Last Updated: 12/4/20 at 11:28am
Many years ago Keel Bolts for boats were made from carbon steel or a basic nickel alloy called Monel, which was a top of the range alloy then (1960-1980). There were numerous failures of Keel Bolts due to localised corrosion in salt water in boat bilges and boaties regularly inspected their keel-bolts for corrosion damage as a matter of course.
Credit: Coyote Point Yacht Club/Flickr
Then the so-called “marine grade” of stainless steel, grade 316, became the holy grail of boaties for Keel Bolts. But lo-and-behold, even 316 Stainless Steel suffered severe crevice corrosion inside keels and some keels failed almost as fast as ordinary steel if seawater got in. It is not well understood in the marine industry that 316 Stainless Steel cannot be used for critical applications immersed in seawater as this grade of Stainless suffers intense localised corrosion (crevice corrosion). If it is located in a fissure (as in a keel) with seawater ingress occurring.
With the advent of modern Duplex Stainless Steels in the 1980s various trials were done with Duplex grade 2205 Stainless Steel for Keel Bolts. The boaties’ holy grail was then discovered and ever since the Duplex Stainless family of alloys have become the preferred alloys for use as Keel Bolts in seawater immersion applications. The two most suitable grades of Duplex Stainless Steel for Keel Bolts are grade 2205 and super-duplex grade 2507. Both these Duplex grades have very high strength and excellent corrosion resistance which are critical properties for keel-bolts in a boat.