By Tom Burden
Stainless steel is an alloy that contains at least 50% iron and 10% chromium, which inhibits corrosion. The more chromium, the more corrosion-resistance, up to a maximum of about 30%. Many other elements are added to enhance the durability 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 austenitic stainless steels, identified by their 300-series designation and used in marine applications. Types 302 and 304 stainless are widely used for rigging, fasteners, fittings and propeller shafts. Type 302 is a general-purpose corrosion-resistant stainless steel with good strength properties. Most 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, satisfying 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 2-3% 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 saltwater 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, with twice the strength of Type 316 and used in rod rigging from Navtec) and Aquamet 22, which is often used in propeller shafts and 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. To prevent corrosion, the passivated stainless steel is immersed in a heated bath of phosphates or salts. This solution forms an oxide film that seals off the iron, preventing it from going into solution in water. 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 invisible cracks 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 both of the lower terminals of swaged-on fittings and 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 saltwater 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 itself 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.
The story you are about to read is true. The names have been omitted to protect the guilty.
We were helping our friends try out their rebuilt Atomic Four engine on their 1976 Catalina 30 on a perfect February day on San Francisco Bay—15 knots of wind, glorious, bright sun, 62° temperature, two foot chop. The engine purred like a kitten as we motored out of Coyote Point Harbor, raised the main and unfurled the jib.
We cut the engine and sailed easily along for a few minutes on a beam reach, and then began grinding in the jib on the Barient winch to head up onto a beat. Let’s see how she goes to windward! Suddenly, there was a loud BANG and, looking up, we saw the top part of the mast bending alarmingly to leeward, resembling the rig on our Laser. Holy cow!
Quick and decisive action, along with a telephone pole mast, allowed us to save the rig. We blew the sheets, went head-to-wind, roller-furled the genoa, restarted the Atomic and lowered the mainsail. Only then did we notice the bottom of the port upper shroud dangling limp with the turnbuckle swinging and clanging about.
The trip back to the slip was drama-free. The cold beers at the dock were most welcome!
This is a classic case of crevice corrosion taking place in an oxygen-starved environment, hidden beneath the sealant. Each time you tack, the chainplate flexes ever so slightly relative to the deck it passes through. This tiny motion breaks down the bond between the sealant and the chainplate, allowing salt water, with its corrosive chloride content, to enter, be held there, and do its dastardly damage.
In general, regular inspection of your standing rigging is the best prevention (as usual on a boat). Use a 50-power 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.