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Five o'clock shadow

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    Five o'clock shadow

    So, here's an odd one. The unpainted metal casing for my Tandy 1000 RSX has always had a more distinct, "duller" finish than say, the comparable RL or RLXs. While looking over the system this evening, I noticed what appeared to be metallic dust on the inside of said casing. On (much) closer inspection, the metal has apparently sprouted a short shag of zinc whiskers. Awesome.

    Heretofore, my only experience with zinc whiskers has been in raised floor data-centers. I've never seen such a thing outside of work. Is this pretty common in the vintage-computer world, where everyone just wipes things down and goes on with life, or should other protective/preventative measures be taken?

    #2
    Are you sure that you're dealing with zinc whiskers? Perhaps you're looking at "white rust" ? But yeah, zinc whiskers do show up on zinc electroplate sheet metal. And they're hell on wheels for modern circuitry with small geometries. Probably not so much with your Tandy HX.

    I've read a few articles that formation can be retarded somewhat by heating the metal in question to relieve stresses, but that sounds a bit extreme in your case. I'd probably go over the surfaces with a good one-step auto paste wax. It's not going to stop those whiskers from growing, but it might contain them a bit.

    Short of stripping the galvanizing from your sheet metal and refinishing in, say, nickel, there's probably not much you can do as the cause appears to be physical and not chemical.

    It's sure lined some pockets of remediation services and sold a lot of replacement data center floors.

    Comment


      #3
      Originally posted by Chuck(G) View Post
      they're hell on wheels for modern circuitry with small geometries. .
      Whiskers are a big issue for high reliability electronics. Typically no pure tin coatings are allowed.

      Here's a website dedicated to whiskers: http://nepp.nasa.gov/whisker/

      Lots of nice hairy pictures.

      Comment


        #4
        Tin whiskers are one thing and unrelated to the zinc whisker issue.

        Zinc whiskers seem to arise from electroplating steel where stresses haven't been adequately relieved. It can take many years for zinc whiskers to appear, if they appear at all. Currently, the only advice given is replacement of the offending elements. Evidently, painting doesn't do much--the micron-thick whiskers will apparently poke their way up through most coatings. They do not occur on hot-dip galvanized steel; only on electroplated steel. It seems that adding a bit of lead to the plating mix prevents them.

        There's been a bit of work done on ameliorating the situation. There's some evidence that heating the offending part slows the growth but doesn't prevent it. But currently, replacement is the only sure-fire solution.

        Comment


          #5
          Originally posted by Chuck(G) View Post
          Are you sure that you're dealing with zinc whiskers?
          Mostly sure. When blown across, the whiskers shimmer like Christmas tinsel (without breaking). Pretty cool effect, despite the gravity of the situation.

          I noticed one other property that I hadn't quite expected, and must have missed in my own readings - the whiskers seem to be completely water soluble. My plan at this point is to remove the system mainboard and other components, wipe down the chassis, and give it a light, once-over with some steel wool. Heating it in the oven would be do-able as well, if I had any idea at what temperature or duration to leave it in there.

          Comment


            #6
            Originally posted by Chuck(G) View Post
            Tin whiskers are one thing and unrelated to the zinc whisker issue.

            Zinc whiskers seem to arise from electroplating steel where stresses haven't been adequately relieved. ... They do not occur on hot-dip galvanized steel; only on electroplated steel. It seems that adding a bit of lead to the plating mix prevents them.
            Hmm. There's a video on the first page (about 3MB .mwv) in the link called "Zinc Whiskers on Hot Dip Galvanized (HDG) Steel and Whisker Bending Due to Electrostatic Attraction, J. Brusse, A. Purves"

            Most of the papers on zinc whiskers imply that tin and zinc whiskers have a similar mechanism. Anyway no one seems to understand the root cause.

            The tin whisker problem became more well known (to me at least) when everyone tried to go "lead-free". Some people tried to replace solder coating with pure tin plating. I recall seeing a PC board drawing with a revision history that started out solder coat, then changed to tin plate, then finally changed to gold flash.
            Last edited by xprt; November 26, 2012, 03:40 PM. Reason: typo

            Comment


              #7
              Originally posted by Chuck(G) View Post
              There's been a bit of work done on ameliorating the situation.
              Having dismantled/cleaned/buffed the chassis, my work is done... for now. I did a pretty swell job, so I'm going rate this effort as a "10" on the Gil Amelio scale.

              We'll see how long it goes before needing another shave. I'll report back in a year or so...

              Comment


                #8
                Originally posted by Cloudschatze View Post
                We'll see how long it goes before needing another shave. I'll report back in a year or so...
                They're baaack...

                Comment


                  #9
                  Yikes--did you try coating the stuff with some paste wax or maybe covering it with with some rattle-can acrylic lacquer?

                  Comment


                    #10
                    Hi Chuck. I've not tried anything beyond the initial "wash" thus far. NASA suggests applying a conformal coating (at least 2 mils) of Arathane 5750. What a bother.

                    Comment


                      #11
                      I'm hoping to have wrapped-up this little annoyance for the remainder of my lifetime - the chassis is now stripped of its zinc, and has been re-plated with nickel.

                      Comment

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