EliminationElimination ofof ChromatesChromates InIn PaintPaint PreparationPreparation Richard H. Buchi, Ken Paterson & Clyde J. Gowers

Reduction and elimination of From a corrosion preventive point in the metals finishing industry where chromate-containing wastes is a of view, continuing to use both the the corrosion protective properties major pollution prevention goal. chromate conversion and the imparted by chromium compounds One of the most pervasive uses of chromate-containing primer would be are unequaled. Treating aluminum material containing chromate is in preferable. Laboratory testing has with chromates to form a chromate the treatment of aluminum with shown, however, that as long as there conversion coating on aluminum is a chromate conversion coating. is chrome in the primer, the corrosion- common way to enhance corrosion Researchers at Hill Air Force Base protective properties of modern resistance and to prepare the metal to in Utah have found an environmen- aircraft systems will suffer little. accept paint. When the coating is tally friendly solution that leaves a A study of a substitute for the scratched—leaving the metal bare and thin film on the substrate, which chromate conversion coating was susceptible to corrosion—chromate passivates the surface and enhances conducted at Hill AFB. The candidate from the surrounding area is believed paint adhesion. The solution has and the process developed for its use to leach into the scratch, providing been accepted for the prepping of were tested extensively against the additional corrosion protection. This aircraft. This paper contains test current process. is a valuable conversion coating data and a model of the new In adhesion and flexibility tests, the function. Chromates in primer coating conversion coating, and covers new environmentally friendly process serve the same purpose. some of the Hill AFB work on the performed better than the chromate Elimination and reduction of cleaning of substrates. conversion coating. It performed chromates in Air Force maintenance equally well in all other testing. In operations has long been a goal of the Because chromium is a confirmed addition, it was found that this environmental community. Modern human carcinogen, reduction and candidate could eliminate two solvent aircraft coating systems consist of elimination of chromate-containing wipe-downs and the corrosion- chromated primer and tough wastes is a major prevention goal. removing compound used in the polyurethane topcoats. They do an Material containing chrome is used conventional paint preparation excellent job for corrosion prevention. extensively in the treatment of procedures. The use of this product As long as the primer contains aluminum with chromate conversion also reduces the need to sand anod- chromate, the paint system will . Chromate conversion ized surfaces before repainting. continue to provide very good coatings help prepare aluminum for Operational tests have been corrosion protection, even if the the application of paint, as well as conducted on several aircraft, and the chromate conversion coating is provide a corrosion preventive barrier. paint shop at Hill AFB is now using eliminated. Many vendors of propri- In aircraft paint systems, chromate this product on its production line. At etary products have been working on conversion coatings are used in the time of this study, more than 50 “non-chromate” conversion coatings, conjunction with modern epoxy aircraft had been painted using this and have made various claims for primers, which also contain chromate, product. The study recommended their effectiveness. Several factors to guard against corrosion. The expanded use of this product to make such non-chromate conversion primers are topped with a tough layer eliminate a major source of pollution coating desirable. of polyurethane paint. Landing gear, and hazardous waste. This new wheels and brakes of military aircraft product simplifies and reduces the • The process by which the conver- are similarly treated. paint preparation steps, saving time sion coating is applied is ineffi- ______and money in aircraft. cient, especially for large areas Editor’s Note: This is an edited (entire aircraft). The chromate- version of a paper presented at the Background containing compound is brushed, AESF/EPA Conference for Environ- Chromate (chromium VI) in all forms wiped or sprayed on the alumi- mental Excellence, held January 17– is known to be a serious environmen- num surfaces. A small portion 19, during AESF Week 2000 in tal pollutant. On the other hand, contacts the metal and forms the Orlando, FL. chromate compounds have many uses conversion coating, but the bulk

10 PLATING & SURFACE FINISHING Preparation Steps 1. Scrub with an alkaline cleaner 2. Hydroblast 3. Dry 4. Wipe clean MEK 5. Sand 6. Rinse with water 7. Apply a corrosion-removing compound 8. Rinse with water The Air Education and Training Command 9. Apply chromate conversion (AETC) tested Product D on various mili- tary aircraft, such as the T-37 (above) and coating the T-38 (right). 10. Scrub it on for five minutes 11. Apply second coat and dwell for one minute of it either drains off, is rinsed off This is a difficulty unique to 12. Rinse with deionized water or is wiped off. The operation aircraft rework, as opposed to 13. Let dry results in a hazardous waste original manufacture. 14. Prime stream that must be collected and Fig. 1—Preparation for paint process. treated or properly disposed. Testing for cleanliness on the • Because chromates are suspected surface of the aircraft to determine well to the fresh, porous anodize carcinogens, personnel must whether the surface is ready for paint coating. Repainting of anodized protect themselves from the is traditionally accomplished using aluminum is a different story. Paint chemicals by wearing protective the “Water Break Test” (T.O. 1-1-8). retained in the pores after stripping clothing and equipment. This test is fairly sensitive to oily adversely affects the adhesion of the • Special facilities are required for contaminants on the metal surface, if paint system. Adhesion difficulties large-scale chromate conversion conducted properly. The presence of were reported when repainting the coating jobs. surface-active agents or flooding of stripped aircraft using conventional the surface with water can give , but the problem intensified History erroneous results. For whatever when environmentally compliant This study represents a multi-year reason, painters were obtaining good paints were applied. The F-16 prime effort at Hill AFB to reduce or water break results, but the surface contractor was asked for recommen- eliminate the use of chromate com- was still not clean enough to accept dations. In 1990, it recommended that pounds in the paint-preparation the new high-solids primers. A joint the anodized coating be sanded off in process for aircraft, especially for the study was initiated with the paint shop order to obtain the required paint F-16s. To understand the approach, to determine when the surface was adhesion. Hill AFB engineers decided priorities and evolution of this project, clean enough for priming. The best that this solution must be a last resort. a brief history is helpful. simple measure of cleanliness came to If at all possible, the anodized coating In 1989, the paint shop at Hill AFB be the reaction of the surface with the needed to be preserved as a corrosion began to use the new volatile organic chromate conversion coating. When barrier. compound (VOC)-compliant primers the color imparted by the chromate Eventually, Hill discovered that and paints (commonly known as conversion coating was uniform and light sanding of the bead-blasted, “compliant” or high-solids coatings). fairly dark (depending on the alloy anodized surface removes residual The first efforts were unsuccessful and previous surface treatment), paint primer in the pores of the anodize. By because the new paints failed to give bonded well. removing the residual primer, good adequate adhesion. The Science and New cleaning and preparation adhesion is obtained. In practice, Engineering Laboratory was asked to procedures were established at Hill however, this “light sanding” is find a cause and cure for the adhesion AFB to consistently give a uniformly difficult to achieve. The visual problem. Use of the compliant colored chromate conversion coating indicator that the primer has been coatings was suspended until the paint (Fig. 1). Painters were taught to clean removed is difficult to see, and only adhesion problems could be resolved. areas again on the aircraft where the an experienced eye can tell. As a Studies conducted between the chromate did not take well, as result, the anodized layer is often laboratory and the paint shop revealed indicated by the color. For this reason, totally removed. Finding a way to two separate, but related, problems: one of Hill’s top priorities for a non- bond environmentally compliant paint chromate conversion coating was a without destroying the anodized 1. The cleanliness of the aircraft visual indication of cleanliness. coating became another priority in the was not sufficient for the The F-16 is manufactured with an search for a non-chromate conversion compliant primer to form a good anodized aluminum aircraft skin. The coating. bond. anodized surface has proven invalu- The non-chromate conversion coat 2. Bonding the primer to the able in the fight against corrosion. project was funded by a Pollution anodized surfaces of previously Initially, painting the anodize coating Prevention Project that began in painted aircraft was problematic. presented no problem. Paint bonds February 1996. The Science and

February 2000 11 Engineering Laboratory at Hill was aluminum panels that had previously commissioned by Hill’s Environmen- been painted and PMB-blasted to Table 1—Product A tal Management directorate to simulate the repainting of F-16s. Test Results conduct the study and manage the Uniform Fail project. Additional funding came Corrosion Resistance Color Consistent uniform color from another Environmental Manage- Preliminary laboratory testing could not be obtained, ment project, which focuses on indicated that none of the candidate especially when the improving the procedures and quality non-chromated products would application temperature of paint operations on the C-130 provide the corrosion resistance in a dropped below 85° F aircraft. This project was a logical salt spray cabinet required by Mil-C- Bonding Does nothing to enhance follow-on to previous laboratory 5541. After consultation, engineering paint bonding over common studies on paint adhesion. and corrosion personnel authorized a contaminants & on corrosion-resistance test on a primer previously painted surfaces Objective system, rather than on bare, conver- Corrosion Not tested The objective of the study was to sion-coated panels. After all, it is the Resistance eliminate the use and subsequent entire system—metal coated with disposal of chromate-containing conversion coating and chromated Ease of Fail materials in the paint preparation primer—that must resist corrosion. A Application Solution needs to be heated process on aircraft by substituting an 90-day test in a five-percent salt spray Hydrogen Not tested alternate, non-chromated material. environment was prescribed for this Embrittlement test. Kapton Wire Not tested Testing & Criteria Adhesion Passed wet tape Ease of Application Mil-C-5541, Class 1A, formed the Crosshatch rating=3b-4b performance basis for the non- Candidate coating products should be chromated product requirements. This as easy to apply and handle as the Flexibility Not tested specification calls for only three tests: current chromate conversion coating Surface Not tested materials. Ultimately, the success of Analysis • Corrosion Resistance any new product or process will • Paint Adhesion depend on acceptance by those who • Workmanship must use it. If the prep for paint Table 2—Product B process is made harder, it will be Test Results To meet the objectives, nine tests difficult to institute. Uniform Fail were conducted and used to evaluate Color No color was imparted to candidate products. Not all candidate Hydrogen Embrittlement the aluminum surface materials were subjected to all the This testing is not normally required Bonding Not tested tests. Indeed, all but one was elimi- for conversion coatings, because they Corrosion Not tested nated on the basis of initial testing. are applied, then given a short dwell Resistance time and rinsed off. This test was Uniform Color added because one of the candidate Ease of Pass Coating must produce some type of materials had a two-hour dwell time, Application color or visually detectable indication and it was deemed prudent to conduct Hydrogen Not tested that the coating has uniformly taken. embrittlement testing. Embrittlement Kapton Wire Not tested Bonding in Presence Kapton Wire Test Adhesion Not tested Of Known Contaminants Engineering requested that this test be Crosshatch rating=3b-4b The desired product should help added for candidate materials, even Flexibility Not tested create an adequate bond between the though the possibility of contact with paint and substrate. Hill had previ- aircraft wiring is remote. Surface Not tested ously completed extensive paint Analysis adhesion testing on the chromate Adhesion Testing conversion coating process. The Engineering asked that “Wet Tape objective of this study was to get Testing” (Fed-Std-141C, Method good paint adhesion with a new 6301.2) be accomplished on painted pared to results with standard chro- product. The paint removal process panels. The laboratory added the mate conversion coating treatment. for F-16s at Hill employs polymetha- “Crosshatch Test” (ASTM D-3359- crylate media blast (PMB). Blasting 93, Method B) to what was called out Surface Analysis with PMB leaves a residue on the in Mil-C-5541E. The final candidate The surface of treated panels was surface of the planes that is very was also tested extensively with the analyzed using electron spectroscopy difficult to remove. Failure to remove Hesiometer. for chemical analysis (ESCA). The this residue results in poor paint purpose was to identify surface adhesion, especially when using high- Flexibility changes in the aluminum substrate solids primers. The laboratory also Mandrel bend testing was performed brought on by the chemical treatment. tested candidate products on anodized on treated painted panels and com-

12 PLATING & SURFACE FINISHING Hydrogen Embrittlement Table 3—Product C Preparation Steps Because Product D requires a two-hr Test Results Uniform Pass 1. Hydroblast soak, it was deemed advisable to test Color 2. Spray on Product D & scrub the material for hydrogen embrittle- with pneumatic sander ment on high-strength steel. Four Bonding Pass 3. Rinse embrittlement coupons were tested Corrosion Fail 4. Apply second application of per ASTM F519-93 and four coupons Resistance Failed on 2024 & 7075 Product D with 180-grit scrub were tested per ASTM F519-97. The Ease of Pass pad (poles) & let dwell on the coupons were cadmium-plated 4340 Application surface for 2 hr (uniform waxy steel. Three of the four coupons film forms) endured 150 hr at a 45-percent load, Hydrogen Pass 5. Spray a third application of and all four coupons endured the Embrittlement Product D & scrub with poles required 200 hr at 75-percent load. Adhesion Pass 6. Rinse with deionized water The specification requirement is for Crosshatch rating=3b-4b 7. Let dry three of four to pass, so Product D 8. Prime would not be considered embrittling Table 4—Product D Fig. 2—Preparation for paint process using Prod- to high-strength, cadmium-plated uct D. Test Results steel. Uniform Pass Comparison of Product D Kapton Wire Testing Color Indicator is not a permanent To Chromate Conversion Coating color, but is a recognizable This test was conducted on Product D Uniform Color waxy film on the surface of as a precaution. Many maintenance Although no color is imparted to the the metal chemicals damage Kapton wire used metal by Product D, the waxy film in F-16s by breaking down the Bonding Enhances bonding of primer produced during the first application insulation and allowing arcing. After to previously painted of Product D (see Fig. 2) proved to be exposure to Product D, Kapton wire anodized surfaces & PMB a reliable indication that the surface is showed no signs of deterioration, and surface contamination ready to accept paint. no failure points were detected with Corrosion Pass impulse dielectric testing. Resistance Anodized, bare 7075 & 2024 Bonding Al-primed panels all passed Product D enhanced paint bonding to Adhesion 2000-hr salt spray surfaces contaminated by bead blast Paint adhesion is excellent on both Ease of Pass residue and oily fluids more than Product D and chromate conversion- Application Eliminates soap wash, current chromate conversion coatings. coated panels. Both passed the wet solvent wipe-down & acid Most important, Product D-treated tape test and both yielded a 5b brightener steps samples show excellent adhesion, (highest rating) on the crosshatch with little or no sanding when paint is Hydrogen Pass adhesion test. applied to previously painted, Embrittlement Further testing was conducted anodized aluminum. using a blade-driven adhesion tester. Kapton Wire Pass This instrument has been employed Adhesion Pass wet tape Corrosion Resistance previously by the Hill Laboratory to Crosshatch rating=5b The panels prepared with Product D evaluate paint adhesion. Tests on two Flexibility Passed 40-in. drop reverse performed as well as chromate different aluminum alloys, anodized impact test at 40% elonga- conversion-coated panels when coated aluminum and on panels that were tion; passed one-eighth in. with a chromated primer. This is an painted and stripped with PMB- mandrel bend indication that the Product D does confirmed paint adhesion on Product provide a corrosion-protective film. Surface Identifiable coating left on D-treated panels were as good as, or Product C, even though it imparts Analysis surface that promotes better than, standard chromate color to the aluminum, does not seem adhesion of primer conversion coating. to enhance the corrosion resistance at all. Flexibility Prior testing at Hill and at Wright Test Results Summary Ease of Application Laboratories has revealed that The laboratory tested four different One of Product D’s big advantages is chromate conversion coatings are not products alleged to be non-chromate the application process. More than extremely flexible. When painted conversion coatings that would give a just a drop-in replacement for a panels are tested to failure on a visual indication that the product was chromate conversion coating, it mandrel bend or a conical bend test, properly applied and the surface was eliminates several steps in the the coating failure usually occurs prepared to accept primer and paint. preparation process (see Figs. 1 & 2). between the chromate conversion The products are referred to in this Most notably, the solvent wipe, coating and the substrate aluminum. report as products A, B, C and D. alkaline soap wash and acid bright- In Hill AFB lab testing, chromate- Testing is summarized in Tables 1–4. ener steps, along with their associated prepared panels failed a one-half-in. rinses, are eliminated. mandrel bend test. On the other hand,

February 2000 13 panels treated with Product D passed to fly the same hours and to be usage of solvents, detergents and acid a one-eighth-in. mandrel bend test. subjected to the same environmental brighteners. On the F-16 aircraft, in conditions. particular, it will help preserve the Surface Analysis The AETC SAS conducted this highly effective, corrosion-resistant Appendix E details the current phase of testing at the 14th Flying anodize surface, because less sanding analysis of Product D-treated alumi- Training Wing, Columbus AFB, MS, is required to obtain the desired paint num surfaces. It shows that the for a period of 12 months. The final adhesion. surface has been modified in such a report was dated June 20, 1996. It is the recommendation of this way that increased adhesion is The results and conclusions of the report that the use of Product D on probable. study were that Product D provides military aircraft be expanded. The superior paint adhesion and equal affected aircraft should be examined Operational Evaluation corrosion protection as the current periodically, although no latent Air Education & Training chromate conversion coating process. defects arising from Product D use are Command (AETC) In addition, the report stated the anticipated. P&SF Prior to Hill AFB testing, Product D following positive comments of the was being evaluated by Air Education advantages of Product D: About the Authors and Training Command (AETC). In a Richard H. Buchi memorandum for HQ AETC/LG, • The paint is more resistant to is a process dated May 8, 1997, AETC reported stains. engineer at Hill on its experience in painting one T-37 • The aircraft is easier to clean AFB, UT, OO- and one T-38 aircraft with Product D during wash. ALC/TIELM, 7278 as a pretreatment. The T-37 was • There is less chipping and peeling 4th St., Hill AFB, painted in August 1996 and the T-38 of the paint surface. UT 84056-5205. in November 1996. AETC felt that • The paint and primer application He received a BS test configuration of this first testing is smoother. in chemistry from of Product D did not provide suffi- • Elimination of several hazardous the University of cient data to make an adequate chemicals. Utah, and has worked for the U.S. Air assessment. An amended test plan • Use same process on both Force for 33 years. During the last 10 was devised. The new test plan aluminum and magnesium. years, he has worked in the field of allowed Product D and a chromate • Less use of paper and tape. cleaning and preparing metal conversion coating to be applied on • Product D is environmentally surfaces for painting. He also has the same aircraft under the same friendly to both people and the experience in electroplating and painting conditions (heat, humidity) environment. landing gear repair. Ken Paterson is Hill AFB Operational Tests the chemical The first week in November 1997, the hygiene officer of right wings of two F-16 aircraft were the Science and prepared for paint using the Product D Engineering process. The remainder of each Laboratory at Hill aircraft was prepared using chromate AFB. He gradu- conversion coating. These aircraft had ated from Weber been fully stripped using PMB. After State University painting, the aircraft were returned to with a BS in their respective units. chemistry. He has worked for the U.S. Hill AFB laboratory representatives Air Force for 11 years, during which have examined these aircraft at six- time he has worked and developed month intervals. The last examination many projects relating to cleaning was September 1999. The entire paint and preparing metal surfaces for job on each aircraft was very good, painting. and the right wings were no excep- Clyde J. Gowers tion. is a lead chemist at Hill AFB. He Production holds a BS in As of September 1999, Hill AFB has chemistry from painted more than 50 aircraft using Brigham Young the Product D process. University, a BS in logistics manage- Conclusions & Recommendations ment from Weber Product D is a viable substitute for State University pollution caused by chromate conver- and a masters in engineering adminis- sion coatings in the paint preparation tration from the University of Utah. procedure for aircraft. It not only He has experience in metal extraction Free Details: Circle 108 on reader service card. eliminates chromates, it decreases the and electroplating.

14 PLATING & SURFACE FINISHING