Coating Al and Cu simultaneously

  1. Hi,

    I hope this question is in the right section.

    I have a heatsink that consists of both aluminium and copper. It's started to corrode, oxidate etc. I've tried different chemicals (such as HCL, Salt and Vinegar, Coke, and a aluminum brighenter that consisted of hydrofluric acid and sulphuric acid) to get it bright and shiny again - but none seem to do the job. The problem is that the fins are very tighly spaced (a few mm's) so I can't get inbetween them to scrub.

    I would ideally like to get it anodized in black, but i've been told copper cannot be anodized, is this correct?

    I took it to a company that does electroplating but was told that the nickel wouldn't go inbetween the fins - they said electroless nickel might work, but they didn't have the means to do it. Should I bother ensuing this method? Also because copper and aluminium are treated/cleaned in different ways prior to electroplating, the acids used may cause a problem.

    Powder coating is another option, but i'm not sure how this would affect the heasinks perfomance. What's the heat conductivity of a thin layer of powder coating? Bad, right? Also would it work, or would the powder simply be attracted to the egde of the fins instead of coating it evenly?

    Lastly I thought of sand blasting it. Would this damage the heatsink in anyway? I've also seen tiny glass beads being used instead of sand, would this be a better option?

    Sorry, lots of questions! I would really appreciate any answers. Thank you!

    Bob
     
  2. jcsd
  3. Gokul43201

    Gokul43201 11,141
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    Do you have a picture of your heatsink (or one that looks like it)? If not, can you make a sketch and upload it (here, or to an image server and link to it) ? Importantly, do you have aluminum and copper parts in contact? And are they separable or not?

    Next, what are the typical operating temperatures? If you have very high temperatures and/or are counting on heat dissipation through radiative emission, that is something to keep in mind. Coatings or surface treatments can drastically change the emissivity.

    I do know of an anodization (patina) dip for Copper that blackens the surface with some kind of oxide/hydroxide layer. We, in fact, are planning to use just such a thing for our copper baffles. Our objective, however, is to modify the emissivity of the surface, not corrosion resistance. I can get you the details if you are still interested. As you must know, aluminum can easily be anodized. But then you'd have to separate the aluminum parts from the copper parts.

    Methinks the dominant cause of corrosion is the contact between dissimilar metals. Is there a specific reason you need both metals in the heatsink? Also, what are the typical environmental conditions that the heatsink is subjected to, and what is it being used for?

    More questions than answers, I know, but heck, that's life, eh?
     
  4. Thanks for the reply :smile:

    It's a computer heatsink. I've posted simliar questions on PC related forums, but I wasn't getting very far.

    From the manufacturer's website:

    http://www.zalman.co.kr/usa/product/view.asp?idx=142&code=009

    Here are some pictures of my particular heatsink:

    [​IMG]
    Fan removed.

    [​IMG]
    Rust on the bolts that hold the fins together.

    [​IMG]
    Bleh.

    As you can see, the copper is in contact with the aluminium. I don't know how difficult it would be to remove the bolts and take it apart. This would also create an uneven base, as the fins wouldn't line up exactly as they where before. I would have to lap the base after reassembly. This is why I want a method of coating the aluminium and copper at the same time.

    Typical operating temperatures are between 20'C and 55'C, relatively low I would guess. I live near the coast, so there's more salt in the air than usual. Also the humidty is quiet high. I live in Durban, South Africa.

    How well does anodizing prevent corrosion? I have a few heatsinks that are anodized and they show no signs of corrosion, so I'm guessing pretty well?

    And if there is a way to get it electroplated, which metal is the best option? Silver's heat conductivity is very good but how quickly would it corrode/tarnish? Would nickel be a better option?

    Yes please :smile:
     
    Last edited: Aug 25, 2006
  5. NoTime

    NoTime 1,570
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    From those pictures.
    I'm thinking you might want to invest in a dehumidifier.
    I doesn't seem likely that the computer is the only thing that is having problems with all that water.
     
  6. Gokul43201

    Gokul43201 11,141
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    Looking at the pictures, I would suggest removing the bolts, putting the aluminum piece in a mild NaOH dip for the right amount of time, then anodizing the aluminum and putting the pieces back together. The last step does not appear to me to be as difficult as you think it is. You only have to machine a little T-shaped section out of something else to use to realign the pieces.

    Also, what are the fasteners made of? If they are regular steel or zinc-coated steel bolts, I would consider replacing them with stainless steel bolts (or even with aluminum bolts) and nuts.

    The cause of the corrosion appears to be the contact between Cu and Al. Coating all but for the mating surfaces, will do little to protect your heatsink from subsequent corrosion. Only, you might not know it's corroding under the coating.

    When copper is by itself, it rarely needs corrosion protection - it forms a very stable, and very strongly adherent oxide coating. Unless it's in a very acidic environment (pH <5), the natural passivation of its surface will protect copper from corrosion. Typically, the layer that forms on copper starts of being a darkish, nearly black color, which later turns green and might eventually end up with a reddish hue. I don't think the passivation layer is thick enough to noticeably impede the thermal disspitave ability (notably the conductance).

    There are some organic coatings for copper that are reported to be very good corrosion inhibitors in extreme environments. You could look into Thiazole derivatives: 5-benzylidene- 2,4-dioxotetrahydro-1,3-thiazole (BDT) and 5-(4′-isopropylbenzylidene)- 2,4-dioxotetrahydro- 1,3-thiazole (IPBDT), and see if you come up with anything there.

    http://www.iupac.org/publications/pac/2001/pdf/7312x1861.pdf#search="copper corrosion"

    This will have to wait till monday. I don't have access to it right now.

    Where's Bystander when you need him?
     
  7. Astronuc

    Staff: Mentor

    I'm guessing coated steel bolts as well as a coupling between Al and Cu. It looks like some rusting on the bolts.

    The objective of anodizing is to put a 'protective' oxide on the surface of the metal, which acts as a barrier to electrical conduction, which is the cause of galvanic corrosion, which is probably what one has here. Galvanic corrosion occurs when two dissimilar metals are in contact and one is reduced while the other oxidizes. One has to electrically isolate (insulate) the metals.

    As Gokul mentioned find an alternative to steel bolts.
     
  8. Bystander

    Bystander 3,291
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    Making rash judgments about the content of the thread from the title --- where else? P&WA is boring today, so I figured I'd give this a peek --- a 70 piece heat sink? !!! If it's for a $1k computer, buy a replacement, wash the computer with i-PrOH, and get a HEPA filter to clean the salt aerosol from the air --- or, run it 'til it quits, and replace it --- lots cheaper.

    The corrosion is going to boost the emissivities, probably improve heat transfer to the air, but also going to interfere with convective transfer --- prolly a wash.

    Remanufacture a two-bit piece? At what cost? Wash it with alcohol, and run. You take it apart, you're going to pick up crap on the surfaces between each "fin sheet," bend fins, have the fins get out of order when the wind blows through the room where you're working (or your shirt sleeve snags the paper towel you've laid them out on --- my trick), have to lap them (bending more fins, and crudding them up with abrasive).

    Gold plate 70 pieces? At what $5 a piece? For a $1k computer?

    You've used HCl, vinegar, and plenty of other corrosives --- that crap's all crawled into 70-1 gaps between "fin sheets" --- see that round, open-topped can next to you? The one that's maybe a foot in diameter, foot and a half high? That's where the heat sink belongs --- you're never going to get the electronics eating stuff cleaned off the surfaces of the fin sheets, or from the holes for the screws holding them together --- even with disassembly. Save the fan --- "parts is parts" --- chalk the rest of it up to a failed experiment in "economy."
     
  9. Thanks to everyone for the replies :biggrin:

    Bystander, you make some good points, especially the bit about tossing it in the bin - i'm really getting fed up with this thing now. But if I go out and buy a new one, in 6 months time i'll have to do the same again...and i've got a side window on my case, and i want it to look nice and prerdy.

    What about powder coating then? Would it work? Would it have a negative affect on thermal preformance?
     
  10. Gokul43201

    Gokul43201 11,141
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    Bob, here's the blackening solution info I promised earlier:

    Ultra-Blak 420 (for copper/brass), from Electrochemical Products, Inc.

    http://www.epi.com/black-oxide/hot-oxide-blackening.html#420

    Edit: Just tried it out on a test piece. Made a uniform, black coating in about 30 seconds (maybe less) over a 2 inch square plate. Feels pretty stable (does not flake or chip off easily; doesn't scratch with a fingernail), but I have no idea if it's any good for corrosion resistance.
     
    Last edited: Aug 30, 2006
  11. Cool, thanks!

    Doubt that sort of thing would be available in my part of the world though.

    edit: And what would NaOH do to copper if i didn't take the heatsink apart?
     
    Last edited: Aug 30, 2006
  12. Bystander

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    NaOH eats Al, anodized or not. Dipping the assembled sink in anything is going to leave "the dip" between fin sheets; from there, it's going to chew on the processor. You want to coat the sink to prevent salt corrosion, keep it shiny --- hit it with a light coat of clear lacquer.
     
  13. But that's want I want it to do, surely this would help clean it up? I would start with a low concentration.

    mmm...how sure are you about that? The fins are heavily compressed together.

    You want to coat the sink to prevent salt corrosion, keep it shiny --- hit it with a light coat of clear lacquer.[/QUOTE]

    Clear laquer has been suggested to me before, but i'm really not keen on spraying it with anything that could negatively affect thermal performance.
     
  14. Bystander

    Bystander 3,291
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    Anything that wets the surface (which you want for cleaning purposes) is going to crawl right between the sheets --- little thing called surface tension --- and it ain't gonna come out --- won't rinse due to another nasty little property of liquids called viscosity --- and the last killer is the distance the nasties in solution have to diffuse --- letting it soak for a week in distilled water won't work.

    Thin film --- you familiar with how thermal conductivity affects heat transfer? Heat flow is proportional to the temperature gradient, which is temperature difference divided by "length" of conductive path (film thickness) --- yeah, it'll boost temperature, and that'll be proportional to the film thickness. Compare this to the reduction you're contemplating by etching the fin --- this reduces it's cross-sectional area, slowing heat transfer along the length of the fin.
     
  15. Can you elaborate on this please. I'm not following :yuck:
     
  16. Bystander

    Bystander 3,291
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    Thermal conductivities are measured in (W/m2)/(K/m), SI; that is thermal conductivity is proportional to the area of the conductor, and to the temperature gradient along the direction heat is conducted. What you'll see in handbooks is "W/(m.), cancellation of units, and in older handbooks cal/(cm.s.K), and a mess of other units, the English "(BTU/hr x x x) being among the worst to handle. Not to matter.

    What's the direction heat moves in the fins? Radially, away from the hot center, and the fan's blowing air radially away past the fins. The temperature gradient in the fin is the difference between processor temp, and fin tip temp divided by the length of the fin. The cross-sectional area at right angles to the gradient is the thickness of the fin times, first approximation, the width, vertical height, of the fin. Drop the fin in NaOH, dissolve some Al, and what have you done? You've reduced the thickness of the fin, hence its x-sectional area --- it conducts less heat for a given temperature gradient, or, requires a larger temperature gradient to conduct the same amount of heat (processor waste).

    I'm just guessing from the pictures you posted, but I don't see a fin thickness much greater than 1/4 to 1/2 mm, .010-.020 inch. NaOH etching is gonna take some of that, how much? Molar volume of Al, 10 cc; fin thickness, .01 cm; fin area, 5 cm; fin volume, .05 cc, .005 moles Al; etch strength .01 molal (?); etch volume, 50 cc to submerge the fin (?), .0005 moles; etch stoichiometry, 4:1 off the top of my head; the phone rings, you can remove 2 1/2 percent of the x-section. The other thing that happens is that it doesn't etch cleanly --- it pits --- finds an "active" spot in the surface, and chews a hole right on through --- 'tain't like electropolishing (which also gives you a mass loss).

    That loss vs. the lacquer loss, and the cleanup mess for chemistry vs. painting? Go with the lacquer --- practice on the old sink first.
     
  17. Wow, that was really indepth - thanks!

    What about Vacuum Coating? I'm not really sure what it is, wiki's explanation is confusing. Could it apply to this situation?
     
  18. Bystander

    Bystander 3,291
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    Wiki's explanation is so confusing it seems to have been deleted between the time you posted and I went to look at it.

    Place item to be coated and coating material in a vacuum chamber; heat the coating, or cool the item, or both; material is transported in the vapor phase. Quality of coating depends on how clean the surface of the item to be coated is --- in the case of your "old" heat sink, a waste of time; in the case of a "new" heat sink --- hard telling --- the stamping process for the fins is going to leave lubricant, shop dirt, who knows on the fins, and it probably needs cleaning first ("trichlor," TCE, degreaser of some sort), might need surface prep, depending on coating.

    This is where you'd go with the Au, and the shape is complex enough that you'll never get a complete enough coating to do you any good. Vacuum coating --- think of getting a "line of sight" arrangement of the coating source to all fin surfaces (vacuum means molecular flow, or near zero intermolecular collision frequency). Might be "purty" once done, but a bottom line waste of time and money.
     
  19. Bystander

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    It's a sandwich of 70 copper sheets, rather than a 24 sheet Cu sandwich between two 23 sheet Al sandwiches --- what coating? You want to plate it? Same problem as the Al-Cu-Al --- wicking of plating solutions between the sheets. Less bimetal problem, if there was one (doubt it), and no Al doing what Al does when in contact with salt air (corrode).
     
  20. It's been coated with something to make it red. Most reviews that comment on the colour say it's been anodized, but copper can't be anodized, so what is it?
     
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