Home Forums Repairs and Maintenance How to attach steel components to carbon

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    Paul White
    Paul White
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      You may have noticed how the rivets holding your shiny new metal fittings on the mast and rudder of your Weta soon start to show signs of corrosion, especially if you sail on salt water.

      The issue is that stainless steel reacts with Carbon.   My tiller extension on my old boat (#325) came off the tiller in one race because the screw had completely corroded where it touched the carbon.

      So what are your options if you need to replace or attach a steel fitting to the carbon?

      The image below shows the galvanic relationship of particular materials (graphite = carbon). Monel is one of the Cu-Ni alloys on that scale. To avoid galvanic corrosion between dissimilar materials, you want to choose materials that are close to each other on this chart. As you can see, Aluminum Alloys are almost as far away as they can get from graphite…the aluminum, being the more active of the two will act as the anode and will corrode at a high rate.

      316 series stainless steel is typically used in marine applications and most common stainless fasteners but even it is a bit far away from the graphite – it will rust if in contact with it.

      You could use platinum rivets…but they are expensive and aren’t easy to come by.

      [Linked Image]

      The other problem is that drilling a hole in carbon weakens the structure – so if you then use a normal stainless steel rivet, the considerable forces involved in fixing the rivet may weaken the structure further and the stress of sailing may cause the fitting to pull out.

      You could use a stainless backing plate or washer to spread the load but if you can’t reach inside the tube, ideally you need to use Stainless Avinox rivets which are designed for thin wall applications.

      During the rivet placing sequence the tail of the rivet body collapses into a bulb shape against the rear sheet. This creates a load bearing surface, which absorbs the force of setting the rivet. Otherwise the setting force, or clamp load, could spread into the application material and might damage the material. This makes Avinox® ideal for use in thin sheet, soft, brittle and low strength materials, as well as resulting in high resistance to pull-out loads.

      Also during installation, the portion of the rivet body contained within the application thickness expands outwardly toward the wall of the application hole until the hole is completely filled by the rivet material. This radial expansion results in a strong, vibration resistant joint and compensates for irregular, oversized, slotted or misaligned holes.

      Eliminating Corrosion
      Galvanic corrosion takes place when the materials start swapping electrons – this leaves the less noble material unbalanced and oxygen atoms step in and start to make new materials from it (oxidation!). If you can keep these two items electrically isolated they won’t swap electrons as readily.  When they’re in the process of corroding you can actually measure a tiny electrical current. This also explains why salt water accelerates the problem – salt water will carry electrical current better than fresh water so when you get salt water mixed between these materials, it accelerates the corrosion because it increases the electrical conductivity allowing the different materials to swap more electrons.

      One way to prevent corrosion is to include an electrical insulator between the carbon and steel.  Tef-Gel Corrosion eliminator paste is designed to provide a barrier between dissimilar materials and also prevent seizure. It’s available from marine suppliers and hardware stores.

      Wall Plugs Solution

      A workaround is to epoxy a wall plug into the hole where you want the fitting and, when the glue has cured, use a screw to anchor the fitting in place. The plug provides something for the screw to bite into and also keeps the screw separated from the carbon preventing corrosion.

      You may need to drill a hole oversize (also useful if you have damaged the original hole or the fitting has pulled out) to fit the wall plug. Most cavity wall toggle fittings won’t work with the thin walls of the carbon tube since they are designed for plasterboard so you will need to use a regular wall plug .

      There are hollow door anchors which are designed for the thin skins of hollow doors and these may be more suitable but I am unable to find an Australian supplier.

      • This topic was modified 4 months, 3 weeks ago by Paul White Paul White.
      • This topic was modified 4 months, 3 weeks ago by Paul White Paul White.
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