Keel bolt #2 - Extraction
Back to metal work!
Keel bolt nr 2 was little more dramatic to extract - this time it made thumping noise when I was pumping it through oak. But luckily that was the only difference, otherwise extraction went smooth.
Judging from the state of this bolt it was high time to change it. Similarly to bolt nr 1 it corroded mostly on interface between oak keel and oak floor - and inside the floor. The part sitting in iron is in quite good shape. That tells me that ballast-to-oak keel interface is still tight (no salt water ingress) and corrosion is mostly caused by tannin acid in oak.
I’m trying to decide what should I use for new bolts. Wrought iron is out of question: there is only one place on the Globe which still sells it (in UK) but even they don’t produce it any longer, they re-use scrap wrought iron. I doubt I can get constant quality on bolts which are 80cm long.
There is also ductile iron (used in sewage pipes under ground) which is still made today. It is very close on galvanic table to cast iron (my keel) and would be very suitable.
But - it is hard to find in Sweden or Germany if you don’t need 10 tons. Apparently this is not a product for small customers.
Then there is mild steel. I could go with that given it is cheap and readily available. Hot dip galvanising could add some corrosion resistance and I can check the state in 5 years. However - my bolts are 3/4 inch in diameter (19 mm). The closest diameter for mild steel is 20mm with tolerances up to 1mm. That means I would need to machine it to required dimensions and that is tricky with so long material.
Björn proposed silver steel. This is sold in 19mm diameter and tolerances are 50 micrometers. Really nice. This is high carbon steel with some addition of vanadium and molybdenum. This alloy is used for making tools like screwdrivers, drills, auger bits etc. Sold as raw it is in annealed state which makes it easier to machine. After machining one can chose to harden it to required Rockwell grade. Very nice material, a bit expensive but not over the top.
There are also super-alloys like alloy C276 or AL-6XN. These are used on oil riggs and are formulated to be extremely resistant to salt water corrosion, SCC (stress corrosion cracking) in chloride and to inhibit crevice corrosion (pitting). Nice, but where do I get one???
I lean towards silver steel. At least Björn has found the source in Stockholm and I found it at my local metal shop.
I will definitely try to shield it from oak with some primer, maybe epoxy tar or something similar. And - I will check the state in 5 years.
So without further due - I started melting the pocket to reach the third bolt.
Sources for more information about super-alloys:
https://www.shallcrossbolt.com/sea-water-corrosion-monel-titanium-al-6xn-to-the-rescue/
http://www.metalcor.de/en/datenblatt/29/
https://www.csidesigns.com/products/superalloys/al-6xn
https://sverige.myjacquet.com/files/JCQsverige-fichetech-c276.pdf
http://forum.woodenboat.com/showthread.php?4719-Fastenings-with-an-iron-keel
Silver steel: