Sunday, 16 June 2013

When Stainless Steel Isn't

On any sea going boat corrosion is a constant companion, but a dangerous one.  Like a sore tooth, it will flare up sometimes with no warning.  We use grade 316 stainless steel fasteners whenever we can, to reduce the impact of corrosion, however every now and then we are reminded how even the 316 has its problems - namely crevice corrosion.

When removing the old Onan generator a while back, we found one bolt head sheared completely (a real knuckle skinner !) as it was loosened. On bolts like this the thread is usually roll formed, while the head is forged, a process called "rolling and heading". Its a cold formed process, and it can clearly lead to problems, specially if sea water collects where the metal is deformed and stressed.  316L stainless steel in an oxygen deprived environment can and will corrode.  Fortunately this bolt didn't cost a lot to replace, but in the past we've had to replace an entire propeller shaft due to crevice corrosion.

2 comments:

  1. It is scary but true that all the stainless parts on the boat are subject to this type of corrosion, especially fabricated or machined parts. We had a backstay chainplate fail catastrophically while sailing. We kept the rig up thankfully, but finally we've now done the hard job of having all the chainplates replaced or renewed. This is recommended for any yacht over 10-15 years of age.

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  2. Hi Neil n Ley, my pleasure to meet you guys up in weld quay restaurant just now, I must say you've got a wonderful life n boat too! Will follow your blog from time to time, looking forward to have a short visit to your beautiful yatch n perhaps I will bring you guys for a fire flies watch in nibong tebal, Penang (mainland)

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