Treating 316 wire rigging with anti-corrosive?

[Hogan]

I'm currently re-rigging my Catalina 30's standing rigging. It's a 1984, and I'm pretty sure it has its original wire.

The bobstay (it's a tall rig) parted a couple of years ago at the lower swage. Wire looked perfect from the outside, but had crevice-corroded away to dust on the inside.

Recently one leg of my split backstay began parting, so that was it - I stood down and began refitting ALL wire - shrouds, stays, and lifelines.

Upon inspection, again I noted that the backstay looked nice and bright and shiny on the outside, but when I started unlaying it's strands, the inside surfaces were very corroded.

So 316 wire rope disintegrates from the inside-out. Very insidious because it's impossible to routinely inspect it. Water and salt become trapped between the strands, passivation breaks down as the wire works, and the water acts as an electrolyte, eating away at your rig, slowly but surely over the years.

My thinking is this:

Why not treat the wire annually with a penetrating anti-corrosive like CorrosionX or Boeshield?

You should inspect your entire rig at least annually, so why not hit the wire (and associated fittings) with AC while you are at it?

Does anyone here do this?

I know 316 rigging is supposed to be replaced every 10 years or so, but how many sailors actually do so? From what I've seen, 316 can last much longer, and it's corrosion, not load cycles that seem to eventually do it in.

Thoughts?




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[roverhi]

Okay, so some people ignore the recommended 10 year life cycle. Make it 15 years. In your case you were asking for a dismasting. When you lost the baby stay a couple of years ago, you should have changed out all the rigging. Ignoring a 2x4 to the head warning like a failed wire makes me wonder about your grip on reality.

Most corrosion resistant treatments are oil based. That means they come with a big 'mess' quotient if you are going to use them in the quantity and regularity for them to have the desired effect. You should also end for end the wire regularly so the anti corrosion treatment will have gravity working for it. Rerig the boat with Norseman/StaLok terminals and swap the wire out at least every two decades. Don't forget the chain plates, they seem to fail more often than the wire.

[Wotname]

Non-expert response

316 requires the presence of oxygen to maintain it's corrosion resistance; using an anti corrosion coating will prevent that oxygen from doing it's magic.

[Hogan]

Yes, I know the rig was / is WAY overdue, and that's why I'm replacing it, so I don't need a lecture about dismasting - I'm well aware of the risks. I have monitored the rig closely and don't sail offshore or inshore for that matter in winds exceeding 15 knots. I reef at 10 knots, and keep the rig tight and properly tuned.

I've now replaced all but the forestay and capshrouds - myself - so it takes time. I'm doing it with the mast up, hoisting myself with a bosuns chair and tackle. It's hard, dangerous, expensive, time consuming work.

The bobstay (not "baby stay" ) wasn't properly toggled by Catalina when they built the boat. The lower swage spends 1/2 it's time immersed in saltwater while underway. It's the most heavily loaded stay on the boat, and subject to mechanical damage from docking mistakes. It's exactly the same wire as the upper shrouds, which don't see nearly the same loads or abuse as that poor bobstay.

So while the rig definitely needed / needs replacing, the risk I took with it were calculated, and guess what?

The mast is still standing. Your results may vary.

Now, back to my original question about Anti-corrosives:

Two recommended by Practical Sailor for above deck are CorrosionX and Boeshield. Both are very sophisticated treatments developed for aerospace and the protection of aluminum and electrical components. Both contain passivating agents designed to block the corrosion process at the molecular level, as well as displace moisture and prevent its entry into assemblies and such.

From what I understand, corrosion is an electrolytic process that requires and anode, cathode, an electrolyte and electromagnetic field. Eliminate any one of these and the process stops.

Stainless steel is an alloy of low carbon steel and various other elements like chromium, nickel, and maganese. It's a chromium oxide surface film that gives it corrosion resistance, and the "oxide" part requires oxygen. When moisture is trapped inside of the wire, the oxygen in it binds with the surface chrome creating the anti corrosive film.

At first. Over time the free oxygen is used up by this passivation, and the film breaks down. Now you have a low oxygen electrolyte that sits inside the wire structure, literally eating it away from the inside out.

Repeated cycles of this weaken the wire, and eventually it fails - usually at a stress riser, like a swage end. My backstay began parting where my tensioning pulley met the wire at an obtuse angle.

SS wire is passivated during manufacture. My thinking is that maintaining this passivation with the appropriate treatment might prolong the life of the rig, keeping it stronger during its service life.

CorrosionX is a lighter oil based treatment than the wax based Boeshield. The main issue I see with treating the wire is that it's a chore, and that Boeshield leaves behind a slightly tacky wax film that could trap moisture inside an already wet wire. CorrosionX does not have that issue, but is less durable.

Again, I'm looking for informed opinions regarding this, not lectures about what a reckless idiot I am for waiting too long to replace the rig. The boat was new to me 3 years ago, and my surveyor thought the rig was serviceable then. It was only later I came to suspect it was unserviceable after the bobstay failure, and began taking steps to correct it. These things take time and money, both of which are in short supply.

Back in the old days, hemp rigging was regularly sized with pine tar to preserve it - why not do the same with this finicky modern material using the fruits of modern organic chemistry?




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[Stumble]

Hogan,

You are conflating two different corrosion methods that effect SS.

Large scale galvanic corrosion does require anode-cathode-electrolyte contact to happen. BUT and its a big but, stainless steel has the anode-cathode pair combined into one alloy. At the granular level SS can be both its own anode and cathode since the microscopic granules act seperatly. So all you need is an electrolyte, which dry salt will provide quite happily. A dry salt crystal is hydrophilic enough to pull water out of the air and start the process. So unless you are sure that the displacement compound you are using is fully embedded across every micro pore of the wire then you can't rely on it to provide protection. Because if a single mm of space is left uncoated that is where the corrosion will occur.

This is called inter-granular corrosion, and it is often the starting place for follow up corrosion.


In addition to this SS suffers from what's called Stress Crack Corrosion. Which occurs when SS is in tension and subject to load cycles. The work hardening from the cycle loading causes micro cracks to form. These cracks can then be penetrated by a salt crystal and you get inter-granular corrosion following. the metal literally rusts from the inside out. Which is why the only accepted methods of testing for this are x-Ray and dye penetration testing.

For rigging these tests are often close to the same price as replacing it anyway, so it really isn't worth the bother.


Finally SS suffers from work hardening. Which actually makes the metal stronger, but extremely brittle. This is often the culprit when a mast comes down without obvious signs of corrosion failure in 30-40kn winds. The rig gets shock loaded and something is too brittle to give just a little, causing a cascade failure. Basically part X breaks because it didn't give in sync wi the rest of the components and overloads since the strain can't be shed to other parts.

The only cure for work hardening is to re-temper the metal. Which is far more expensive than replacing it.



A corrosion inhibitor really only works to retard the first failure method, it can't do much to stop the other two. There is a reason no reputable rigging supplier recommends more than 10 years between replacement of standing rigging. At a minimum the entire rig needs to be professionally inspected at ten years, and every year after that.

Or you can do what we did and switch to Dynex rigging instead. Instead of inspecting it, we are just planning on replacing it every five years. But at a substantially reduced price.

[hellosailor]

"316 requires the presence of oxygen"
I was fairly certain that was *only* a problem in a DAMP or WET anoxic environment. Keep the water off and out, and it doesn't require constant oxygenation, and doesn't have the same DAMP crevice corrosion problems.
So for standing rigging? Should not be an issue. Finding a trained monkey to slide down the wires every month or so while sparingly applying Boeshield...somewhat more difficult.(G)
If there's a rope (fiber) center and the rigging is not all stainless strands, that can also hold moisture much longer. Perhaps that was an issue?
And changing the rigging at 10 or 20 years instead of 40...Just how much Boeshield (and monkey chow) would that save anyway?

[Hogan]

Thanks - that's the sort of informed reply I was seeking.

I figured since SS is an alloy, it can and does act as its own anode/cathode in the presence of an electrolyte and the earths magnetic field.

My reasoning behind the periodic application of a penetrating corrosion inhibitor is not necessarily to extend a rigs service life, although stopping or slowing one source of potential failure seems better than ignoring it completely.

The reasoning is to potentially enhance its performance during its service life - and given the fact that most of the boats I see around here do not follow recommended replacement intervals, it seems like regular rig maintenance like lubrication, tuning, and yes, corrosion inhibition might make these boats a little bit safer, or perhaps even much safer.

Kinda like airbags: You still need a seatbelt for optimal chance of survival, but airbags are better than nothing.

Since wire rigging, especially low on the boat, close to the spray rigging like bobstays, lower swages, and lifelines are subject to periodic salt water baths, anything that is designed to penetrate and displace water / salt and then bond to and properly passivate the steel can't hurt anything. A thicker wax based coating on brand new wire might keep that salt water out altogether, at least for a while.

Maybe it's not worth the trouble -

As you point out load cycles tend to embrittle stainless steel, and crevice corrosion is more a function of finishing processes and metallurgy than the large scale corrosion I'm seeing between unlaid strands on the old wire.

The question comes down to what the most likely mode of failure is, and that comes down to a variety of factors surrounding use, maintenance, and rig tuning.

I keep my rig bar tight - as dictated by tuning to just remove all slack from leeward cap shrouds under full press of sail on flat water. This limits the shock loading you speak of when things get nasty. It also limits metal fatigue and work hardening by limiting spar deflection, twisting, bending, and shearing load cycles and range of motion.

Walk around your dock sometime and check the tension on neighboring rigs by pulling on a cap shroud or two. My bet is that many boats will have fairly slack rigs, a result of old ideas about rig tension that applied mainly to traditional wood boats with wood spars subject to creep and deformation under load.

Anyway, thanks for the advice. I'm gonna cut open a few of my old swages and rigging just to see what 30 years of neglect did in highly loaded low in the boat SS.

FYI -the first thing I replaced were the lifelines - with uncoated 316. I can't believe they are still available with PVC coatings - that's just about the stupidest thing ever invented for sailboats.

Synthetic rigging is slowly gaining acceptance, but it's still challenging to find all the right bits to do it properly, or riggers familiar with it, at least around here. I like the fact it's light and strong, and has none of 316s aforementioned vices.

It's only concerns are UV degradation which can be mitigated by using covered line, and chafe, which you can at least SEE taking place, giving the prudent boat owner plenty of warning prior to failure. Some high tec line like Vectran are especially tough. Vectran is made of Kevlar, the same stuff in carbon fiber composites and bulletproof vests.

It would be great if people installing these newer materials were to have them destructively tested after replacement with load cells in a lab. It would be great to build a database of how much ultimate strength deterioriates over time and use and lattitude with such materials. I'm guessing they will outperform stainless steel.


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[barnakiel]

Originally Posted by Hogan

(...)

to be replaced every 10 years or so, but how many sailors actually do so?

(...)

Thoughts?

I know someone who does.

He will stick with his schedule too.

b.