|From Ocean Navigator #115 |
Stainless steel is the name for a group of steel alloys. The most common metals added to steel are chromium, nickel and molybdenum.
Steve Babcock,Islamorada, Fla
Answer: A full answer to this question gets pretty technical. For those interested, the Boatowner’s Guide to Corrosion, by Everett Collier, a recent publication from International Marine, gives quite the best explanation that I have seen of corrosion, and of how different metals are alloyed and react to different situations.
The short answer is that most stainless steel in marine use is, in fact, some variant of what used to be known as 18-8 (the base steel is alloyed with18% chromium and 8% nickel), of which the most common variant is 304 (18%-20% chromium; 8%-10.5% nickel). Greater corrosion resistance is given by increasing the nickel content to between 10% and 14% and adding nitrogen and 2% to 3% molybdenum (316, and the less common 317); 316 (16%-18% chromium, 10%-14% nickel, 2%-3% molybdenum) is twice as resistant to pitting and crevice corrosion as 304; 317 (18%-20% chromium, 11%-15% nickel, 3%-4% molybdenum) is even better (it was developed for use in the highly corrosive pulp and paper industry).
If any of these alloys are to be welded, any residual traces of carbon left in the alloying process need to be removed (it leads to weld corrosion), which is signified by adding an “L” to the alloy number – e.g., 316L.
The next step up in corrosion resistance is a little more chromium, nickel, and molybdenum to create alloy 22-13-5 or similar alloys that go under such proprietary names as Nitronic 50 (used, for example, in Navtec rod rigging) and Aquamet 22 (used, for example, for high-quality propeller shafts). Nitronic 50 and Aquamet 22 are pretty much the same alloy with 22% chromium, 12.5% nickel, and 2.25% molybdenum (alloy 22-13-5 has a fraction more molybdenum). Additional corrosion-resistance can be imparted by adding more molybdenum – what are known as the “6% mollies” to form such “superaustenitic” alloys as 254 SMO (20% chromium, 18% nickel, and 6.1% molybdenum), AL-6XN (20% chromium, 24% nickel, and 6% molybdenum), alloy 926 (20% chromium, 25% nickel, and 6.2% molybdenum), alloy 31 (27% chromium, 31% nickel, and 6.5% molybdenum), and 654 SMO (24% chromium, 22% nickel, and 7.3% molybdenum). To quote Everett Collier, “The superaustenitic stainless steels are truly outstanding marine metals.”
A final point worth making is that in the old days the alloying process could not be controlled with the degree of precision that is now possible. Manufacturers would tend to err on the high side of the allowable ranges for the various ingredients in an alloy in order to ensure that the alloy qualified for the desired designation. Today, processes are precise enough to be able to shoot for the low end of the range and still fall within the necessary parameters. As a result, a particular grade of metal today, such as 316, may not be as corrosion resistant as it was, say, 30 years ago!