It is often heard in sailing circles that “stainless steels fail without warning” or “cannot be relied upon.” This is not true. Stainless steel grades are widely used in industrial situations where total reliability is required. Provided its limitations are understood, a suitable grade will almost always be available.
Corrosion
Most yacht fittings are made from grades in the 300 series, which are highly resistant to general corrosion, but tend to be susceptible to chloride pitting. 316 grade, or A4, has molybdenum added to reduce this tendency, whereas the other commonly used grade, 304 or A2, has none. The problem with pitting is that it can reduce strength without being highly obvious, but close inspection will identify it. There can be problems if the pitted area is hidden from view
Most metals are susceptible to crevice corrosion, a condition that occurs in joints and close fits between two components when exposed to something as corrosive as seawater. Stainless steels are susceptible, although not to a marked extent, accounting for the rusty stains that occur at bolted joints, for example.
The best way to avoid both crevice and pitting corrosion is to exclude seawater by the use of lanolin or grease, although good quality sealants can be effective. I warm my swaged fittings each season and melt waterproof grease into them, to exclude water.
When making up Stalock or Norseman fittings I always cover the cone area with silicone before tightening the screw so that it extrudes out after filling the fitting. Waxoyl seems to work especially well in excluding seawater.
Rigging
By far the biggest problem that both wires and screwed fittings face is not corrosion but fatigue. Displacement is the main killer, so the answer is to minimise it by keeping rigging tight. Corrosion enters the picture because a pit is a stress-raiser, so fatigue will tend to start at pits before it would on a smooth, uncorroded surface.
A simple little demonstration can illustrate this. Bend a piece of aluminium welding rod, or some other metal if you are strong enough, back and forward until it snaps. Now make a little nick in another rod and do the same again. The nicked one will fail normally in less than half the number of cycles. This is a perfect illustration of the effects of fatigue, stress-raisers, work hardening and many other phenomena.
Cables never fail with a single fracture in my experience. Each strand develops its own little fatigue fracture, until the strength of the remaining part is insufficient to carry the load. It then fractures in overload. Ultimately the remaining strands are unable to sustain the load and they all fail in overload. Every rigging wire failure I have seen starts when one strand unravels. Although pits are a common source, mechanical damage can be another, so poorly made-up swages can have a very short life.
I have yet to see a cable that failed in normal service without the involvement of fatigue. Only one other mechanism is realistically possible. The classic situations in which overloads occur are either impact or some other effect by which actual loads exceed design loads, for example 360 degree capsizes and colliding with bridges, or reduction in cross-sectional area by general corrosion. But stainless steels do not suffer from general corrosion.
A common argument is that galvanised steel wires survive when stainless ones do not. There are several reasons for this:
- Galvanised wire tends to be used on older or traditional craft, where specific loads are lower
- Pitting tends not to occur with galvanised wire, and general corrosion is inhibited for long periods
- The strength, and therefore the fatigue endurance limit, is higher for carbon steel than for stainless
Fittings
Do you remember the infamous Chay Blyth British Steel forestay fractures? The forked cable end at deck level was incorrectly manufactured, causing it to foul the fixed part, inhibiting full movement of the fitting and over-stressing one side of the cable. A fatigue fracture began at the highly stressed point.
All fittings must be free to accept design movements of the cable without lateral restrictions. Modern fittings should be designed to minimise local stresses, but older ones may have sharp internal edges, section changes and other nasties. All can start fatigue cracks.
Some older T-ball fittings on masts were not well designed and the swaged part formed a slight angle with the cable, a prime source of fatigue. One of my lowers failed here about five years ago. I replaced the whole lot with the later type, which is of a better design.
Finally
Regular inspection is the key to reliability. Any evidence of pits, cracks or stress is sufficient to condemn the item. Areas that cannot be inspected should be protected if corrosion is likely to be a problem.
