bonded aluminum construction

Discussion in 'Sheet Metal' started by Georden, Dec 5, 2006.

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  1. Dec 5, 2006 #1

    Georden

    Georden

    Georden

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    looking for more information on this type of construction, advantages, disadvantages, etc.

    I'm working on the design for my ultimate personal plane, which was going to be built from composites, but now a future move looks like it may have to be stored outdoors in a hot place so I am concerned about the durability of composites and considering switching to aluminum construction.
     
  2. Dec 5, 2006 #2

    wally

    wally

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    One of the many wonderfull things about building your own airplane is you can do it any way you want.

    However I am not sure that bonded aluminum would be the best for a homebuilt. I have worked in and around aerospace industry for a good while and I would say use rivets. You can work as slow or as fast as you want and stop and start at any point. With adhesives, once you start an assembly, you either finish it or you have a big mess. And with riveted aluminum, you could literally go fly after you pound the last rivet. No need for special paints or cure times or even any paint at all!

    The metal preparation and elevated temperature cure times to structurally bond aluminum make it a challenging undertaking for even a large company. The special adhesives are expensive and must be carefully mixed and cured as well.

    If you want slick on the outside, you should see the Van's RV planes the builders are finishing. Flush riveted (relatively) simple to build and most will fly over 200mph.

    And for inspections, making repairs and/or changes, riveted construction makes so much more sense. You can build a very fine airplane that is fast as you want with rivets and will be durable outside as well.

    Just my .02 worth.

    Best wishes and enjoy.
    Wally
     
  3. Dec 5, 2006 #3

    Georden

    Georden

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    thanks for the info. i was a bit suspicious after searching for myself and turning up nothing, got me thinking there was probobly a reason it's not done more often.
     
  4. Dec 5, 2006 #4

    orion

    orion

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    Bonded aluminum has not been utilized in the homebuilt industry simply because the processes were somewhat complex, the chemicals used were environmentally unfriendly and also, quite expensive. The combination of these factors limited the bonding of aluminum only to very special projects, usually handled only by the major aerospace organizations.

    Today though there are more efforts being made at cleaning up the processes and simplyfying the procedures through the development of unique bonding agents that are capable of achieving some rather impressive properties, even with little or no surface preparation. The primary result of these developments that could have application to the homebuilder are adhesives formulted specifically for aluminum, namely the Methylcrylates. This is an epoxy like product (color, handling, cure, etc.) that forms a very strong and tough bond with aluminum by chemically reacting with the surface, thus achieving the prep and the bond in one step. I am not privy the the specific chemical reaction(s) that take place but having tested a couple of these products within the last two years or so, I am very encouraged that bonding of homebuilts is possible.

    But in doing so proper design is still required, as are proper procedures for jigging and assembly. Most still recommend a few rivets in order to minimize the chance of edge peel and to act as sort of a bit of extra insurance. The leading product line for this application are the compounds developed by Extreme Adhesives and by Plexus, although I have not done any aluminum testing with the latter. The bonds achieved with the Extreme products were however very impressive.
     
  5. Dec 7, 2006 #5

    pilot103

    pilot103

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    What kind of adhesive is used on planes like the Kitfox? They bond plywood ribs to an aluminum spar. And this summer I saw a Bede exhibit at a flyin. It looked to me like they were bonding a sandwich rib to a tubular aluninum spar. And I think the skins were bonded too.
     
  6. Dec 7, 2006 #6

    George Sychrovsky

    George Sychrovsky

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    Just because they are doing it doesn’t mean it’s the ok and nobody will get killed some tome later because of it. Think about it

    George
     
  7. Dec 19, 2006 #7

    BD5builder

    BD5builder

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    Does everyone here forget the BD-5?

    The ENTIRE aircraft is assembled using "pro-seal B2" a aviation fueltank and pressurized bulkhead sealant / bonding agent. Granted it isn't assembled with just the pro-seal it also uses the Avex style pop rivets. It is an extremely simple method (ok, its still messy as hell), but is extremely strong if properly engineered. As in the case of the BD-5, there has NEVER been a structural failure of the bd-5. There is even one case of Bob Bishop pulling approx. 14+g's on the backside of a highspeed loop when the canopy opened and it inflickting a permanet 3' (YES FEET!) deflection of the wings at the wingtips and he landed the plane safely...

    Soo.. now whats so bad about a properly designed bonded structure?
     
  8. Dec 19, 2006 #8

    orion

    orion

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    There is nothing wrong with any properly designed bonded structure, be it composite, wood or aluminum. The trouble is that today there is vey little or no information available for proper aluminum bonding procedures, nor for any anticipated bonding properties - the candidate materials that one could possibly investigate for this application are not at all documented for the process. Although the Methylcrylates I mentioned above were formulated for this and are now commonly used in the automotive industry, even for highly loaded parts (door hinges attached to kingposts, body panel joints, hood and trunk reinforcement, etc.), there is little or no publically information available that could be used for responsible design.

    There is no reason this type of process couldn't be utilized for the construction of an airplane. Grumman did it years ago, although their process is rather expensive and requires baking at high temperature. I personally did start down the road of testing the various products and aluminum alloys but as of yet, I do not have sufficient data to risk someone else's life.

    Regarding the BD5, yes it does use ProSeal throughout but keep in mind that the product is a sealant, nothing more. It's actually there more as an insurance policy since it provides good damping to the sheet metal and thus reduces the chance of the rivets from shaking loose. But most of the BD5s and BD5Js I know are actually assembled with normal structural rivets through out, not pop rivets.

    About three years ago I did a series of bonding tests that worked at joining a glass/graphite laminate with a coupon of aluminum. The test utilized several bonding agents, including three varieties of ProSeal. The report however does not list the ProSeal results since in each case once a very minimal amount of edge peel was initiated, the laminate simply fell apart with no additional help. As such, the numbers were so low I didn't feel the need to even mention the product.

    Regarding the pullout, it is amazing that the more a story gets told the bigger it gets. I remember when this happened but I don't recall the exact figures (but they weren't even close to those magnitudes). I got a call in to his organization - I'll post the actual figures when I get them. However, be assured that the BD5 is incapable of loads much beyond normal category. Besides, it doesn't have enough wing to actually pull 14 Gs, even on the back side of a loop and, if the wings did actually bend 3', the airplane would have left a pretty big hole in the ground just due to the lack of resultant lift.
     
    Last edited: Dec 19, 2006
  9. Dec 21, 2006 #9

    orion

    orion

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    OK, just got a call back from a friend of Bishop's who is familiar with this incident - according to him (his words) the maneuver most likley resulted in 14 turds in his pants, not 14 G's on the airplane.

    The actual load was a something over 4 G's, resulting in about an additional three inch dihedral, although my source did not have the actual measurement with him. Given the 3.8 G limit design load on the airframe, it is logical that a yielding somewhere in the structure would occur at over 4 Gs. The 3" bend in the wing however does sound a bit excessive - I would've expected something on the order of one inch or so, so close to the limit load.
     
  10. Dec 21, 2006 #10

    Falco Rob

    Falco Rob

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    Orion !!

    Do I detect a sense of humour lurking somewhere in all that techno talk ??
     
  11. Jan 23, 2007 #11

    MalcolmW

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    Aluminum bonding should have a prominent role in the construction of experimental aluminum aircraft, however, it plays a distant second fiddle to riveting. In commercial and military aircraft, the mighty rivet sings a faint song to the chorus of adhesives and composites. Why, in the case of experimental aircraft, which should be the cutting edge of general aviation, does this role reversal take place for fastening and assembling materials?

    Many say that adhesives are difficult to work with and are not as strong as rivets and mechanical fasteners. Yet the military and commercial aviation fields have adopted them with, yes, enthusiasm. An adhesive bond between two pieces of metal distributes the load more or less uniformly, with out any stress concentration or stress risers. Inherently, this produces a longer lasting and much stronger joint.

    However, that’s not the only benefit, for a lap joint bonded with adhesives is weather-proof (no fluids can leak between joint), is smoother and aerodynamically more efficient. In addition, if properly engineered, aircraft constructed using adhesives for the majority of the mechanical joints, will be lighter in weight than a comparable aircraft built using rivets.

    As for the difficulty of using adhesives, much work has been done to remove the mystery from the technique. Proper surface preparation is the single most important factor to obtain a strong, long lasting adhesive bond.

    MalcolmW
     
  12. Jan 23, 2007 #12

    MalcolmW

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    Aluminum honeycomb panel is a case of using adhesives by default. For example, the F-111 jet fighter used aluminum honeycomb panels throughout its structure, bonded together with an epoxy-novalac adhesive that withstands the elevated temperatures experienced in its flight regime. This adhesive, produced by the BFGoodrich Company, bonded so strongly that at room temperature, the aluminum honeycomb fails in a peel test.

    Today, there are multiple varieties of honeycomb composite panels available from many vendors, ranging from stainless steel, aluminum, fiberglass, nomex and paper composites. The range in properties and strengths is very wide. In addition, the range of adhesives available to bond aluminum continues to grow.
     
  13. Jan 23, 2007 #13

    orion

    orion

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    I agree wholeheartedly. I would however distinguish between the bonding agents offered for joining cored panels and other aerospace components (which often require elevated temperature cures) and those that are available for retail sales. There's not too many homebuilders who have access to the tooling and ovens necessary to cure an entire bonded assembly, using these materials.

    As I mentioned in an earlier thread, yes there are potential bonding agents that have excellent properties and exhibit great potential for aluminum bonding. The problem however is that there is no published data that you can reliably use for the design of said structures. With a rivet, you know exactly the strength, its assembly requirements and how it will do in service. There is however no similar database for any adhesive that would allow me, as an engineer, to write a specification that would assure a builder that his or her structure will last the next twenty or thirty years.

    Now, that's not to say that the structure won't last that long, but to date there is no data to verify these properties. Furthermore, given the cost of some of these adhesives and the equipment you might need to dispense them reliably, it is likely that at least for now, builders would more likely select the rivet than the bonding agent for their aluminum aircraft.

    Personally, I'm still playing with the idea and as soon as I can free up some time I'll get back to my own project, but for now, it doesn't look like too many kit producers will be going this way any time soon.
     
  14. Jan 23, 2007 #14

    MalcolmW

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    Hmm, well, if you have to bond up the honeycomb panels, I would certainly agree with you that is a difficult task. However, there are many vendors of panels (already assembled) with known engineering properties. As for the strength of lap joints (the most common adhesive configuration), that is not a difficult task to calculate its strength and load bearing capabilities. Sure, elevated temperatures speed the cure and produce more strength. There are many epoxy adhesive systems that will produce a high strength bond using a room temperature cure.

    I believe that in experimental aircraft, adhesives can produce lighter and stronger aluminum aircraft. With proper preparation, aluminum can be bonded with readily available adhesives to produce strong and durable joints. The role of adhesives in aircraft can be expanded through the use of prefabricated honeycomb composite panels which have very high strength to weigh ratios, and in properly designed configurations, outperform riveted assemblies.

    The majority of honeycomb composite panels are bonded together under carefully controlled conditions, producing uniformly strong and dependable building materials. Aircraft built using these panels can have significant weight savings over sheet metal riveted to stringers. In addition, the hand labor component of assembly, i.e., construction time, should be much reduced. In the case of kit built aircraft, the use of CNC cut honeycomb composite panels should reduce kit costs. Consequently, the time for adoption of adhesives in the construction of experimental metal aircraft seems to me, long overdue.

    Aluminum honeycomb (.020
     
  15. Jan 23, 2007 #15

    MalcolmW

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    Hmm, well, if you have to bond up the honeycomb panels, I would certainly agree with you that is a difficult task. However, there are many vendors of panels (already assembled) with known engineering properties. As for the strength of lap joints (the most common adhesive configuration), that is not a difficult task to calculate its strength and load bearing capabilities. Sure, elevated temperatures speed the cure and produce more strength. There are many epoxy adhesive systems that will produce a high strength bond using a room temperature cure.

    I believe that in experimental aircraft, adhesives can produce lighter and stronger aluminum aircraft. With proper preparation, aluminum can be bonded with readily available adhesives to produce strong and durable joints. The role of adhesives in aircraft can be expanded through the use of prefabricated honeycomb composite panels which have very high strength to weigh ratios, and in properly designed configurations, outperform riveted assemblies.

    The majority of honeycomb composite panels are bonded together under carefully controlled conditions, producing uniformly strong and dependable building materials. Aircraft built using these panels can have significant weight savings over sheet metal riveted to stringers. In addition, the hand labor component of assembly, i.e., construction time, should be much reduced. In the case of kit built aircraft, the use of CNC cut honeycomb composite panels should reduce kit costs. Consequently, the time for adoption of adhesives in the construction of experimental metal aircraft seems to me, long overdue.

    Aluminum honeycomb (.020” skins, ½ inch thick) has sufficient strength and stiffness for fuselage and wing rib construction. For example, depending upon wing loading and length, .064” skin aluminum honeycomb should be strong enough for wing struts.

    Let's face it, commercial and military aircraft have used these materials and techniques for many years, and their structural reliability is very high. Sure, no amateur constructor will go to the same degree as a, say Boeing, however the techniques for adhesive bonding aren't that difficult to master. Certainly banging rivets takes some practice (or so I've heard).
     
  16. Jan 23, 2007 #16

    MalcolmW

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    Ooops! Looks like I've got to learn how to use this system. My apologies for the double entry.

    Back to the adesives... I don't believe exotic adhesives are needed. After all, experimental aircraft don't go fast enough to get skin heating... well, at least not yet.

    There are several families of adhesives that work well with aluminum, and the preference of use depends upon the builder.

    If ultimate strength is required, then the epoxy family, combined with a chromate conversion surface preparation would probably be the best choice. There are many epoxies that have high strength, good impact resistance and cure at room temperatures for this role. To fully exploit the performance of epoxies, the use of a few pull rivets to locate pieces and ‘clamp’ them in place during the cure simplifies the process. To permit an unrushed assembly of specific components, pre-mix (B-stage) the epoxy resin and hardener and refrigerate it to slow or extend its cure time.

    For a more forgiving and less demanding method, a simple surface preparation (Jasco Prime & Prep, or similar) combined with a polysufide adhesive is recommended. This is the well-known ‘Pro-Seal’ material, which comes in a number of viscosities from a number of vendors. The key to a strong bond is uniform application to a clean surface, followed by clamping during the cure. Again, the use of a few pull rivets can line up the pieces and hold them in position.

    Examples of experimental aircraft constructed using aluminum honeycomb panels and adhesives are far and few between. Jim Bede is using this technique with the BD-18 aircraft, which is not yet available in kit form. Also the Grumman series of aircraft (Yankee, Lynx, Tiger) use aluminum honeycomb panels in their construction. There may be others.

    An excellent example of the potential of adhesives in aircraft construction is given in Barnaby Wainfan’s P.A.V.E report (NASA) : http://members.aol.com/slicklynne/pavereport.pdf . It is a design for a lifting body aircraft using honeycomb composite panels in its construction. This design illustrates the reduction in hand labor and lower weight of a structure that exploits adhesives bonding aluminum and other materials together. To my knowledge, Mr. Wainfan has not advanced this design into production in either prototype or kit form.
     
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  17. Jan 23, 2007 #17

    Captain_John

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    HA! This reminds me of the time Spock made a joke on Star Trek!

    :roll: CJ
     
  18. Jan 23, 2007 #18

    orion

    orion

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    I agree that using prefabricated sandwich panels is a good basis for aircraft construction. Most people around our industry however do not consider those panels a great benefit since any structure made from the material ends up being rather boxy. Several years ago my company investigated an alternative to the boxy structures with the incorporation of inner skin kerfing (to fiberglass based sandwich panels - my regular supplier is Teklam). This technique produced a series of tapering and/or closely spaced (~3/4") longitudinal cuts that allowed us to form the material to nice rounded shapes (for forming the fuselage). A problem though evidenced itself when we went back to restore the inner skin integrity by laminating over the kerfs - the weight penalty was beyond justification since excess resin had a tendency to pool in the grooves. While this might have been solved eventually, the extensive amount of work involved was determined to be beyond the interest of most homebuilders (project was scrapped).

    But in general I am a supporter of this type of application and am still working toward the incorporation of this type of structure to fuselage assembly (albeit slowly due to other work). As you say, the Grumman line of aircraft was a great example of the type of simple structure that can be made but again keep in mind that the surface prep of the Grummans was rather expensive, and required oven curing of the adhesive. Long term use has also shown areas of bond failure, which now has to be repaired using rivets.

    In short, to date I have not seen any evidence that any epoxy generally available would work in this application, regardless of the surface prep you might want to use. I am very familiar with the numerous materials that are used within Boeing (used to work there) an othe aerospace applications and I would not recommend any to the homebuilt arena.

    The major issues in bonding aluminum include of course surface prep, but also longevity under GA service environments (vibration, temperature swings, hangar rash, etc.), inspectability, repairability and of course, human factors (some of the good bonding agents and surface preps are really caustic). The biggest problem evidenced so far is edge peel and degradation of bond strength as a function of edge contamination. So far I've investigated and tested at least two dozen different materials for bonding and in all that have come up with only one formulation that I would come even close to trusting over the long haul. And no, it isn't an epoxy.

    Also a word of caution, while ProSeal is an excellent sealant, we have tested four different variations of the PolySulfide about six years ago - none came even close to being something that I would recommend even for a trial. It's a good sealant but nothing you would want to use for bonding of a primary structure.

    So yes, bonding an airplane together, even an aluminum one, is sort of the Holy Grail of the industry. To date though, I have seen little evidence that would allow me to conclude that this is something I would want to try in the GA and/or homebuilding environment. Obviously we have systems that work well for composites - but for aluminum I think we still have a bit of work to do before we come up with a system that will enable the average homebuilder to apply the technology with minimal risk.
     
  19. Jan 23, 2007 #19

    MalcolmW

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    Yes, I agree, flat panels are not conducive to slippery shapes. However, there are ways to combine panels with sheet metal to reduce the ‘boxy’ appearance. Composites certainly are the way to go for aircraft with a ‘svelte’ look. You make a very good point about surface preparation

    I do have a little experience with aluminum bonding, primarily from the adhesives side, and this work did employ environmental and physical property testing. I believe we learned that bond failure was generally due to contaminants under the joint. Chromate conversion works well and I suspect its inherent corrosion resistance adds to the durability of the bond. I don’t know what procedures Grumman employed, however, here’s a couple of suggestions:

    To properly adhesive bond aluminum, the surface MUST be clean and chemically treated. The highest performance adhesive bonding includes degreasing and chromate conversion. Improper surface preparation is the major cause of adhesive bond failure.

    Remove ink and markings from the surface of the aluminum with a light wash using MEK or acetone and white paper towels, followed by a degreasing (vapor phase degreasing preferred. However a rinse using brake parts cleaner works well, which comes in spray cans and is available from most auto supply stores). Perform all of the above operations in a well ventilated area (outside or fan blowing) and wearing a respirator.

    It is not necessary to ‘buff’ or abrade the surface of aluminum to obtain a high-strength adhesive bond. Forget the Scotchbrite scrubbing pads. Adhesive bonding is a chemical bond, not mechanical (Van der Waals forces).

    Etch or chromate convert the surface with a chromic acid solution by immersion at 65-70 degrees Celsius for 5-10 minutes. A commercial chromate conversion solution is Iridite 14-2, which is very similar in composition to chromic acid (see below).

    Rinse the metal thoroughly with distilled water and dry well. Do not use compressed air, because there is oil in most air compressors which will contaminate the clean surface.

    If you do not wish to work with chromate solutions (I can provide a suggestion for safe disposal), you can use a phosphoric acid cleaner (Jasco Prep & Prime) to etch the aluminum surfaces which then passivates the aluminum with dichromate. Clean aluminum as above (no abrading) and immerse the aluminum in the Prep & Prime for thirty minutes (check time). Remove, rinse with distilled water (rain water – see note below) and air dry. All cleaned parts cannot be touched by hand; use fresh, white cotton (mickey mouse style) gloves.

    For best results, parts should be coated or bonded immediately after cleaning or chromate conversion. If you do not plan to bond the parts together within twenty-four hours, wrap in fresh ‘butcher’ paper to keep clean.
    There are companies which perform chromate conversion processing, and will provide this service on a tolling basis. Prior arrangements must be made to ensure the processed parts remain clean and suitable for adhesive bonding. This may be a way to go if a builder wants to use adhesives and obtain excellent corrosion protection.

    Chromic acid cleaning solution:
    10 parts/wt. Sodium Dichromate.
    30 parts/wt. 96% Sulfuric Acid.
    100 parts/wt. distilled water.
    (Dissolve the dichromate in the water, then add sulfuric acid slowly, stirring carefully.)

    Iridite 14-2 chromate conversion concentrate is available from McDermid, Inc., #8659. Alodyne solution from Aircraft Spruce - same thing, only diluted.

    Distilled water produces best rinse results. However, rainwater caught from the roof is very low in mineral ions and is an acceptable substitute (discard the first five minutes of runoff – that is the roof's rinse water & is dirty).

    When working with chromic acid or Iridite solutions, wear nitrile rubber gloves (Harbor Freight – low cost). Also a long apron and shop face shield – safety first and last.

    Since the original opening of this thread was a request for information, here are a couple of other cleaning solutions (suitable for Pro-Seal):

    Jasco Prep & Prime – caution, this contains both phosphoric acid and dichromates. Use full strength on aluminum at room temperature for thirty minutes, rinse well with distilled water and air dry.

    Dupont Quick-Prep - Auto paint supply stores which sell Dupont paint. Quick Prep is a mixture of phosphoric acid and alcohol. When spread over bare metal it reacts with the surface to form a light coat of aluminum phosphate that prepares the surface for bonding. Most finishes can be applied directly over a surface that has been coated with metal prep. Quick Prep is soluble in water and is generally only mildly toxic unless vaporized by heat. It is always advisable to wear nitrile rubber gloves and protect eyes from the irritating effects of contact.

    Sherwin Williams Metal Prep - Auto paint supply stores sell Sherwin Williams paint. Metal Prep is a cleaner and conditioner for bare metal. It is composed of phosphoric acid and glycol ether. See Quick-Prep above for use directions.

    Anko Phosphoric Acid Aluminum Brightener – This is available over the internet and is a cleaner and brightener for bare metal. It is composed of phosphoric acid and glycol ether.

    I'm enjoying this discourse, for it forces me to remember work from long ago. You obviously have a lot of experience in this field.
     
  20. Jan 23, 2007 #20

    MalcolmW

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    Dichromate Disposal

    Dichromates are potentially hazardous substances. Care should be taken in handling them and the disposal of solutions containing dichromates. Ingestion and inhalation of dichromate dust is toxic and should be avoided. Putting bare skin in dichromate solutions can contribute to the development of an allergic sensitivity and should be avoided. The following is a method to dispose of used dichromate solutions:

    Chemically, the following procedure should work. Add a strong alkaline (baking soda, lye [sodium hydroxide], etc) to the dichromate solution until it becomes alkaline. Determine this by using litmus paper, which should be available from many sources, including Edmund Scientific, if you are in the US. This will cause the dichromate to convert to chromium hydroxide (very insoluble).
    Either filter or let stand for a week to let the precipitate settle out. The supernatent liquid (the liquid above the sludge) can be washed down the drain with 50 times its volume of water. The precipitate (Chromium Hydroxide) can be washed with hot water to remove the sodium sulfate, dried, packaged and labeled, and then taken to a toxic waste disposal site or company.

    If you choose to filter the solution to remove the sludge, a double layer of coffee filters should work, though lab grade filter paper would be best. (*Don't* use your coffee maker for this step!). At that point, you should be able to dispose of the liquid which should be chromium free and give the filtered sludge to a place that handles hazardous waste. The trivalent chromium is less toxic (according to my sources) but I still wouldn't just toss it down the drain.

    For an alternative means of disposal of the chromium hydroxide sludge: Mix the wet sludge (precipitate) with dry concrete mix (right, the bagged stuff from Home Depot, Lowes, etc.) to form a stiff mix. Use for setting posts, making blocks, etc. This will tie up the chromium in a very insoluble form and thus render it harmless to the environment.

    That's about it. I offer this in the spirit of information sharing and NOT as expert advice. If anyone can and wants to add to or correct any of this, I would welcome it.
     

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