March 2012 Issue
Table of Contents
Where Credit is Due
Mailport: March 2012
In response to the December 2011 article on replacing stainless chainplates with titanium: While I applaud the author’s attempt to “think out-of-the-box,” I think he started off with a flawed understanding of the failure modes involved. In my opinion, the failure had nothing to do with galvanic reaction of the stainless. The photos show that the chainplate failed due to a combination of pitting as well as stress corrosion cracking. The author does not mention the particular alloy involved; however, given the age of the boat, I would bet that it was 304.
While the particular alloy used may indeed have a limited lifespan in this application (my guess is that the chainplate was in service for at least 30 years), this does not mean that all grades of stainless would be wholly inappropriate for this application. After all, I would be fairly certain that the propeller shaft on his boat is made out of Aquamet 22 (a super-austenitic alloy that is identical to the Nitronic 50 used for rod rigging), and it sees full immersion service in salt water and marine growth.
Through-deck chainplate tangs must be resistant to pitting with the understanding that there will be some concentration of chlorides/salts that will eventually build up on the surface. More importantly, the chainplate is obviously subject to constant tensile loads in this chloride environment. This turns out to be the perfect recipe for stress corrosion cracking. Therefore, the simplest answer would be to choose the appropriate stainless alloy that will resist both pitting as well as SCC. In this case, AL6XN would perform admirably. While this alloy was not available when the boat was originally built, industrial demand since has driven mills to produce significantly higher grades of stainless. While the material is harder and somewhat more difficult to work with than 304, it would prevent no fabrication problems for a proper machine shop.
The idea the author would go through the bother of sourcing titanium in China seems impractical to say the least. At the risk of being politically incorrect, I would underscore that quality assurance in terms of alloy grade would be virtually nonexistent. Given the critical application, I would want to know specifically what I am getting and from where. Domestic mills provide alloy certification forms with every load of material that is shipped out. At the same time, having custom hardware produced by a domestic machine shop would allow a closer business relationship with a fabricator who may have a more intimate understanding of the application.
While titanium is an intriguing metal, one has to be very careful in selecting the application. Although titanium itself is extremely noble and resistant to corrosion, it must be matched to the fasteners used. If not, then the mass of titanium will tend to induce pitting corrosion in the common stainless fasteners he may have reused.
The author would have done far better to have his chainplates fabricated in AL6XN stainless. The job would’ve been less expensive, virtually immune to the original corrosion issues, immensely strong, and would not set up a whole host of new potential corrosion issues. Non-insignificantly, he would’ve been giving his business to a domestic steel mill and machine shop.
Marine Surveyor, NAMS-CMS