Control No. 17 Garson P. Shulman Alumitec Products Corp Del Am0 Blvd. Torrance, CA (3 10) FAX(3 10)

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1 1. Control No. 17 Organic Acid Sealants for Anodized Aluminum, A.J. Bauman & Thomas D. Brown+, Torrance, CA, & +Douglas Aircraft Co., Long Beach, CA Following the 1963 discovery by Kramer (U.S. Patent No. 3,510,411) that long chain carboxylic acids applied in the hot molten state greatly increased fatigue life and corrosion -resistance of anodized aluminum, we have developed products which provide corrosion protection with slight improvement in fatigue life when applied at ambient temperature (Shulman & Bauman, Metal Finishing, 93, No. 7, 16, July, 1995). A summary of our work was presented at last year's meeting in Denver, but based on discussions at that meeting, we recognized that further work was needed to solve certain problems and concerns. Conventional sealing of anodized aluminum results from conversion of an oxyhydroxide, boehmite, to aluminum trihydroxide. This process increases the volume of the oxide layer, closing the pores which make up about half its volume. Carboxylic acid sealants form a water repellant aluminum soap on the surface, while the pores remain open, but filled with residual acid. For high copper corrosion-prone alloys (e.g and/or 380 diecast), superior corrosion resistance as measured by ASTM B-117 salt spray testing of 2000 to 4000 hours can be obtained using isostearic acid. Certain conventional tests, Dye Stain and Acid Dissolution, which measure the integrity of a solid oxide coating, are not applicable to this organic type of sealing. Electrical Impedance Spectroscopy and Galvanic corrosion tests are somewhat better at measuring performance of our coatings. Fatigue data reported last year showed extremes in random scatter. This may have been due to wear on the sample holder of the Sonntag tester used. Since the sample holder was replaced, data scatter has been within acceptable limits of the ASTM procedure. Carboxylic acid sealed anodized specimens retain the fatigue properties of raw aluminum; chromate treatment causes a significant decline. We believe that the residual porosity of the organic filled layer makes it more flexible than a non-porous hot water sealed aluminum trihydroxide coating. Scanning Electron Microscopy (SEM) showed that the occasional very high fatigue values occurred in samples which had not delaminated from the metal surface. This delamination, rather than cracking in the oxide layer was the principal failure mode. A layer of carboxylic acid coated the aluminum after delamination. Liquid high molecular weight acids are similar to light mineral oil in their physical properties. After immersion of the aluminum in sealant, an oily film adheres to the surface. In the laboratory this film can conveniently be removed by wiping with a soft cloth or paper towels--this method is obviously not satisfactory for large scale production. N-methylpyrrolidone (NMP) is a good solvent for the excess acid, but used full strength it also removes material from the pores. A 1

2 Control No. 17 mixture of 30 to 40% water in NMP allows for excess removal without extracting liquid from the pores. Recovery of the dragout acid is achieved by adding water, then skimming the oily liquid which separates. This liquid may be returned to the sealing tank. The aqueous phase can be heated and/or sparged to remove water, then the residual NMP can be recycled. Alkaline cleaners will also remove the excess. Special care-control of concentration, temperature, and contact time-is needed to prevent extraction from the pores. For small parts, tlunbli withabsorbent~materialshas proven satisfactory. Mineral absorbents (Jonny Cat cat litter), sawdust, and ground cork have been used successfully. In addition to the standard Douglas AircraR exterior paint procedure, methods for coloring and protecting interior surfaces were developed. Following up on the NMP excess removal, it was found that 10% to 25% solutions of isostearic acid in NMP could be used to seal anodized aluminum, with some loss of corrosion resistance. The NMP can be removed with a water rinse. Surfaces treated in this way will accept epoxy primers or paints without further treatment. A preferred method of imparting color is with use of water soluble dyes before sealing. The amount of dye and sealant needed to fill the pores of an anodized coating 0.3 mil thick is the equivalent of 0.15 mil, compared to 1 to 2 mil for paint. The saving of 85% to 95% of the weight of the paint amounts to over 1000 pounds saved in coloring the interior (often unexposed) surfaces of a large transport aircraft. Salt spray life afker dyeing and sealing is at least 1600 hours, roughly equivalent to that of a painted surface. In response to a request for a sealant which would not smudge oil soluble inks and dyes, we formulated an emulsion containing only 10% isostearic acid. Use of 83% water in the product obviously reduces cost per gallon significantly. Adequate (480 hour) salt spray resistance is retained. Searching for a method of reducing costs, we investigated coating by passing aluminum through a layer of isostearic acid floating on water. Good coatings were obtained in this way. Salt spray exposure of 1938 hours without failure resulted. 2

3 Control No 17 Alumltec Products Corp. ALLOY SEALANT TEMP SALT SPRAY FAILURE fhrs) (million cycles) 2014T6 W a t e r 180-F Stearic Acid 130-C T6 W a t e r 180-F Stearic Acid 130-C 7075T6 W a t e r 180-F Stearic Acid 130-C IR Kramer & CF Surrows. U.S. 3, TABLE 2. SALT SPRAY TESTING OF 2024 SOFT ANODIZED ALUMINUM TREATED WITH CHELATING AGENTS DISSOLVED IN ISOSTEARIC ACID CHELATING AGENT AMOUNT TIME TO FAILURE (%) (5 Pits) (hours) BZA* BZT+ BZT BZT BZT BZT COBRATEC BZA PRETREATMENT BZT** saturated SATURATED * = BENZOTRIAZOLE-5-CARBOXYLIC ACID + = BENZOTRIAZOLE ++ = PROPRIETARY TRIAZOLE CHELATING AGENT ** = SANODAL DYED BEFORE SEALING 3

4 Control No. 17 Garson P. Skulman ACCELERATED TEST1 NG (2024) METHOD RESULT ELECTRICAL IMPEDANCE SPECROSCOPY M EGOHM/CM' DYE STAIN SOME PASS/SOME FAIL ACID DISSOLUTION 416 MG/DM' SALT SPRAY HRS 4

5 FATIGUE SAMPLES RUN AT DOUGLAS LONG BEACH. FATIGUE RESULTS OF ANODIZE SEAL B, C 792 D t CYCLES AT 25,000 POUND AVERAGE KEY: A. Baremetal. B. Anodized and chrome sealed.* C. Highest lot of organic seal. D. Lowest lot of organic seal. * Present production method. 800 n w - 0 v 4 9 w CI 0 3 rc -! 0, z P

6 Control No. 17 Carson P. Shulman Alurtitec Products Corp Dei Am0 Blvd FATIGUE DATA SOURCE TREATMENT CYCLES #of TESTS APC DANKO-APC 522K 5 MD DANKO-APC MD NONE MD Cr Deox-H,SO,-APC MD MD MEG-APC MD NONE MD Non-Cr Deox-H,SO, APC MD MD Paint Prep Danco = Nitric Acid Deox-NaOH rinse-h,so, Anodize MD Manufacturing = Chromate Deox-Chromic Anodize-Chromate Seal 6 6

7 Control No. 17 ISOSTEARIC ACID & NMP RECYCLING I I WATER ALUMITEC D EVAPORATE 7

8 DOUGLAS EXTERIOR PAINT PROCESS Control No SOLVENT WIPE-MEK/IPA 2. SCOTCH-BRITE ABRASION 3. BRUSH ALODYNE 4. WATER RINSE 5. DRY 6. PAINT: EPOXY PRIMER, URETHANE TOPCOAT PAINTABILITY OF ANODIZED ALUMINUM SEALED WITH ISOSTEARIC ACID-NMP SOLUTIONS ALUMITEC D SALT SPRAY WET TAPE TEST (%I (-1 (E ) PASS PASS PASS FAIL FAIL COLOR ANODIZE/SEAL VS PAINT WEIGHT SAVING: 94% FOR 0-25 MIL COMPARED TO 2 MIL PAINT COST SAVING: $1.60/1000 SQ FT COMPARED TO $50 AT 50%VOC LABOR/TIME SAVING BETTER PERFORMANCE: SCRATCH/GRAFFITI RESISTANT ENVIRONMENTAL IMPROVEMENT: NO VOC/NO CONVERSION COAT 8

9 Control No. 17 CO-AUTHORS A.J. Bauman 524 Oakdale Drive Sierra Madre, CA Phone (818) FAX (310) Thomas D. Brown Douglas Aircraft Co. MS Lakewood Blvd. Long Beach, CA

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