aqueous degreasing of skins and honeycomb core

Aqueous Degreasing of Skins and Honeycomb Core by T. A. Woodrow, E. Barnes, and S. I? Evanoff, Lockheed Fort Worth Co., Fort Worth, Texas

L o&heed Fort Worth Co. (LFWC) has replaced a trichlo- roethylene (TCE) vapor de-

greaser used to clean aluminum and composite skins and honeycomb core with a system that uses an aqueous alkaline cleaner. This project began in 1987 as a laboratory investigation of alternate cleaning methods.’ The goal was to develop cleaning methods with minimal hazardous material use and emissions, as part of the LFWC long- term goal of “zero discharge” of hazardous waste and emissions.233

Startup and shakedown of the new system began in January 1992. As of May 1994, 100% of all aluminum and composite skins and honeycomb core used at LFWC are being cleaned in the aqueous cleaning system. The materials used for the skins are aluminum, fiberglass composite, and graphite composite. The honeycomb core is constructed from aluminum or from fiberglass composite. Implementation of this aqueous system eliminated ap-

proximately 26 tons of air emissions and solid waste per year that were normally produced by the vapor degreaser that it replaced.

BACKGROUND

The aqueous alkaline cleaning system that was implemented for the cleaning of skins and honeycomb core used in the construction of F-16 aircraft bonded assemblies consists of one alkaline cleaner tank, one rinse tank, and two drying ovens. Each tank is constructed of stainless steel and holds 5,500 gallons of liquid. Each tank is serviced by a 250-gpm centrif- ugal pump. The fluid in each tank is circulated through 14 eductors on the bottom of the tank, which increase the flow of liquid in each tank to 1,000 gpm. The liquid in each tank is main- tained at 150 + 5°F. The cleaner tank is charged with a 25% solution of a proprietary aqueous cleaner in deion-

ized water, while the rinse tank con- tains deionized water. The contamina- tion level in the rinse tank is monitored with a conductivity meter, and the rinse tank is automatically flushed with deionized water when the conductivity rises to a predetermined level.

The core and skins to be cleaned are placed into stainless steel mesh baskets that are then lowered into the tanks by an overhead conveyor. The skins are suspended from supports on the top of the baskets using wire. The honeycomb core details are laid flat in the baskets so that the cells are oriented vertically. This allows the core cells to fill and drain freely. In addition, the core is covered with a plastic mesh to hold it in place during the cleaning process. The core and skins are held in the cleaner tank for 15 minutes and then in the rinse tank for 10 minutes. Spray nozzles are located on the sides of the cleaner tank freeboard and on the sides of the rinse tank freeboard so that the core and skins can be sprayed

ALKALINE DEGREASER

150-F

D.I. RINSE 15O*F

NITRIC CHROMIC

HF PRECLEAN

R.T.

1WF OVEN

RINSE suLFuRic, D.I. RINSE DICHROMATE DEOXIDIZER

: 140.17CF

Figure 1. Schematic of aqueous cleaning system.

40 0 Copyright Elsevier Science Inc. METAL FINISHING . JULY 1995

Lapshear Specimen

+= kk 3 Load

Load

Pi Flatwise Tension Specimen

Figure 2. Specimens for mechanical testing.

with deionized water as they are removed from the tanks, After rinsing, the core cells are blown out with fil- tered, compressed air and the core is dried for 30 minutes in an oven held at 250°F.

After aqueous cleaning, aluminum skins are chemically precleaned, rinsed, deoxidized, rinsed, and then dried for 30 minutes in an oven held at 140°F.

Fiberglass and graphite composite skins are not normally cleaned in the aqueous system before bonding. These skins are only cleaned in the aqueous system when their faying surfaces are accidentally contaminated after the peel plies are removed. Contaminated composite skins are cleaned in the aqueous system as described above, but are not exposed to the subsequent chemical cleaning and deoxidizing steps that aluminum skins receive. Af- ter being cleaned in the aqueous system, composite skins are dried for 30 minutes in an oven held at 140°F.

A schematic of the aqueous cleaning system is presented in Figure 1.

MECHANICAL TESTING

During the implementation process, mechanical testing was used exten- sively to determine if the faying surfaces of skins and of honeycomb core cleaned in the aqueous system were sufficiently clean for adhesive bonding.4 Two types of test specimens were used for this testing (see Fig. 2). Lap- shear specimens were used to demon- strate that skins weR sufficiently clean for adhesive bonding, while pi flatwise tension specimens were used to dem- onstrate that honeycomb core was sufficiently clean for adhesive bonding.

Aluminum, graphite composite, and fiberglass composite skins are used to construct bonded assemblies for the F-16 fighter at LFWC. Skins constructed from each of these materials were cleaned in the aqueous cleaning system and then assembled into lapshear test specimens (see Fig. 2). Each of the lapshear specimens was constructed using an inner set of spacers sandwiched between outer sets of skins. For this testing, the materials used for the spacers and for the outer skins were usually the same. One set of specimens, however, had aluminum spacers sandwiched between graphite skins (i.e., graphite/aluminum/graphite, or Gr/Al/Gr). All of the specimens were representative of bonded assemblies actually used on the F-16 fighter.

A thermoset adhesive was used to bond the outer skins to the inner spacers. The adhesive was cured by placing the specimens in a cavity press or in an autoclave and ramping the temperature up to the cure temperature of the adhesive, while maintaining pressure on the specimens. After the cure cycle, holes were cut into each end of the test specimens to accommodate a test fix- ture and five specimens of each type were tested at 75 or 270°F. The results of the lapshear testing (an average of five specimens of each type) are presented in Tables I and II. The lapshear specimens had to fail above a predetermined requirement in order to pass the test. These requirements were established based upon years of testing using lapshear specimens whose details were cleaned in a vapor degreaser using TCE.

Aluminum, graphite composite, or fiberglass composite skins are com- bined with aluminum or fiberglass-re-

Table I. Results from Testing of Lspshear Specimens st 75’F (Average of Fhre Specimens)

Specimen Test(@,J Requirement (psi)

Al/Al/Al 5,669 5,000 GrlAllGr 3,399 2,800 GVGVGI 2,451 1,650 GrlGrlGr 964 670

Al, aluminum: Gr, graphite; GI, @s.s.

inforced honeycomb core to construct bonded sandwich assemblies for the F-16 fighter at LFWC. Skins and core constructed from each of these materials were cleaned in the aqueous cleaning system and then used to produce pi flatwise tension test specimens (see Fig. 2). The flatwise tension specimens are cut from a sandwich panel that is made by bonding two skins onto the faces of a section of honeycomb core. For this testing, aluminum, graphite composite, or fiberglass composite skins were usually bonded to aluminum core. One set of specimens, however, had fiberglass-reinforced core sandwiched between fiberglass composite skins (i.e., fiberglass/fiberglass/ fiberglass, or Gl/Gl/Gl). All of the specimens were representative of bonded assemblies actually used on the F- 16 fighter.

A thermoset adhesive was used to bond the outer skins to the inner core. The adhesive was cured by placing the sandwich panel in a cavity press or in an autoclave and ramping the temperature up to the cure temperature of the adhesive, while maintaining pressure on the sandwich panel. After the cure cycle, cylindrical flatwise tension specimens were cut from the sandwich panel. Aluminum test blocks were bonded onto the faces of each flatwise tension specimen and five specimens of each type were tested at 75 or 270°F. The results of the flatwise tension testing (an average of five specimens of each type) are presented in Tables III and IV. The flatwise tension specimens

Table II. Results from Testing of Lapshear Specimens at 27O’F (Average of Five Specimens)

Specimen Test (psi) Requirement (psi)

Al/Al/Al 4,926 4,500 Gr/Al/Gr 2,536 1,560 GllGllGl 2,065 1,500 GrlGrlGr 1,155 660

Al, aluminum: Gr, graphite: GI, glass.

METAL FINISHING ?? JULY 1995 41

Table III. Rssults from Testing of Flatwlse Tension Specimens at 75’F (Average of Five Specimens)

specimens Test (Ibs) Requirement (IbsJ

AVAIlAl 5,666 5,000 Gr/A!lGr 3,000 GrlAUGr E 1,900 Gl/Gl/Gl 21877 2,000

Al, aluminum; Gr, graphiie; GI, gbss.

had to fail above a predetermined load requirement in order to pass the test. These requirements were established based upon years of testing using flatwise tension specimens whose details were cleaned in a vapor degreaser using TCE.

CORROSION TESTING OF CORE

During the implementation of the alkaline degmasing system for cleaning core, it was also necessary to dem- onstrate that the corrosion-resistant coating applied to aluminum core by the manufacturer was not damaged by the aqueous alkaline cleaning proce- dure. To accomplish this task, aluminum core that had been cleaned in the alkaline system was exposed to a 5% salt spray test in accordance with MIL- C-7438G.’ The amount of corrosion produced was visually compared with that produced on control specimens that had been vapor degreased using TCE. A gravimetric analysis on the experimental and control specimens was also conducted.

Five 3 X 3 X 0.750-in. specimens of aluminum core were cut with a band saw from a larger section of core and each was engraved with an identifica- tion number.

Three of the specimens were cleaned in the alkaline system as pre- viously described. In order to simulate the worst possible case, the core cells were not blown out with compressed

Table IV. Results from Testing of Flatwise Tension Specimens at 270°F (Average of Five Specimens)

Specimens Test (Ibs) Requirement (Ibs)

AVAIlAl 4,937 3,550 GrlAVGr 3,108 2,000 GrlAVGr 2,661 1,570 Gl/Gl/Gl 2,257 1,600

Al, aluminum; Or. graphite; GI, glass.

Table V. Salt Spray Corrosion Testing of Aluminum Core

Core Sample” Wt. After

(~0.0001 g) wt. Loss [Gain] Wt. Loss [GainJ Per Sq. Ft.

(2 0.0002 g) h@fV

A-l 15.0407 15.0426 A-2 14.0004 14.0040 KFl:; A-3 14.7050 14.7074 [o:oG24] T-l (control) 14.8583 14.8571 0.0012 T-2 (control) 14.5139 14.5120 0.0019

‘A, cleaned in akdirm system; T. deansd in tridbroeft@ne vq.w degreaser. bPass: Weight loss of 125 RN@’ of less.

air before being dried. At the time of this test, the rinse tank contained ap- proximately 0.06% of alkaline cleaner due to drag-out. After rinsing, the grossly wet specimens were dried for 30 minutes in an oven held at 250’F.

The remaining two core specimens (controls) were degreased for 10 minutes in a vapor degreaser using TCE.

All five specimens (three test specimens and two control specimens) were then dried in an oven at 250’F for 24 hours and were allowed to cool in a desiccator over calcium sulfate. The specimens were weighed to the nearest tenth of a milligram and placed flat (W-L axes parallel to the bottom of the chamber) in a salt spray chamber. The specimens were exposed to a 5% salt spray for 30 days in accordance with MIL-C-7438G. At the end of 30 days’ exposure, the specimens were removed and rinsed thoroughly in clear running water. Immediately after rinsing, the specimens were stripped of aluminum oxide corrosion by immersion in a phosphoric-chromic acid solution for 5 minutes at 212°F. The solution con- sisted of the following 103 ml of phosphoric acid, 76 g of chromic acid (CQ), and distilled water to make 3.75 L.

The specimens were removed from the solution, washed in distilled water, dried at 250°F for 30 minutes, and allowed to cool to room temperature in a desiccator over calcium sulfate. The specimens were then reweighed.

The loss of weight as milligrams per square foot of exposed foil area was calculated as follows:

M = 36C(O-A)A’LW

where M = Weight loss per square foot of exposed foil area in milligrams

C - Nominal cell size, ‘/s in. T = Thickness measurement in direc-

tion of cell axis, in. L = Ribbon length direction, in. W - Transverse direction, in.

0 = Original weight of specimen in milligrams before exposure

A - Final weight of specimen in milligrams after stripping

According to MIL-C-7438G, the core should not show a weight loss greater than 125 mg/ft! of exposed foil area.

RESULTS AND DISCUSSION

All of the lapshear and pi flatwise tension specimens passed their test requirements. This indicates that alkaline cleaning is capable of producing faying surfaces clean enough for adhesive bonding.

The results of the corrosion testing indicated that alkaline cleaning had no adverse effects upon the corrosion re- sistance of aluminum honeycomb core. Visual inspection after salt spray exposure, but before stripping, showed no corrosion on any specimens except for trace amounts of oxide on the edges that had been cut with the band saw. Oxide was present on both the test and the control specimens. After stripping, no visual evidence of corrosion was present on any of the specimens even when viewed under high magnifica- tion.

Gravimetric analysis of the specimens exposed to the salt spray indicated that none of the specimens (test or control) lost anywhere near the allowed weight loss of 125 mg/ft* of exposed foil area. The actual weight change per square foot of exposed foil area for each specimen is shown in Table V. Curiously, the specimens cleaned in the alkaline system actually showed a very small weight gain, whereas the specimens cleaned in TCE demonstrated a very small weight loss. It is unlikely that the weight gain seen with the alkaline-cleaned specimens was due to absorption of water during the cleaning process, as both the test and control specimens were exposed to

42 METAL FINISHING . JULY 1995

a high humidity environment for 30 days after cleaning. Although the ob- servations cannot be adequately ex- plained at this time, the magnitude of the weight changes is so small that they are of little significance to this study.

CONCLUSIONS

The following conclusions can be drawn from the experimental data:

1. The results of the lapshear testing and of the pi flatwise tension testing indicate that alkaline cleaning is capable of producing faying surfaces clean enough for adhesive bonding.

2. Based upon the results of salt spray testing, cleaning aluminum core in the aqueous alkaline system does not damage the corrosion-resistant coating on the core. The effects of a 30-day salt spray exposure upon core that has been cleaned in the alkaline system and upon core that has been cleaned in a TCE vapor degreaser were negligible in both cases.

References 1. Weltman, H.J. and S.P. Evanoff, “Re-

placement of Halogenated Solvent De- greasing with Aqueous Immersion Cleaners,” Proceedings of the 46th An- nual Purdue Industrial Waste Confer- ence, Lewis Publishing Co., Chelsea, Mich.; 1991 Evanoff, S.P., “Hazardous Waste Re- duction in the Aerospace Industry,” Chemical Engineering Progress; April 1990 Woodrow, T.A. et al., “Aqueous Alka- line Degreasing of Aircraft nbing,” Proceedings of the 1994 International CFC and Halon Alternatives Confer- ence, Washington; October 24-26, 1994 FPS- 1028D, “Standard Test Procedures for Reinforced Plastic, Adhesives and Core, Structural Potting Compounds, and Thermoplastic Sheet Material,” Lockheed Fort Worth Co., February 16, 1978 MIL-C-7438G. “Core Material, Alumi- num, For Sandwich Construction,” June 19, 1985

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aqueous degreasing of skins and honeycomb core

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