Stainless Steel Rigging
Stainless steel is an alloy that contains at least 50% iron and 10% chromium. The chromium inhibits corrosion and thus plays a part in defining stainless steel. The more chromium, the more corrosion-resistance, up to a maximum of about 30%. But chromium is not the only factor in corrosion resistance. Many other elements are added to enhance the properties of a particular grade and type of stainless steel.
Stainless steel alloys are grouped according to the structure of their crystals. Adding nickel creates the structure used in marine applications, called austenitic. Austenitic stainless steels are identified by their 300-series designation. Types 302 and 304 stainless are widely used for rigging, fasteners, fittings and propeller shafts.Type 302 is a general purpose stainless steel with good strength properties, which is resistant to many corrosives. Most of the stainless produced today is type 304, a low-carbon variation of 302, also called 18-8, because it's made of 18% chromium and 8% nickel. There are many 304 sub-alloys formulated for specific applications. Type 304 has good resistance to corrosion by a great number of chemicals. Consequently, it satisfies a broad demand for adequate performance at an affordable price. But there are a number of marine applications where 304 and other 300-series types are inadequate. For instance, 303, which contains sulfur or selenium for easy machining, has poor corrosion resistance in the marine environment.
By adding more nickel and 2% molybdenum to 304 stainless, you get type 316, which has the best corrosion resistance among standard stainless steels. It resists pitting and corrosion by most chemicals, and is particularly resistant to salt water corrosion. There is a trade-off, however. Type 316 is only about 85% as strong as 302 or 304. Among the best austenitic stainless alloys for rigging are the "super stainless" variants: Nitronic 50 (also called 22-15-5) and Aquamet 22, which contain nitrogen and vanadium. Their "super" status is, of course, reflected in their price.
When stainless steel is produced, the chromium forms an outer oxide layer. As long as that layer remains intact, the stainless remains passive. Stainless steel that has been passivated is immersed in a heated bath of phosphates or salts which form an oxide film to seal off the iron to prevent it from going into solution in water, which is what produces corrosion. Once the oxide layer begins to break down, the stainless steel becomes active and its corrosion resistance is reduced. Rust is the obvious, visible evidence of corrosive activity.
There are a variety of ways in which the oxide layer is compromised. These include pitting and crevice corrosion caused by microscopic water-retaining cracks or scratches, microscopic impurities, galvanic corrosion, corrosion fatigue and stress fatigue cracking. For rod rigging, corrosion fatigue is the biggest enemy. Stress on rod rigging is concentrated at the rod head, which eventually suffers cracks you can't see without disassembling the rig, so failure is difficult to predict. In wire rigging, the stainless wire is subject to stress and fatiguing every time the boat rolls which, over a 10-year average lifespan, adds up to literally millions of stresses on the rig. The insides of the lower terminals of swaged-on fittings and the insides of barrel-type turnbuckles collect water. The corrosion that results will likely be invisible or difficult to detect. Unfortunately, a failure may be the first indication that your stainless is deteriorating.
Rigs constructed of 316 stainless will generally outlive those built of 302 or 304, especially in warm tropical waters, where salt water corrosion is a formidable adversary. If you select 316, consider increasing your wire one size to make up for the reduction in strength. But price carefully. Stepping up one size in wire will increase your rigging strength 15-16%, but your cost may increase exponentially, since the wire is not the expensive part of the bargain. The larger turnbuckles, jaws, eyes, clevis pins, etc. required by the larger diameter wire can wreak havoc on your budget.
Regular inspection is the best prevention (as usual on a boat). Use a 50x pocket magnifier if you really want to see what's happening on the surface of your stainless rigging and fittings. For preventive maintenance, polish the surface of your stainless with a stainless steel polish like Wichard's Wichinox. Wichard achieved its reputation for superior polished stainless finishes by polishing their hardware twice as long as anyone else in the industry. Polishing creates a smooth metal surface and helps minimize the number of pits, valleys and microscopic cracks where moisture can reside and create the environment corrosion loves.