I found the recent article on attaching a keel by Michael Carr (“Keel attachment,” Issue No. 77) quite timely. I just replaced the keel nuts on my Beneteau 345. The original nuts were carbon steel, the new ones are “stainless.” The keel bolts also are a stainless steel, grade unknown.
In the article, Mr. Carr writes, “Nitronic 50, [is] a stainless alloy known for its strengthit is commonly used in propeller shafts.” This is not quite correct. Let me explain the differences.
Although the chemical composition of Nitronic 50 and “the alloy commonly used in propeller shafts,” Aquamet 22, is essentially the same, there is a significant difference in the thermal processing of these two products. Aquamet 22, the boat shaft (propeller shaft) alloy, receives no thermal processing following the conversion from billet to bar stock. Nitronic 50, on the other hand, is solution-annealed at some temperature between 1,850° F and 2,200° F, after rolling, to place the material in a condition of ultimate corrosion resistance. So, even though the two productsAquamet 22 and Nitronic 50have the same chemical composition, they have significantly different thermal processing. The combination of chemical composition and thermal processing is necessary to give Nitronic 50 its superior corrosion-resistance properties.
Aquamet 22 is the higher-strength product and does not possess the same superior corrosion resistance as Nitronic 50. That is why it is used in shafts and bolts for marine applications. Nitronic 50 is used in applications requiring ultimate corrosion resistance with some lesser-strength considerations such as boiling water nuclear power equipment and hardware. I note in passing that high-strength Nitronic 50 is exactly the same as Aquamet 22.
In either case, it is true that Nitronic 50 is superior to Type 316 and almost any other alloy, stainless or otherwise, used in the marine environment. The addition of nitrogen rather than carbon to the iron-chromium-nickel alloy system is a what gives the family of nitrogen-strengthened stainless steels its superior properties.
It has been 12 years since I left ARMCO Steel Corp., Baltimore, Md., where I was the supervising metallurgist, responsible for the manufacturing of these alloy systems. Aquamet and Nitronic are patented trade names for the ARMCO products. Because of the financial direction ARMCO moved into, it no longer may be possible to purchase these alloy products. Carpenter Steel Co., Reading, Pa., had a similar family of alloys sold under the trade name of Marinalloy.
John R. Coleman lives in Timonium, Md.
