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Stainless Bolts On Aluminum

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  • Stainless Bolts On Aluminum

    ok I just found out today that...

    you cannot use stainless bolt for attaching aluminum parts.

    I used stainless bolts to attach the brackets for my router extension on my ts3650.

    I was reading a magazine and it mentioned something to that effect.. then I remembered someone telling me this a long time ago..

    so I need to replace the bolts /screws now that are attaching to the aluminum. can I use regular steel bolts? or do I need aluminum bolts?

  • #2
    Re: Stainless Bolts On Aluminum

    Stainless bolts are used on aluminum all the time. There is no problem with it. Heck, I was just working on a boat loaded with aluminum attached by stainless screws. I know you definately shouldn't use titanium fasteners with aluminium. Titanium violently accelerates galvanic corrosion on aluminum. Stainless steel can also cause galvanic corrosion on aluminum but the quantity of stainless steel needs to be higher that that of the aluminium. For example a stainless screw in a block of aluminium will not corrode. However an aluminium screw in a block of stainless steel would rapidly corrode. In galvanic corrosion the most noble metal (highest resistance to corrosion) will attack the lesser noble metal. When not sure and you want to play it safe a dab of grease or anti seize on the fastener will insulate the two metals and solve the problem.
    Last edited by Velosapien; 06-13-2007, 08:26 PM.

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    • #3
      Re: Stainless Bolts On Aluminum

      ok so I guess I don't need to worry! I thought maybe over time the stainless hex heads would eat through the rails of the aluminum on the ts.

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      • #4
        Re: Stainless Bolts On Aluminum

        You should be fine. If there was a problem you should immediately notice the white dusty fungus like corrosion of the aluminum forming around the stainless steel part and take them off. Just keep in mind there are many grades of stainless steel and aluminum alloys. Just saying aluminum and stainless doesn't necessarily mean anything. Some react more than others depending on their galvanic potential. The closer the galvanic potential between the two metals the less reaction there will be. As long as you keep the proportion of the anodic metal (Alu in this case) much higher than the cothodic (stainless) the lower the corrosion potential. The loss of atoms on the anodic metal is spread over a larger area. It's directly proportional.

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        • #5
          Re: Stainless Bolts On Aluminum

          Yes, you probably will get some corrosion around the fastener heads because of dissimilar metals. However, the SS bolts won't cause you much more problem than would common steel, and aluminum won't give the strength needed. SS, titanium, and steel fasteners have been used in aluminum acft skins for years. There has been $millions of tax dollars spent in researching and developing corrosion mitigation techniques, so you may as well benefit. The solution is to insulate the two or prevent any electrolyte (moisture) from completing the corrosion cell.
          Basic galvanic corrosion theory:
          The corrosion cell requires 4 things:
          1. Anode (metal that corrodes, in this case the aluminum)
          2. Cathode (A metal higher on the galvanic scale, in this case the fastener)
          3. Electrical path (in this case the two metals in direct contact with each other)
          4. Electrolyte (in this case it could be moisture in the air, or the air itself if the oxygen concentration level is different throughout the depth of the hole and under the fastener head)

          Remove one of the four and it stops the corrosion cell. Choices:
          1. Change the aluminum to stainless - not a good option due to cost and difficulty)
          2. Change the bolts - to get less of a galvanic difference, you could use galvanized fasteners because the zinc coating is very close to the aluminum on the galvanic chart. A galvanized washer under a bolt head would help (the washer corrodes instead of the aluminum).
          3. Eliminate the electrical path - On aircraft, the fasteners are "wet installed" by dipping into wet epoxy or zinc chromate primer during installation. Messy but effective if you have the primer. Non conductive washers under bolt heads (nylon, etc) insulate that connection but the shank in the hole may still make contact.
          4. Eliminate the electrolyte - easiest and simplest. Spray the areas around the bolt heads with a corrosion preventive spray (CPC) like Boeshield T9, LPS3, and even WD-40. All displace any moisture and will remain in the contact area. If you loosen the fastener, spray under it and retighten, it will give the longest protection. You could also spray it with clear lacquer, or wipe with penetrol, which will seal it, but any movement will likely crack the coating after it dries, eventually allowing moisture into it.
          If it were me, I would use the Boeshield, LPS-3, Amal-Gard, or any of various-brand name water displacing corrosion preventive sprays or liquids. They dry to a flexible coating and won't collect as much dust as WD-40 residue will.
          While you're spraying, hit the underside or the saw table top, the planer bed, your handsaw blades, battery terminals in your vehicle, etc. The stuff has a lot of uses.

          JMTCW

          Go
          Last edited by Gofor; 06-14-2007, 12:37 PM.
          Practicing at practical wood working

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          • #6
            Re: Stainless Bolts On Aluminum

            After living aboard our sailboat for 10 years, I learned a thing or two about corrosion. I assume you are not going to use your TS in a marine environment. That makes a big difference. You said you used bolts which means the bolts are fastened with a (S/S?) nut. So, mechanically you are good. If you had tapped the aluminum and threaded the bolts to the aluminum, that would be a different story. I would say you are okay the way it is. But, if you are a perfectionist like me, I would put a bit of anti-seize compound on the threads -- the kind that has fine particles of zinc in it to act as the sacrificial metal. The zinc corrodes, not the aluminum.

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            • #7
              Re: Stainless Bolts On Aluminum

              I ride mountains bikes which are made of all sorts of exotic materials and alloys and ridden in dirty muddy and wet conditions, not to mention hosed off afterwards. They are made of carbon fiber, titanium, fancy aluminum alloys with very high strength to weight ratio's and magnesium. The biggest drawback of those ultra light and rigid aluminium alloys is the are very brittle and highly susceptible to corrosion. Carbon fiber and titatium attack the aluminum very violently, quite more than stainless steel. Usually plain old quality grease is perfectly suitable to getting the job done and isolating the parts. For stuff like bolts its usually more than enough. If you want to go a step further use anti-seize but its probably not even necessary.
              Many aircraft actually use neutralized varieties of stainless steel as an isolator between carbon fiber and aluminum parts.
              Last edited by Velosapien; 06-14-2007, 03:57 PM.

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