ELECTRODEPOSITION OF PRIMER FOR AIRCRAFT DETAIL PARTS L7 $7 7 BdF

Robert R. Johnson

McDonnell Douglas Corporation

8th Annual Aerospace Hazardous Materials Management Conference

October 26-28, 1993 Sheraton San Marcos

Phoenix, Arizona

Hosted by Allied Signal

ELECTRODEPOSITION OF PRIMER FOR AIRCRAFT DETAIL PARTS

I. INTRODUCTION

The Clean Air Act Amendments of 1990 are going to limit the use of Volatile Organic Compounds (VOC's) and Hazardous Air Pollutants (HAP'S) in painting processes. Continued use of spray applied coatings subjects the user to repeated reformulation and requalification of new coatings. Furthermore, RCR/A and CERCLAl make the disposal of hazardous waste very costly and subject the generator to permanent future liability. The best solution is a technology which eliminates concern over future air reg ul at i ons, minimizes hazardous waste, while minimizing aircraft weight and improving quality.

The McDonnell Douglas Corporation (MDC) believes that one such technology is the electrodeposition of paint. Since the beginning of 1992, they have had an Independent Research and Development (IRAD) program to develop this technology for aircraft use. As will be seen, the electrodeposited coating, referred to as E-coat, shows great promise for the aerospace industry.

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11. THE PROCESS

Electrocoating is a well established technology, in use since the '60's in the automotive, appliance, and other high volume painting industries. It is a bath process whereby an organic is plated onto a conductive substrate. Parts are immersed in a bath of coating and electricity is applied across the bath.

All E-coats fall into two major categories, anodic or cathodic, depending on whether the part being coated is the anode or the cathode. Originally, all E-coats were anodic, but over the years cathodic E-coat was developed and has become much more prevalent in industry. It is commonly held that

RCRA is the Resource Conservation and Recovery Act dealing primarily with the regulations concerning the creating, handling, and disposal of hazardous waste.

concerned primarily with allocating liability for the cleanup of sites containing hazardous waste.

CERCLA is the Comprehensive Environmental Response Compensation Liability Act 1.

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cathodic E-coats provide better corrosion protection, especially on steel substrates. MDC did consider an anodic E-coat but the bulk of the work has been done with the cathodic type.

Figure 1 shows the chemistry behind cathodic E-coat. Basically, the induced voltage causes the ionized coating to migrate to the part. Electrodeposition of the coating onto the part is extremely rapid, with the entire part having some coverage in seconds. Immersion time of a part is usually 1-1/2 to 3 minutes. After the first initial seconds, the rest of the immersion time is used to build coating thickness and to compact the coating, squeezing out water. After the required immersion time, the part is immediately rinsed and oven cured.

In production, an E-coat system utilizes ultrafiltration to achieve closed loop rinsing. Figure 2 shows the basic components. In production, the bath is agitated by a pump-around loop. Part of the stream goes through an ultrafiltration unit which separates out a stream of permeate, which is clear, colorless, and 95+% water. (The non-permeate returns to the tank.) This permeate stream is then used as the next to final rinse, and previous rinse stages are fed by the rinse tanks overflowing back to the main tank. Because the bath itself is used to do the bulk of the rinsing, up to 98% utilization of coating is attainable.

In. BENEFITS

E-coating offers many inherent benefits to the aerospace industry.

A. Air Emissions - The baths are water based emulsions of 10 to 20 percent solids. The coating currently being considered contains about 85g/l VOC (with all VOC's also being on the HAP list), and environmental pressures on the E-coat industry are forcing research to ever lower VOC levels. Per square foot of surface area coated, spray painting emits at least 20 times as much HAP/VOC to the air as E-coat does.2

B. Waste Minimization - Since it is an immersion process with closed loop rinsing, it is common to achieve transfer efficiencies above 95% with E- coating. In contrast, the absolute best transfer efficiency that can be expected for the spray painting of detail parts is around 40%. This means

Assumptions: Mil-P-85582 primer at 340 g/l VOC content, 487 ft2mil theoretical coverage, 27% overall transfer efficiency, 46 wt. percent water. bath content, 279 ft2mil theoretical coverage, 95% overall transfer efficiency, half of VOC's are emitted. rest to drain.

E-coat at 85 g/a VQC

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Anode Reactions 2H20 - 4H+ + 02T + 48- Fe 4- Fe+++2e- R - COO-+ H+ .+- RCOOH

Cathode Reactions 2H20 + 28- - H2T + 20W

(Soluble) (Insoluble) c

d - FIGURE 1

I I ULTRAFILTER L -

DRAIN

FINAL RINSE

PERMEATE

that there is a lot of material being deposited on the paint booth and filters, resulting in the generation of hazardous waste. In addition, current primers and topcoats are two-component systems with limited pot life; all unused material is also hazardous waste. Assuming an average of 30% transfer efficiency and a 10% loss of paint due to pot life for spray painting and 95% transfer efficiency for E-coat, spray painting generates at least 5 times as much hazardous waste as E-coat per square foot painted.3

C. Non-chromated Protection - The electrodeposition process results in a tightly compacted and extremely adherent barrier film. As a result, it is possible to have a non-chromated coating that gives superior barrier corrosion protection and satisfactory protection when scribed. E-coats tested to date have far exceeded military and MDC salt spray requirements even with no chrome in the system.

D. Weicht Minimization - Complex aircraft parts which require spraying from several angles will end up with areas of excessive film thickness. E- coating gives a very uniform film thickness on the most complex part due to its nature: as an area is coated, i t eventually becomes too resistive and further film build is stopped automatically. Current follows the part to its inner recesses until complete coverage is attained. After this, further time at voltage will make the coating more and more uniform in thickness, until no more film build takes place. With sufficient immersion time, coating thickness is entirely controlled by the voltage applied to the part. Thus, E- coating applies only the amount of paint film required for corrosion protection and no more.

It is estimated that if all detail parts were E-coated on an F/A-18, weight savings would be at least 25 lbs per aircraft. This assumes that E-coat is equivalent to spray applied primer with 0.4 mils less coating, and that on average, 0.2 extra mils of coating are applied due to part complexity.

E. Material Cost Savings - Naturally due to much better transfer efficiency, there are substantial material cost savings with E-coat. Using current prices for E-coat and waterborne primers, the primer costs at least 7 times

Assumptions: Mil-P-$5582 primer, 4.3 lbs solids/gallon, 487 ft2mil theoretical coverage, 27% overall transfer efficiency. Waste doubled due to dry filters, kraft paper, trays, etc. E-coat at 279 ft2mil theoretical coverage. Racking stripped every time for anodized parts, every fourth time for conversion coated parts. racking is 1/4 area of parts on rack. stripper used.

Area of Stripped paint volume is quadrupled by bath

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as much as E-coat for the same amount of area covered.4

IV. MDC RESEARCH PROGRAM

MDC's objective is to find an E-coat which can be used to coat detail parts. The coating must be non-chromated and lead-free. The qualification issues for E-coat are:

a. The dry film must meet the requirements of MIL-P-85582, MIL- P-23377 and MDC internal material specifications MMS 423 and MMS 425 if no quality is to be compromised.

b. Film must be compatible with primers, topcoats and sealants currently in use.

c. Cure temperature must be 300 F or lower. (Most parts are a luminum)

d. Thick sulfuric acid anodize would have to be replaced with thin sulfuric. E-coat does not work well above 0.5 mil anodize.

e. Will rack marks need touch-up?

The bulk of the E-coating to date has been carried out in a one gallon laboratory process. Hundreds of test panels with various pretreatments have been coated and tested.

MDC also has a 100 gallon pilot facility for E-coating complete with ultrafiltration (Figure 3). In the big tank the primary thrust is to learn more about bath stability, throwing power, and develop racking techniques.

V. TESTRESULTS

A. G28AD012, a cathodic epoxy type. internal paint spec tests which this product has PASSED to date (7/20/93):

Standard MDC Material Tests - Most of MDC's work has been with BASF The following summarizes the MDC

1, Cross Hatch and Wet Tape Adhesion 2. Reverse Impact Flexibility ("80 in-lb. on thin sulfuric anodize) 3 Room Temp. Flexibility (1/4" mandrel) 4. Heat Resistance Flexibility, 1" mandrel after 12 hrs. at 350 F

Assumptions: W aterbome primer at 487 ft2 theoretical coverage, 27% overall efficiency. E-coat at 279 ft2 theoretical coverage, 95% overall efficiency.

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5 . 6 . 7 . Intercoat adhesion to primer (scribed tape test) 8 . 24 hr. Water Immersion 9. 30 day Water Immersion 10. Mil-H-83282 Immersion, 30 days, Room Temp. 1 1 . Lube Oil Immersion, 30 days, Room Temp. 12. TT-S-735 Immersion, 30 days, Room Temp. 13. Accelerated Salt Spray, S02, unscribed, 500 hours 14. Heat Exposed, Scribed Salt Spray, 336 hours 15. 3000 hour Scribed Neutral Salt Spray 16 . Humidity Resistance, 30 days at 120 F, 93+% humidity

Low Temp. Flexibility, Thermal Shock, -65 F to 120 F, four cycles

1" mandrel after 5 tars at -70 F

* It is worth noting that this product passed reverse impact flexibility at 60 in-Ib on deoxidized alclad. This illustrates the tremendous adhesion possible with E-coat.

B. requirement, and that is a 2000 hour filiform test on deoxidized alclad 2024-T3. conversion coated alclad, G28AD012 was tested to that and it did oass. Work continues to see just how long it will go in both tests.

Filiform Corrosion - G28ADO12 has failed only one MDC material spec

However since the military requirement is only 1000 hours on

C. Militarv-Onlv Tests - There are a few tests in MIL-P-85582 and/or MIL-P-23377 which are not done per MDC's internal material specification. G28AD012 has passed them all:

1. 2. 3. 4. 5. Dissimilar Metal (MIL-P-85582, 4.6.9)

Strippability (MIL-P-85582, 4.6.7, and MIL-P-23377, 4.7.7) Hot MIL-H-83282 Resistance (MIL-P-85582, 4.6.11, 150 F, 24 hrs)

Hot Lube Oil Resistance (MIL-P-85582, 4.6.11, 250 F, 24 hrs) Hot Water Resistance (MIL-P-85582, 4.6.6, 120 F, 4 days) V

It is interesting to note that even the dissimilar metal test was passed; more on this test later.

D. Non-chromated Pretreatment of Aluminum

One of MDC's ultimate goals is to have a non-chromated pretreatment system to go along with E-coat. sulfuric anodize, three alternative pretreatments have been screened on both bare aluminum and IVD aluminum:

To date, besides extensive work with thin

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1. Parker Cobamine 2. Sanchem 4000 3. Zinc Phosphate

G28AD012 has passed 3000 hours in scribed neutral salt spray with Thin Anodize (water sealed), Cobamine, and Sanchem 4000 on bare 2024-T3 aluminum. after 15 months (10,950 hours), and it shows no blistering or corrosion outside of the scribe. to meet the 3000 hour requirement.

In fact, there is still a 2024-T3 panel with Sanchem 4000 on it

The zinc phosphate panels, on the other hand did fail

Likewise, on IVD aluminum, Cobamine and Sanchem passed 3000 hours in scribed neutral salt spray while the zinc phosphate failed.

E. Non-cadmium Pretreatments on Steel

The same three non-chromated pretreatments have been screened on IVD aluminum on steel: week scribed salt spray test.

All three pretreatments passed a heat exposed, two

Three pretreatments were tested for 1500 hours in scribed neutral salt spray on bare 4130 steel. plating allowed the substrate steel to corrode. However, in the case of the Tin/Zinc, the corrosion did not spread beyond the scribe line, nor did i t blister the E-coat along the edge of the scribe. The third pretreatment, Corroban Nickel/Zinc with a chromate coating passed 1500 hours with no corrosion to the steel, blistering, or loss of E-coat adhesion.

In this test, zinc phosphate and a TinEinc

F. SO2 Salt Spray Testing

Unscribed - There is currently no military requirement for SO2 Salt Spray testing in either Mil-P-23377 or Mil-P-85582. The internal requirement for MDC's waterborne primer currently being used is 240 hours, unscribed. BASF G28AD012 has proven itself to be far superior to this primer in unscribed SO2 testing. Figure 4 shows a comparison between two sets of 4340 steel panels. Both sets were pretreated with a thin coating of IVD aluminum, and then Cobamine was used to seal the IVD. They were then exposed to SO2 salt spray for 500 hours. As can be seen, the waterborne primer chalked and flaked off, while the E-coat is unfazed. Similar results were obtained with other pretreatments.

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FIGURE 4: Comparison Between G28ADO 12 and Mil-P-85582 in SO2 Resistance

Scribed - There is currently no military or internal epoxy primer standard calling for scribed SO2 salt spray resistance. Nevertheless, MDC wanted to see how well E-coat would do compared to waterborne primer. It was found that E-coat on bare or clad 2024-T3 and bare 7075-T6 with either chromate conversion coating or thin sulfuric anodize, will go at least 7 days with very little corrosion of the scribe and no blistering of the E-coat. On thin anodized/water sealed 2024-T3 or 7075-T6, there is minor corrosion in the scribe and a few minor blisters (<1/16" dia.) next to the scribe, after 500 hours exposure. (It is important to note that essentially all non-clad 2000 and 7000 series alloys must be anodized on the F/A- 18 aircraft, per the Finish Specification). On conversion coating, there is corrosion and blistering up to 1/4" beyond the scribes after 500 hours.

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Figure 5 shows these panels.

By comparison MDC's current waterborne primer performs well on any alloy/substrate combination after 7 days. However, i t is badly deteriorated in scribed or unscribed SO2 after 500 hours on any substrate. On the other hand, a new "low density" Mil-P-85582 primer from Courtavlds went 500 hours in scribed SO2 with very little corrosion and no blistering, chalking, or loss of adhesion.

All of this adds up to G28AD012 being superior to the present waterborne primer in any SO2 resistance, but inferior to a new waterborne in scribed resistance. The question is, what should the requirement be for E-coat, and on what substrate/pretreat combination?

G. Dissimilar Metal Testing

All mil-spec epoxy primers are required to pass a dissimilar metal test where an anodized and scribed aluminum panel is mated against carbonlepoxy composite, and placed in neutral salt for 500 hours.

In previous reporting on BASF E-coat, it was stated that G28AD012 could not pass the military's dissimilar metal test. This statement was based upon an extrapolation of different dissimilar metal testing done at MDC, Le., the exact military test was not actually done. This year, MDC started looking at various formulations from BASF with additional galvanic corrosion protection directed at passing the dissimilar metal test. Upon passing easily with the first two formulations, it was decided to try the original G28AD012 again and it passed. The testing was repeated using a different composite with the same positive results. Thus, it can safely be said that G28AD012 will pass the dissimilar metal test. Figure 6 shows a

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panel after testing. It should be noted that the test was performed on thin sulfuric anodize rather than thick, and water sealed instead of dichromate. The composite face was also sanded down to be sure that fibers were exposed.

H. Compatibility Testing

G28AD012 has been tested with waterborne primer, Mil-C-83286 polyurethane topcoat, and Mil-S-83430 polysulfide sealant for compatibility. The following tests have been passed:

Primer:

Topcoat:

Sealant:

G28AB012 has Fluid. This test

Cross Hatch Adhesion Scribed Wet Tape Adhesion Room Temperature Mandrel Flexibility (1 inch mandrel)

Cross Hatch Adhesion Scribed Wet Tape Adhesion Humidity Resistance R.T. Mandrel Flexibility (0.5 inch mandrel) Low Temp. Flexibility (-65 F, 1 inch mandrel) Reverse Impact (60 in-lbs)

Salt Water Exposure Distilled Water Exposure

failed sealant adhesion when exposed to Jet Reference is being repeated at this writing.

I. Lower Curine! TemDerature

All test results shown are with BASF G28AD012 curing at 300 F for 30 minutes. Even though Alcoa strength data says there is no loss of strength at 300 F for any temper of 2024, 7075, or 6061 for a half hour, there is a general apprehension in the aerospace community about 300 F for aluminum. MDC would like to see the cure temperature lowered to at least 275 F. Preliminary work indicates that G28AD012 cannot go below 290 F without losing some MEK rub resistance.

J. Additional Galvanic Corrosion Protection

As a barrier to corrosion, G28AD012 is clearly superior to spray primer. scribed neutral salt spray it has gone over a year on Sanchem 4000. passes the military's dissimilar metal corrosion test easily.

In It

Even so, from

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testing on various substrates in scribed neutral and scribed SO2 salt spray, it seems that G28AD012 may not protect as well as a chromated primer. In order to close the gap, it is desirable to add more galvanic corrosion protection.

A fifth formulation from BASF using zinc technology is currently being screened in scribed S02.

K. Rack Mark Testing;

In May 1993, panels were placed in neutral salt spray with untouched-up rack marks. Figure 7 is an example of the type of rack marks left by an alligator clip on a 'test panel. The panel shown was anodized using the clip, and then E-coated without moving the clip. The resulting bare areas are less than 10 mils diameter. Similar panels which were conversion coated are also in test. The plan is to wait for at least a year and then analyze the extent of corrosion using metallography.

L. Practical Concerns

It is one thing to coat panels out of a one gallon beaker and quite another to use a 100 gallon tank. The first two times BASF G28ADO12 was in the 100 gallon system, the bath lasted only four weeks before it started to destabilize. It is now felt that microbes from the deionized water Source caused the bath pH to rise and thus destabilized the emulsion. Excessive shearing due to pump speed and pump inlet piping probably also played a part.

Since being filled the third time on 6/25/93, the tank has been used to study throwing power and racking technique. There are many questions to be answered in the 100 gallon tank before further scale-up:

1. What voltage and immersion times work best for each substrate/pretreatment combination? 2. How sensitive is thickness to anode to cathode ratios? the minimum and maximum part loading on a rack? 3. How often must racking be stripped? Can an E-coated rack go through chemical processing? 4. What types of racks are required? Shape? Metallurgy? 5. dumping any E-coat?

What is

What must the bath turnover be to sustain indefinitely without

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FIGURE 7: Rack Mark left from Alligator Clip on Thin Sulfuric Anodized Panel

Magnification: X8

A, I. c C'

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So far we have learned what microbe contamination can do to destabilize a bath. We have found that anode to cathode ratio becomes less important with longer immersion time at voltage. Most types of racks can be used repeatedly without stripping. In the E-coat industry, racks are used over and over until no-coats occur. At this time, we do not know whether we can allow a part to no-coat and just rinse and reapply E-coat. This is an important question to be answered.

VI. NAWC EVALUATION

At this writing, the Naval Air Warfare Center has agreed to evaluate G28AD012 test panels which MDC is supplying. If their testing is positive, actual aircraft trial parts would be the next step.

VII. CONCLUSION

In conclusion, MDC is looking at the electrodeposition process to solve the environmental problems associated with the spray painting of detail parts. Part of the goal is to find a coating and pretreatment system which is non- chromated, non-cadmium, and lead-free. Besides offering environmental solutions though, electrodeposition promises quality improvements in barrier protection, adhesion, and weight savings.

Testing results to date are extremely encouraging. Every single internal material specification test has been passed except filiform corrosion, and the military's version has been passed. Performance in SO2 salt is far superior to the waterborne primer currently being used at MDC, further illustrating the superior barrier to corrosion. Panels have been in neutral salt spray for over a year on Sanchem 4000 treated 2024-T3 with no blistering of paint on scribed panels.

Future work continues in the E-coat industry to lower the VOC and HAP content. MDC intends to influence the search for a lower curing material and the addition of corrosion inhibitors to provide additional sacrificial corrosion protection.

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