavy ayer of dried salts. There was a verydiscernible H2S odor during spray application of the seawater to the hot specimens,indicating a n appreciable thermal breakdown of the Na2S04 constituent. There was noevidence of galvanicattackor corrosion pit ting of aluminum at any emperature,indicating hat hebrazealloy is not usceptible ohot-saltcorrosion at servicetemperatureand hat he2-hourmaximumexposure oaqueoussaltsduringeachapplication was insufficient to init iate the conventional orrosion mechanism.Exposure oseawater at 700 and 755 K (800O and 900 F) resulted nextensivehydrogen pickup by the ti tanium core and a significant reduction in flatwise tensilestrength,with all failuresoccurring n he itanium honeycomb core. The loss inflatwise tensile strength proved to be due to hot-salt pitting corrosion of the titaniumcore (see fig. 8). In the presence of stress, the occurrence frequently included planarcracks perpendicular to the direction of s tress . At 755 K (900F), pitting occurred atbare itanium areas and at the root of cracks hroug h he aluminide ayer (fig. 9).Hot-salt corrosion of titanium has been encountered previously, and is accompanied by

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Note: During 5000-hour seawater exposureDilute Keller's Etch

589 K (600 )No attac'k oncell walls

700 K (800' F)Shallow planar cracksin cell walls

755 K (900' F )Deep pits and cracksin cell walls

Figure 8. - Hot Salt Corrosiono f Titanium Core

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Note: During 5000-hour seawaterexposure at 755 K (900 F)

Cross section of core wallKeller's Etch

Cross. section of core nodeKeller's Etch

Fillet areaDilute Keller's Etch

Figure 9. - Hot Salt Corrosion Penetration Through Alum inide Layer

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hydrogen production (report NASA TN-D6779). The hot-salt corrosion mechanism hasbeen shown (reportsNASA TM X-68015 and NASA TM X-3145) to be stopped by coolingandnot obecumulative.Theparticularcooling requencyused n hese tests,approximately once a week,magnified thedegree of hot-saltpitting. The frequentcooldowns encountered inaircraftenginesalmost otallyeliminate hisattack,andthere have not been any service problems with titanium engine components that couldbe attributed to hot-sa lt corrosion. It should be noted that this att ack was inh ibited orstoppedalmostcompletelywhen he eawaterwasmixedwith heengine uel,lubricants, and other components in the synthetic airline service fluid.EXCESSIVE THERMAL EXPOSUREExposure for 5000 hours at 755K (900 F) in each of the service media resulted in thedisappearance of many of the aluminum braze f illets. Oxygen discoloration was evidentin even he nnermostcells of thestructural honeycombsandwichspecimens. Thisdegradation in properties was caused by solid-state conversion of the aluminum brazealloy o TiAl3. Thecontrollingeventoccurringduringbrazing is the conversion ofaluminum and titanium to titanium aluminide: Ti + 3A1-c TiAl3. The growth of theTiA13 layer continues by solid-state diffusion at temperatures below the melting pointofthe aluminum braze metal. The TiA13 growth occurs by progressive thickening of theoriginal aluminide layer at the expense of the adjacent aluminum (see fig. 10). "he5000-hourexposure at 755 K (900F) wassufficient oconsume all of thesurfacealuminum and most of the braze fillets, greatly lowering the flatwise tensile strengthandpermitting oxygen topenetrate nto he nterior of thestructural honeycombspecimens. Figure 11 is a composite of data from various Boeing test programs showingthe onset of solid-state TiA13 formation as a function of time and temperature. Fromthis curve and from figure 10, t can be seen that the ra te of TiA13 growth is very slowand would fulfill any expected aircraft life requirements at temperatures up to 700 K(800OF). Above 700 K (800OF), the life expectancy decreases rapidly with progressivelyhigher temperatures. However, the system is shown to be capable of withstanding someexposure to temperatures approaching the melting point of the braze alloy, 2 hours at920 K (1200O F), such as might be encountered ina n engine fire.

TEST RESULTS AND DISCUSSION(SUBSEQUENT ENVIRONMENTAL TESTING)

Table 5 shows the synergis tic effects of various environments on corrosion resistance asa function of flatwise tensile strength.SPECIMEN SENSITIVITYThe stress concentrations and failure mode of flatwiae ensile specimens made hemespecially sensitive to any damage to the outerraze fillet. Reduction in flatwiae tensilestrengthserved as an excellent test for the onset of corrosion damage by greatlymagnifying the effects of damage. Reduction in flatwise tens ile strength da ta must notbe used to assess corrosion damage to actual structure.The percent of damaged fillets inflatwise ensilespecimenswill be many imes hat of actualstructure,andactualhoneycomb structure is rarely critically stressed in flatwise tension.

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589 K (600 )TiAI3 = 0.0013 mm(0.00005 in.)

700 K (800' F)TiAl - 0.003 mm

- (O .OOOI in .)

755 K (900' F)TiAI3 = 0.03 mm(0.001 in.)

. I ,,rt"r' ." ... .- - ....

I .

. ., .. I

I .. ..Figure 10.- Growth o f Aluminide Layer

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100000

10000

1000Time,hr

I oa

10

1

t

I -

l -

I -

-

10 000 hours at 700 Kwith no discernable effects

0 T h i s contractI npublishedBoeingdataA FAA-SS-72-03 report

8\\\

Unstable(Properties degradewith additionaltime)

properties) \I I I I I

500 600 700 800 900 K

400 600 800 1000 1200 FTemperature

Figure 7 7. - Metallurgical Stability ofA.BTi

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Thermalexposuremedia

Air

SyntheticairlineservicefluidSyntheticseawater

Table5. - Effect of Service Exposure andEnvironmental Testing on Strength

Type ofspecimen

Structural

AcousticStructural

AcousticStructural

Acoustic

Condition

StressedUnstressedUnstressedStressedUnstressedUnstressedStressedUnstressedUnstressed

Flatwise tensile strength atioa after 5000 hours thermal exposure505 K (45OOF)

bndensinghumiditytes t0.890.850.460.580.420.170.620.800.18

Alternateimmersiontes t0.420.3600.270.110Nottested

589 K (600OF)Condensinghumiditytes t1.051.050.360.820.740.28Nottested

Alternateimmersiont e s t0.520.4400.540.2200.590.280

T 700 K ( 8 O O O F )Condensinchumiditytest1.090.930.440.900.500.180.540.530.18

Alternateimmersiontes t0.550.2500.460.170Nottested

755 K (900OF)t

Condensinghumiditytest0.050.0500.050.050Nottested

Alternateimmersiontes t00000000.050

aRoom-temperature flatwise tensile strengthatio, average values: after exposuref before exposure. Average FWT aluesbefore exposure: structural specimens= 12.64 MPa (1834 psi); acoustic. specimens= 8.62 MPa (1250 psi).


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Interpretation and analysis of the flatwise tensile resulta require consideration of th especimen geometry during the corrosion test. Corrosion of the peripheral fillets tendedto disproportionately lower the appare nt stre ngth . In the case of the 5.08- by 5.08-cm(2- by 2-in.) unstressedstructural honeycombspecimens, theouterdirectly exposedperipheral row of filletsamounts oapproximately 40% of the total illets n thespecimens. n hecase of th e 5.08- by 30.5-cm (2- by12-in.)stressedstructuralhoneycomb specimens, the directly exposed peripheral fillets represent approx imately20% of the total fillets. Acoustic specimens, because of the perforated face sheet, haveall fillets directly exposed to the corrosive media.GENERAL OBSERVATIONSThe increased strength after 5000-hour thermal exposure at 589 and 700 K (600O and800F) was still apparen t after corrosion testing. There was no indication that thisstrengthening wasa surface effect or that t contributed to anycorrosion mechanism.There was no evidence of stress corrosion or of stress-accelerated corrosion during anyfthe exposures. Sufficient creep occurred during the 5000-hour thermal exposure thatthe pecimenswereessentiallyunstressedduring he ubsequentcorrosion tests.Previous testa (report FM -S S-7 3- 54 indicated that there was no stress corrosion orstress-accelerated corrosion of ABTi pane ls that had ot been thermally exposed.EFFECTS OF THERMAL EXPOSUREThe thick, uniform oxide film tha t formed on the aluminum braze fillets during thermalexposure in air , especially at 589K (600O F), provided increased corrosion protection tothe braze filleta. This oxide film protection is similar to that provided by the chemicalandanodicconversioncoatings oraluminum. As with all aluminumstructures,additional corrosion protection to th e exposed aluminum should be provided by the useof supplemental protective coatings.

The specimens exposed for 5000 hours at 755 K (900F) exhibited little or no resistanceto corrosion. The thermally diminished aluminum braze fillets did not have sufficientcontinuity to prevent solution penetration, or suffticient thickness to resist 180 days ofsubsequent corrosion attack. This temperature is far beyond the design extended-servicelimit or he ABTi system. Any diminishedbraze illets hat would be caused byprolonged ocal overheatingduringservice, would be readilydetected by ultrasonicpulse-echo or eddy current inspection longbefore they become discontinuous.CORROSIVENESS OF TEST MEDIAAlternate mmersion esting at room temperature proved to be a fa r more corrosiveenvironment than 311 K (1000 F) condensing humidity. The 50-minute drainage timeduring each alternate immersioncycle was insufficient to permit theecessed aluminumbraze fillets to dry out and reforma protective oxide film, particularly in the interiorofthe acoustic specimens.

23


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Thecorrosiveness of the alternate mmersionandcondensinghumidity estswasincreased by the deposits rom thesyntheticairlineservicefluidand hesyntheticseawater.Duringalternate mmersion esting, hesedepositsretainedmoistureandinhibited the drying portion of the cycle. During condensing humidity testing, the saltportion of the deposits ionized and accelerated the corrosiveness f the water, especiallyinside the cells of the acoustic honeycomb specimens.ACOUSTIC HONEYCOMBCorrosion of the acoustic pecimenswasmore evere han hat of thestructuralspecimens. The large holes oupled with the 35% open area permitted free access to thecorrosive test solutions anda high concentration of dissolved oxygen. Salts, leached fromthesyntheticfluidlthermaldeposits,were effectively retained n he nner cells andcaused extensive corrosion. The corrosion tes t results indicate that a thin-film coatingshould be used to protect interior surfaces of acoustic ABTi structure against corrosion,especially when the airplane is not in daily service and where the structureoes not dryout in service.

24


30/42

SHORT-TERM EFFECTS OF COATINGS

The effect of coatings,designed oprotect itaniumfromphosphateesterhydraulicfluids,on hecorros ion esistance of acoust ic ABTi sandwichwas nvestigated.Figure 12 shows he coating ests and est sequence: tests to select the best of thecandidate coatings, evaluation of the water eaching and corrosion resistance of thecoatings, and thermal-phosphate ester fluid exposure and subsequent corrosion testingof coated acoustic honeycomb specimens.

TEST SPECIMENS AND PROCEDURESCOATING SELECTIONSpecimens were 10- by 15- by 0.10-cm (4- by 6- by 0.040-in.) Ti-6A1-4V or commerciallypure itanium.Surfacepreparationconsist ed of nitric-hydrofluoricacidetching,abrasive blasing with 150-mesh aluminum oxide, abrading with Scotchbrite typeA finealuminumoxidepads,orphosphate-fluorideconversioncoating,dependingon hecoating suppliers recommendation or Boeing experience. Coatings were spray applied tothe required thickness, ir dried, and cured.Thedrippingphosphateesterscreening estwas accomplished with the specimensmounted on a hot plate inclined at an angle of 0.79 radian (45O). Contact thermocouplesand a recorder-controllerwereused omaintain hespecimens at a temperature of505.2 f 5.6 K (450 f looF). The phosphate ester fluid, generally Skydrol500B, wasallowed to drip on th e specimens at a rat e of one drop approximately every 3 minutesfor 96 hours. At the conclusion of the test, the charred deposit s were removed with asolvent or a paint stripper and the specimens were examined for coating integrity andfor etching or crackingof the titanium.COATING PROPERTIESSpecimens for coating leaching studies were5- by 5-cm (2- by 2-in.) Ti-6A1-4V or 3003-0aluminum. The specimens were reated with two coats of monoaluminum phosphate(with and without a wetting agent) or with Kolene Kov Kote, and each coating wascured 10 minutes at 590 K (600O F).The eaching test was accomplished by immersingeachpanel n a 250-ml beakercontaining 200 ml of distilled water. Periodically, during exposure pH measurementsweremade.The beakers were ightlycoveredwith0.05-mm 0.002-in.) hickpolyethylene during the 19-day exposure.Specimens for salt spray testing of coatings were 10- y 15- by 0.02-cm (4- by 6- by0.01-in.) 3003-0 aluminum.Specimenswerealkalinecleanedanddeoxidized(Amchem 6-16) for 2 minutesprior oAlodine1200 reatingand/orcoatingwithmonoaluminum phosphate or KoleneKov Kote.


31/42

Coating selection--- - + Coating properties--- w Extreme service evaluationTitaniumsheetspecimens

96-hours exposureto drippingphosphate esterhydraulic fluid

AluminumsheetspecimensIpray apply coating

19-day waterleach a t roomtemperature90-daysa l tspray corro-sion tes t

Periodic pHmeasurements examination

AcoustichoneycombsandwichI ~ ~~~Vacuum/ambient pressureapply coatingI

I168-hours exposure t700 K (800' F) in air

96-hours exgosure t45 0 K (350 F) withperiodic immersion nphosphateesterhydraulic fluidIII

120-daysalt spraycorrosion test -I Iisual examinationI 1latwise tensilees t

Figure 12.- Outline o f Coating Tests


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Salt spray testing wasaccomplished per specification ASTM B-117, us ing a 5% solutionof neutral sodium chloride at 308K ( 9 5 O F). Test specimens were oriented 80 as toprovide themostsevere,uniformexposure of the aluminum o he salt fog. Sheetspecimens for screening este were nclined 0.105 radian ( 6O ) from vertical. Acoustichoneycomb specimens were exposed in the horizontal position, with the perforated facesheet up to permit maximum exposure of the int erior cells. Salt spray test exposureswere for periods of 90 days for sheet specimens and 120 days for acoustic honeycombsandwich specimens. Industry standard exposure periods for structural aluminum parteare: 14days orpartsprotectedwith a specification MIL-C-5541 chemicalcoating,21 days for parts protected with specification MIL-A-8625 anodic coating, and 30daysfor painted parts.EXTREME SERVICE EVALUATIONAcoustic honeycomb sandwich specimens were fabricated as previously described, andthe exposed honeycomb edges were abrasive blast-cleaned with aluminum oxide. Thephosphate ester protection coatings (one coat of Andrew Brown B-2000 or two coats ofHanovia Gold no. 6854) were applied to the nonperforated face and cured. Secondarycoatings to provide corrosion protection for the aluminum braze alloy (monoaluminumphosphate or Kolene Kov Kote) were applied by immersion vacuudamb ient press urecycling three times. An Alodine 1200 pretreatment was not used prior to application ofthe second coatingfor woreasons: (1) hecontrolsfor he Alodine solution wouldrequire modification to accommodate the long immersion period during vacuumcycling,and (2) the need for a n Alodine pretreatment had not been onclusively demonstrated.The coated and cured acoustic honeycomb sandwich specimens were thermally exposedin a forced-air furnace at 700 K (800O F) or 168 hours. The temperature in the furnacewas then lowered to 450 K (350OF) nd thermal exposure was continued an additional96 hours. The 450 K thermal exposure was interrupted nine times; the hot specimenswere immersed in phosphate ester hydraulic fluid (Skydrol500B) and returned to thefurnace. The specimens were then cooled to room temperature and salt spray tested for120 days.

TEST RESULTS AND DISCUSSION (COATINGS)DRIPPING PHOSPHATE ESTER FLUIDTable 6 shows the re sults of the phosphate ester screening tests. Heavy, black depositsof fluid breakdown products formed on all of the specimens. Only two of the coatings(AndrewBrown B-2000 nd Hanovia Gold no. 6854)proved capable of satisfactorilyprotecting titanium from he dripping fluid. The monoaluminum phosphate coating wasporous, permitting the hydraulic fluid to spread laterally, retaining the corrosive fluidbreakdown products, and generally aggravating etching of the titanium. The KoleneKov Kote did not provide adequate protection against he dripping fluid when usedalone. The Kov Kote did not degrade the performance of the gold coating and exhibitedpotential as a topcoating oprovidecorrosionprotection oassembledacoustichoneycomb parts.

n


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Table 6. - Resistance of Coatings to Dripping Phosphate Ester Hydraulic Fluid

ExposureC o a t i n gt 505KT ian i um Sur fac eh ickness,yd rau l i c450F).Co a t i n ge s i g n a t i o nu b s t r a t er e p a r a t i o n mn ( i n ch )o a t i n gu r el u i do u r se su l t sNone

Advanced Coatings26W-1. ( F1 uoro pol yme r)AdvancedCoatings26W-1 (F1uoropolymer)AdvancedCoatings26W-1 (oldb a t c h )(F luo ropo lymer )Advanced Coatings26W-1 (new batch)(F1 uoropolymer)HanoviaGold#6854HanoviaGold#6854HanoviaGold#6854Serma-Lon(Polyphenyl enes u l f i d e )Sermete l W(A1 umi num +phospha teb inde r )Teflon-S(F1 uoropolymer)958-208

CP

CP

CP

CP

CP

CP

CP

CP

T i 6A1-4V

T i 6A1-4V

T i 6A1-4V

A1 kal ine clea ned

HN03-HF et ch

HN03-HF et ch

Grit b l a s t e d

Grit b l a s t e d

HN03-HF etch

HNO -HF etch;Sco&hbr i teabradedHNO -HF e tch ;S co zch b r i t eabradedGrit b l a s t e d

G r i t , b l a s t e d

N o n - g r i tb l a s t e d

0.025(0.001)

,(0.001 )0.025

,O. 025(0.001 )0.01 0(0.0004)0.010(0.0004)

0.001(0.00005)0.001(0.00005)0.001(0.00005)

0.1-0.2(0: 05-0.01 0)0.04-0.09(0.0015-0.0035)0.025(0.001)

Baked a t 535K(500F) f o r 30mnu tesBaked a t 535'K(500'F) f o r 30minu tesBaked a t 535K(500'F) f o r 30minu tesBaked a t 535K(500F) f o r 30minu tesBaked a t 535K(500F) f o r 30minu tesBaked a t 700'K(800F) f or 12mnu e sBaked a t 715'K(825F) f o r 15minu tesBaked at 7159K(825F) f or 15m i nu es( P r o p r i e t a r y,process)Baked a t 605K(625F) f o r 30minu tes + g l a ssbead eenBaked a t 575K(575F) f o r 19minu tes

Skydro l 5008

Skydro l 008

H y j e t I V

Skydrol 500B

Skydro l 5008

Skydro l 5008

Skydro l 5008

Aerosafc2300WSkydrp l

Skyd ro l

2

$008

500B

Skydrol 500E

96

96

96

96

96

96

96

96

96

96

96

Subst ra tee tchedseve re ly

Coating removed; .substratee tched eve re ly

t o s u b s t r a t eCoat ing removed;no damage

Coat ing removed; su bst rat ee tched eve re lyCoat ing removed; subs tra te. e t ch e dse ve r e l ySome co ati ng em ov al; somesu b s t r a t e e t ch i n g.ppm.H2 = 38Some co at ing em ov al ; somes u b s t r a t e e t c h i n gppm H2 = ,3 0S l i g h t coa t ing remova l ;nos u b s t r a t ee t c h i n gppm H2,=3 3Co at in g .removed; bad. ss u b s t r a t ee t c h i n gCoat ing removed; sub str atee tched eve re ly

Coat ing removed; ads u b s t r a t e e t c h i n g


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Table 6.- Concluded)

.. ExposureCoat ing . a t 505KTi tan iumur faceh ickness ,ydrau l ic ' (450aF).C o a t i n ge s i g n a t i o nu b s t r a t er e p a r a t i o n mn ( i n c h )oat ingu re , , f l u i d h o u r sesu l t s ,Andrew BrownB-2000( S i l i c a t e )

Andrew Brown8-2000(Si .1 i c a t e ) .DeSoto F1 uorop on(F1 uoropol mer) ,Dow Cor ning XR-62205( S i l i c o n e )DeSoto763-003( S i 1i one)

Repairedpanel2coats,HanoviaGold168542 coats mon,oaluminumphosphate70 % .A l ( H PO ) + 10%MgNO ,+4Pd% Cr03]2 coatso lene .Kov Kote2 coatsHanoviaGold16854 + 2 coatsmonoaluminumphosphate2 coatsHanoviaGold16854. + 2 coatsKolene Kov Kote

CP . P04-Flreatment

CP . PO4-F1 treatmentCP Grit b l a s t e dCP Grit b l a s t e dCP ' . PO4-F1 treatment

CP HN03-HF e t c h

CP HN03-HF e t c h

CP HN03-HF e t c hCP HN03-HF e t c h

CP HN03-HF e t c h

0.041(0.0016)

0.041(0.0016)0.01 00.025 .(0.00041(0.001)(0.0022)0.056

0.0002 '(0.00001)

0. OB(0.003)

0.0008(0.00003)0.08(0,003)

0.001(0.00004)

Dry 1 hour + bake'SRydrol 5008a t 450K (350F)for, 1 hour .Dr y 1 hour + bakeHy je t IVa t 450K (35OOF)f o r 1 h o u rBake a t 535'K(5OOoF) fo r ' 1 hourBake a t 535PK Skyd ral 500B(500F) f o r 1 hourBake a t 360K , Skydrol 5008(190F) +.535'K(500F) f o r 1 houreachBake a t 700OK' Skyd rol 5008(800'F) f o r 12minutesaf te re a c h a p p l i c a t i o nBake a t 590K Skyd rol. 5008( 6 O O O F ) f o r 1 0mnu tes

Bake a t 590OK ' Skydrol 5008( 6 O O O F ) f o r 10 ..minutesGoldakes + Skydrol 5008MAP bake'

- . .

. .

Gold bakes + Skydrol 5008Kov Kotebake

96

96,Te s t n o t u n

4896

.. 96

96

96

' 96

96

Some coating removed; somes u b s t r a t e e t c h i n gSome co at in g removed; nos u b s t r a t e e t c h i n gCoa t ing sof ten ed' at '500K(44OOF)Coat ing removed; s ubs tra tee tchedsevere lyCoat ing removed; 'substratee tchedsevere ly

No coating removed;nosubstrate damage- ,

Coating removed (a);subs t ra te e t c h e dsevere ly. . '

Coating removed (b) ;s u b s t r a t e e t c h e d s e v e re l yMAP coa t in g removed . (a) ;some go ld removed; somesevere e tch ingKov Kote removed ( b ) ;no sub s t ra te . e t ch ing

'2;5-cm (1-inch)wide band o f MAP coa t in g removed.b0.8-cm (0.03-inch)wideband o f Kov Kote removed.


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The dripping phosphate ester fluid test simula tes the most Bevere conditions that couldbe reasonably encountered in service. A slower drip permits corrosion fluid breakdownproducts such as phosphoric acid to evaporate. A faster driprate flushes corrosive fluidbreakdown products away. A lower temperature retards therate of fluid breakdown. Asigeifkantly higher temperatu re permits the fluid to evaporate without remaining inintimate contact with the titanium. The older fluids (e.g., Skydrol500B) do not containinhibitors and are much more corrosive than the newer fluids like HyjetV.

SALT SPRAY TESTINGTable 7 shows the result s of salt spray screening tests of coatings on aluminum panels.The monoaluminum phosphate and Kolene Kov Kote coatings were applied as aqueoussolutions and did not readily form continuous films on bare aluminum surfaces; twocuredcoatswere equired orcompletecoverage. Good filmswere ormedoverAlodine 1200 conversioncoatedsurfacesand,presumably, would be formedover aproperly applied anodically or thermally oxidized aluminum surface.Before curing, hemonoaluminumphosphatecoatingdried as a glossy, ransparentcoatingcontainingabout 30% water as a gel.Thegeldehydratedduringcuring,producing a porous white coating with a marked tendency toward chalking. The levelofchromates in thi s coating provided a significant degree of corrosion protection duringthe 90-day salt spray exposure.The Kolene Kov Kote air dried as a flat to semiglossy, transparent coating containingabout 5% water. The cured coating remained hard and transparent with no tendencytowardchalking,buthadneither he hicknessnor hecorrosion nhibitors oadequately protect bare aluminum rom corrosion.Both themonoaluminumphosphateand heKolene Kov Koteprovidedadequatecorrosion protection when applied over he Alodine 1200 conversion coated aluminum.WATER LEACHINGTable 8 shows the resu lts of water leaching screening tests of the cured monoaluminumphosphate and Kolene Kov Kote coatings. The changes in pH can be related to severalcompeting chemical reactions. The decreasing pH values indicated that the water wasbecoming acidified, partly by carbon dioxide diffusing hrough the th in polyethylenecover. A stronger source of acidity was chromic acid leached from the monoaluminurpphosphate coating. The increasing pH values indicated that the water was becomingmore alkaline, partly by aluminum reacting with the water. A much stronger source ofalkalinity was potassium hydroxide leached from he Kolene Kov Kote coating.The pH changes observed in these tests indicated that very little leaching occurred withany of the coatings. In the absence of a wetting agent, the chromates leached from themonoaluminumphosphatecoatingwouldnot be sufficient t o completely inhibitcorrosionon thealuminumsubstrate. Increasedchromateand ncreasedcorrosioninhibition would be provided by the use of a n Alodine 1200 conversion coating, as canbe seen in table 7) . The amount of hydroxide leached from th e Kolene Kov Kote would

30


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Table 7. - alt Spray Testing o f Alu min um Panels

" ~Immersiontime, days

"

Processing~

Alkaline clean and deoxidize onlyOne coat of monoaluminum phosphate plus10-minute cure a t 590 K (6OOOF)Two coats of monaluminum phosphate plus10;minute cures a t 590 K (60OOF)Two coats of Kolene Kov Kote plus 10-minute cures a t 590 K (6OO0F)Alodine 1200 (1-minute mmersion)Alodine 1200 plus tw o MAP coats wi th10-minute cures a t 590 K (60OOF)Alodine 1200 plus two Kov Kotes wi th10-minute cures a t 590 K (6OOOF)

~~ ~

Appearance after 30-daysalt spray exposureHeavy general corrosionVery light corrosion (16 pits)

Traces of corrosion ( 5 pits)

Moderate corrosion (80 pits)Very light corrosion (13 pits)No corrosion

No corrosion

Table 8. - pH Afte r Immersion in Distilled Water

6.50 I 5.45 I 5.15 I 5.25 I 5.45 I 7.65 I 7.95 I

31


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not be sufficient to ini tia te corrosion of the aluminum (report FAA-SS-73-5-6 showedthat a pH of about 11 was equired to initiate corrosion) oreven omaintainanappreciably alkaline solution inair containing carbondioxide.Carefully controlled leaching tests were not performed on the phosphate ester lubricantprotection systems. The chemistry of these coat ings indicates that the Hanovia Goldno. 6854 would form an all metal coating with no harmful, water-soluble constituents.The Andrew Brown B-2000 coating contains sodium and lith ium silicates which wouldbe subject to water leaching and would raise the pH until accelerated corrosion of thebraze alloy wouldoccur.ACOUSTIC HONEYCOMB TESTSThe acoustic honeycomb sandwich specimens were unaffected by the thermal/phosphateester fluid exposure. Visual examination showed no pitting, etching, or embrittlementto the aluminum braze metal, the titanium face sheets, or the titaniumoneycomb core.Residual phosphate ester fluid and brown varnish-type fluid breakdown products wereon most surfaces of the specimens. Fluid breakdown did not progress to the point offorming the heavy black deposits noted in the 96-hour dripping test. All of the coatingsappeared tobe intact.Table 9 shows that all of th e acoustic honeycomb sandwich specimens were corrodedduring the subsequent 120-day salt spray exposure. Salt spray corrosion progressed tothe point that flatwise tensile tests could only be made on those specimens that hadbeen protected with monoaluminum phosphate as a second coating. The flatwise tensilestrength of the specimens coated with gold plus monoaluminum phosphate was reducedto less than 5% . The flatwise tensile strength of those (nongold) specimens protectedwithmonoaluminumphosphatewas educed o anaverage of 60%. Thedegree ofcorrosion penetra tion of the honeycomb outer braze fillets during 120 days f salt sprayexposurewas nagreementwith he indings of thepreviousprogram reportFAA-SS-73-5-6).The corrosion damage o nteriorbraze illetswasverymuchgreater han hatpreviouslyencounteredwithacoustic honeycomb. Table10shows hat hepreviousinvestigation was accomplished using acoustic honeycomb specimens with the same sizeholes, but with the number of open holes after brazing reduced from the current 17 percell to only one per cell. During prolonged salt spray exposure, specimens with only oneopen hole per cell had restricted solution circulat ion and a tendency for precipitatedsalts to block th e hole, so that the corrosion rate for interior cells was much less thanth at for exterior cell braze fillets. The specimens with 17 holes per cell used durin g thepresent investigation allowed re latively free circulation of oxygen and corroding mediain he nteriorcells, so that he nteriorandexteriorbraze fillets corroded atsimilar rates.The 120-day salt spray exposure proved to be too severe for evalua ting the rela tive lyminordifferencesbetweencorrosionprotectioncoatingsor ordetermininganyacceleration in corrosion caused by the phosphate ester protection coatings. A 60-daysalt spray exposure would be more appropriate for any fur the r tes ts of coatings on

32


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Table9. - Salt Spray Exposure On Acoustic HoneycombSandwich Specimensa~

First coating

NoneAndrew BrownB-2000Hanovia GoldNo. 6854Hanovia GoldNo. 6854NoneHanovia GoldNo. 6854None

Second coating

NoneNone

None

Kolene Kov Kote

Kolene Kov KoteMonoaluminumphosphateMonoaluminumphosphate

Results of examination

Very severe corrosionVery severe corrosion

Very severe corrosion

Very severe corrosion

Very severe corrosionSevere corrosion

Moderate corrosion

F W T ~strengthratio00

0

0

00.05

0.60

al0-days sa l t spray. Face sheet perforated with 1.3 rnm (0.050 in.) holes, 35% open area.bRoom-temperature latwise tensile strength ratio, average values: after exposure before exposure. AverageFW T values for acoustic specimens before exposure: 8.62 MPa (1250 psi).

Table 10. - Effect ofGeometry on the Number o f Open Holes perCell

Previousinvestigation(report FAA-SS-73-5-6) - ~~-~Presentinvestigation

Potential acoustic1 designs, similar1 holes' Potential acousticdesigns, argerholes

" - _.Holediameter. ~ . .

1.3 mm(0.050in.)"1.3 mm(0.050in.)1.3 mm(0.050in.)1.5 mm(0.060 in

""1!

Honeycombcore

SC 4-20 NM

" ~.SC 6-20 NM

SC 6-20 NM

SC 6-20 NM

Openarea

5%

35%

8%

8%

Open holes per cellBefore

brazingrazingAfter

1.6 1.1

25 17.5

5.7 4.0

4.0 2.8

33


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acoustic honeycomb sandwich panels. The 120-day salt spray exposure-is ot, a valid testfor evaluating the corrosion resistance of acoustic ABTi structure for commercial airlineservice where the structures will be heated and shielded from direct contact with theweather.The results of the salt spray tests provide a strong indication, however , that . corrosionprotection finish is needed for ABTi acoustic honeycomb sandwich structure. Two coatsof monoaluminum phosphate provided a reasonable degree of corrosion protection at aweight penalty of 0.1 kg/m2 (0.02 lb/ft2). Two coats of Kolene Kov Kote, weight penaltyof only 0.02 kg/m2 (0.005 lb/ft2), did not provide appreciable orrosion protection to barealuminum. A comparison of the results from ables 7 and 9 indica tes ha tanAlodine1200 pret reatmen t would havegreatly mproved the corrosionprotectionafforded by either the monoaluminum phosphate or the Kolene Kov Kote coatings. Thecontrols for the Alodine 1200 solution, or other MIL-(2-5541 colored coating solutions,would require adjustment to extend the immersion time rom the present 1 o 3 minutesto the 10 to 30 minutes requ ired for vacuum-ambient pressure cycling. The applicationof a heavy-duty phosphate ester fluid/corrosion protection system to a n ABTi acoustichoneycomb sandwich assembly could be achieved by the following procedure:1. Applycorrosionprotective finish to theacoustic honeycombassembly so as toensure penetra tion of all interior cells.

a. Rack the assembly so thatrotationorotherpartmovementwillorient heperforated face sheet up during solution immersion andown during drainage.

b. Subject th e assembly to at least three vacuudamb ient pressure cycles duringeach immersion-processing step and to at least five vacuum/ambient pressurecycles during each rinsing step and the coating drainage step. An absolutepressure 5% of ambinet pressure is a satisfactory vacuum.

c. Alkaline clean and rinse at room tempera ture if the interior cells have beenexposed tooils, oilyfumes,orcuttingfluidsother handistilledwater orFreon TB-1. The Greater Mountain Chemical Co.GMC 528B or WyandotteAerowash cleaners diluted with four parts water re satisfactory.

d. Deoxidize and inse if oxides or other ontaminantson hebraze filletsprevent formation of a proper chromate film in step (e). Wyandotte 2487 at60 g/l (8 oz/gal), 30% nitric acid, or similar deoxidizers hav ing an etch rateless than0.003 mm per hour (0.0001 in/hr) on aluminum are satisfac tory.

e. Apply Alodine 1200 or similar colored MIL-C-5541 conversion coatings. Adjustthe solution composition so that a nonpowdery, colored coating will form in10 to 30 minutes andwill pass the MIL-C-5541 corrosion test.

f. Immersionpplyheorrosionrotectionoatingndrain. hemonoaluminumphosphatesolutioncontains, by weight, 26%Al(H,PO4)3 +3.7% MgNO3 + 7.6% Cr03 in water. The Kolene Kov Kote solution is used asreceived.

34


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g. Air or vacuum dry the coating s required to prevent blistering during curing.h. Cure he coating . An overnight cure at 405 to 425K (265O to 3000F) or a

10-minute cureat 590 K (6000F) s satisfactory.i. Reapply and cure coating per steps (0o (h).

2. Apply protective finieh to surfaces th at could be exposed to wetting by phosphateester hydraulic fluid during elevated temperature service.a. Abradesurface6withScotchbrite.b. Spray apply a coating of Hanovia Liquid Bright Gold no. 6854.c. Cure 10 to 15 minutes at 715 -t 14K (825O t 25O F).d. Reapply and cure gold coating per steps 2(b)and (c).

3. Apply corrosion protection finish to exposed outer edges of honeycomb core.a. Mask perforated face sheet and gold coated face sheet as required to prevententrapment of abrasive partic les or damage to the coatings .b. Dryabrasiveblastwith ine 150 o 200mesh)aluminumoxide a t

approximately 0.14MPa (20 sig) preseure.c. Spray pplywo oata of protective inish nd ureperhe uppliers

recommendation.Finch 454-4-1 poxy primer is satisfactory orextendedservice at temperatures up to 420K (3000 ).DeSoto 763-003 ilicone primeris satisfactory for extended service between420 and 505 K (3000 and 450F).Sermetel W aluminum filled silicate coating is satisfactory for extended servicebetween 505 and 75 5 K (450 nd 900F).

d. Cure he inishper hesuppliers ecommendation.TheSermetel W finishrequires a 30-minute cureat 605 K (625O F) after each coat.

e. Glass bead peen the Sermetel W inish.

36


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. . .. .. .. . .

'CONCLUSIONS. . . .

Extreme service evaluations show th a t aluminum-brazed titanium (ABTi) honeycombsandwich is not subject to att ack by most service chemicals and that cqat ings can beused to provide additional protection against the.most severe environments.'The following specific conclusions were reached:' . . - .. . , .1. Bothstructuralandacoustic ABTi arestrengthenedandcorrosionresistance is

increased by prolonged thermal exposure, particularly exposure. in the range of590 K (600O F). . . .

2 . Bare titanium is susceptible to hot salt pittingy seawater at tempera tures of 700K(800O F) and higher.3. The ABTi system is not susceptible to stress corrosion, katress-accelerated corrosion,

or iquidmetalembrittlementdue t o pressurecontact of t i taniumwithsilver-plated nuts at temperatures up to 755 K 900F). . ' . .

4. The .ABTi system is not suscept ible to attack by the fuels, engine oils, deicers, orcleaning materials that will be encountered in ambient or elevated temperaturecommercial airline service.

5 . The titanium is subject to etching and hydrogen embrittlement when. exposed todrippingphosphateesterhydraulic luids at elevated emperature.Excellentresistance to this attack s provided by two coats of Hanovia Gold no. 6854.

6. Exposed aluminum braze fi llets can become corroded during prolonged service in amarine environment and should be proGcted with a suitable corrosion-resistantfinish.

7. Accelerated orrosion tests ndicate ome nherentenvironmentaldegradationmight occur during long-term use. Identification of proper inspection techniquesand/or corrosion protection maybe required for some specific applications.

8. The orrosionesistance of ABTi oneycom bandwichtructurewithnonperforated ace heets is totallyadequate orcommercialairline ervice.Satisfactory finishes have been developed to provide complete corrosion protectionto exterior braze fillets under ll anticipated commercial airline service conditions.

9. The inherent corrosion resis tance of acoustic ABTi honeycomb sandwich with oneor more perforated face sheets is satisfactory for commercial airline service wherethe structures will be frequently heated and shielded from direct contact with theweather.

10. Additionalwork is needed on finishes or nteriorbraze illets nacoustichoneycomb sandwich to .permit service n an unshel tered environment.

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1. Report No. 2. Government Accession N o. 3. Reclplents Cdtalog No .NASA-CR- 2 9 4 3I 4 . Tl t le and Subti t le 5. Reporta teEFFECTS OFSERVICE ENVIRONMENTS ONI ALUMINUM-BRAZED TITANIUM (ABTi)

7 . Author(s1 8. Performlng Organvation Report NoW. L. Cotton D6-44435-2

9. Performing Organization Name and Address 10. Work Unl t No.Boeing Commercial Airplane CompanyP.O. Box 3707Seattle, Washington 98124National Aeronautics and Space Administration Phases 11,111, and IVLangley Research Center 14. Sponsorlng Agency CodeHampton, Virginia 23665Langley Technical Monitor: W. Barry LisagorTopical Report

1 1 . Contract or Grant No.NAS1-13681

13. Type o f Report and Perlod Covered12. Sponsorlng Agency N am e and Address

15. Supplementarv Notes

1G AbstractThis programcontinued he ong-term lightserviceand etengineexhaust estsini tia ted under he DOT/SST follow-on program.Additional estswereperformed toevaluatedegrada tion of ABTi stru ctur eduringprolongedexposure oextremeenvironments: elevated temperature exposure to airline service fluids, hydraulic fluid,andseawater, followed by laboratory corrosion tests. Solid-face and perforated-facehoneycombsandwich panelspecimens,stressedpanelassemblies,andfaying-surfacebrazed joints were tested.This report covers the results of the extreme service tests, phases 11,111, and IV. Theextended service evaluations, phase I, ar e still underway, and are not treated in thisreport.This investigation showed tha t he corrosion resistance of ABTi is satisfactory forcommercial airline service. Unprotected ABTi proved inherently resistant to attack byall of the extreme service aircraft environmentsexcept: seawater at 700 K (800OF) andabove, drippingphosphate ester hydraulic luid at 505 K (450O F), and a marine

~ environment at ambient emperature. The natural oxides and depositspresent on1 titanium surfaces in airline service provide protection against hot salt corrosion pitting.

Coatings are required oprotect itaniumagainstdrippingphosphateesterfluid atelevated emperaturesand oprotectexposedacoustichoneycombpartsagainstcorrosion in a marine environment.

17. Key Words (Suggested by Author(s1 I 18 .Distribution Statement ~~ ~~ ~1 Titanium CorrosionHoneycomb sandwichProtective oatingsAluminumrazingkydrolBrazingervice Unclassified - unlimitedSubject Category 26Phosphate ester Hydraulicluid

19. Secu rity Classit. (of this rep ort) 21 .o.f Pages 22 . Price0. Secu rity Classif. (o f this page)

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Effects of Service Environment on Aluminium-Brazed ABTi