Stainless steel and Maillons

[Simon Wilson]

The photo is of a zinc plated Maillon that was used for long term rigging in a wet location. You can see it has shiny zinc plating on the thread and that it is corroded at the top where it was in contact with the stainless steel resin anchor. It was brand new and in place for about two months. It is corroded to the extent of having it's diameter reduced by 0.6mm close to where it was in contact. This is the effect of galvanic corrosion.

In the past I just kept replacing Maillons but I have decided to invest in some stainless steel Maillons which should soon pay for themselves. Available from all good caving shops such as : https://starlessriver.com/shop/maillons_and_karabiners_rigging/maillon_rapide_7mm_stainless_long_opening

 

[topcat]

What's happening at the other end, ie the s/s bolt?

 

[Simon Wilson]

That is a good question. There is an effect on the anchor and I have been told that it is little more than surface discolouration. I shall consult my metalurgist friend and find out more.

 

[aricooperdavis]

So is it the zinc plating that's causing the bulk of the galvanic corrosion, or the steel underneath?

 

[D.Snaith]

So is it the zinc plating that's causing the bulk of the galvanic corrosion, or the steel underneath?

It would be the steel underneath...

 

[aricooperdavis]

Cheers, okay. So is the galvanic corrosion caused by the differences in the electrode potentials of the steel in the maillon and the stainless steel in the anchor? Sorry if these are simple questions, I just find it remarkable how 2 subtly different steel alloys can have such a big impact! :blink:

 

[bograt]

So is it the zinc plating that's causing the bulk of the galvanic corrosion, or the steel underneath?

I suspect the hardness of the stainless anchor caused mechanical erosion of the zinc plating, causing ingress of acidic water (electrolyte) to the dissimilar metal (zinc / steel) junction, allowing galvanic (electrolytic) corrosion to occur in the steel of the link.

P.S., posts crossed 

 

[aricooperdavis]

That makes sense; would that suggest that maillons which are going to be wet for most of their life would be better off not zinc coated, to reduce the electrolytic corrosion, or would they then just rust?

 

[cavermark]

There would still be a galvanic reaction between the carbon steel and the stainless steel if there was an electrolyte.

 

[Simon Wilson]

I only used the term 'zinc plated' because that is what the makers P?guet use to distinguish these Maillons from the Maillons made from other materials and I was avoiding using the term 'carbon steel'. But let's use 'carbon steel'.

There will be some very minor electrolitic action involving the zinc plating but we can ignore it. The zinc plating gives minimal corrosion resistance and it's function is to give a uniform shiny finish that looks good in the shop. It is so thin that you can rub it off with your fingers.

What we need to be bothered about is the loss of metal from the Maillon by corrosion of the carbon steel and in this case that corrosion is increased by electrolysis between the two types of steel.

 

[topcat]

That is a good question. There is an effect on the anchor and I have been told that it is little more than surface discolouration. I shall consult my metalurgist friend and find out more.

Given the number of long-term rigging on P bolts with rusting maillons there could be some significance to the answer!  Some pitches in the Dales have been rigged for years, maybe decades!

 

[Simon Wilson]

That is a good question. There is an effect on the anchor and I have been told that it is little more than surface discolouration. I shall consult my metalurgist friend and find out more.

Given the number of long-term rigging on P bolts with rusting maillons there could be some significance to the answer!  Some pitches in the Dales have been rigged for years, maybe decades!

That is why I am highlighting the problem here. If anybody has concerns about a particular site then they need to either sort it out by removing the rusty items or inform the CNCC.

 

[pwhole]

Isn't part of the problem that stainless maillons often get pinched in public systems? And even if they don't, at a fiver a pop, it's an expensive business rigging anything extensive for a long time. I've recently helped replace a bunch of old galvanised maillons that had been in a private site for years, and quite a damp one at that, but every one unscrewed straight away, though they hadn't got muddy whilst in there. But the same corrosion on the contact point had taken place, though no visible decrease in diameter. We did replace them all with stainless though, as it will be intended to last a long time, and the bolts and hangers already were stainless. When we've got over the shock of the cost of this installment, we might look into replacing the ropes too!