UKC

I'm looking at replacing some perma-draws some chain draws. As galvanised steel maillons and aluminium carabineers are significantly cheaper than their stainless equivalents, I'd like to save some money if possible.  The bolts and chains they'll be attached to will both be stainless, so ideally... or rather: most cheaply,  the set up will be:

SS expansion bolt - Galvanised maillion - SS chain - Aluminium crab.

A quick google suggests that this won't cause the stainless components to corrode, but that it may increase galvanic corrosion on the galvanized and/or alu parts.

So question is: Is buying cheap buying twice? i.e.Will the maillon and crab last until they wear out, or will they need replacing due to corrosion in a year or two?

For info... The draws will be outside and exposed to the UK's glorious weather, but aren't anywhere near the sea.  

I think it should be fine. My understanding is that the presence of stainless will cause the aluminium and the zinc to corrode faster. I would probably use galvanised chain rather than stainless - it'll last plenty long enough and be a significant cost saving. Easily replaced too when the time comes, unlike the bolt (which should be stainless).

As I understand it from the one corrosion course I've done with work, the galvanic corrosion will concentrate all the corrosion that would have happened on the less reactive metal into the more reactive one that is bonded to it.

So your alu and galv bits will take more hammer if connected to a long ss chain than a short one.

I also got the impression that a continual state of damp is worse than being fully submerged as the damp lets the ions move around while the air exposure gets the oxidation going.

So while not by the sea you might want to consider whether it will be quick drying when it does get wet, and when it does get wet is it just rainwater or will it be getting exposed to some ground runoff with dissolved ions that might speed up corrosion. As well as keeping the chain length frugal.

They didn't teach us how to guess at the corrosion rates though so feel like I can't help there!

Edit following above reply:

As I understand it a galv chain would share the load a bit over what would have been attacking the maillon. I can't remeber offhand what wins in the fight between alu and zinc. If zinc wins the fight then it might be better go galv chain, ss carabiner?

Yeah, I think minimising the quantity of stainless will help reduce the overall galvanic 'charge', as well as reducing the cost! Examples I have come across - a stainless bolt in aluminium structure won't cause too much trouble (possibly some local corrosion in the alu around the bolt), but alu rivets in a stainless structure will disappear worryingly quickly. It's about relative quantities of the different materials.

Forgetting the economy for a moment (sorry, that's your actual question) and looking at safety: Worst-case is corrosion where it's not obvious  (like stainless everything but a galv bolt in the rock). Since the bolt/hangar you say are stainless then any corrosion in other parts may be easier to spot.  From a safety perspective you should be OK

If it's a crux or loweroff then an alu krab might wear out faster than it corrodes (depending on...). If this was marine we could nerd out about mixing stainless types and galvanic series potentials, but if it's not then personally I'd chill.

It's a personal irritation of mine that stainless krabs are so expensive in UK when they seem less extortionate in the US (https://usstainless.com/hardware/spring-hooks/stainless-steel-316-spring-ho...)

In reply to jdh90:

Zinc is the winner of the zinc versus aluminium fight (i.e. Zinc corrodes) would link to galvanic series table but all the ones I could find are based on seawater so didn't link them as this isn't marine. Stainless welded long link chain (if that is what OP was thinking?) is also annoyingly expensive so your suggestion to save money on the chain is a good one

Why not make the whole system plated or galvanised steel? Then you wouldn't create the potentials in the first place?

Complain to the government for applying a special import tariff to stainless products and then adding 20% VAT on top of everything!

Agree all stainless is better. But if the bolt or hanger is stainless, that means the whole lot has to be stainless which is expensive.  Depending on how many draws were talking, if it's just a few can't OP have a whip-round and get others to chip in, to enable stainless everything.

In reply to mmmhumous:

Does the BMC have a fund that could help towards a re-equip  (or is that walking into a bureaucratic quagmire). Have you asked about for contributions, I mean you're doing a public service  (Good on you)

Based on others replies, theres it looks like we could do galvanised/plated for the chain. For the bolt (and hanger) though we really need stainless to ensure they last a decent length of time. We're bolting (and re-bolting) with stainless, as the nonrmal steel bolts on the earliest lines at the crag needed replacing after 11 years. 

I assume the bolts were galvanised? If so, logically, galvanised chains / krabs etc. will also need replacing after 11 years.  316 SS will last 'forever' (assuming it's not nicked), thus galvanised stuff is probably false economy, especially if you consider re-equipping time?

Even stainless through bolts can come loose and need replacing, although more commonly in softer rocks than Matlock limestone? So I wouldn't say "forever"

You've not heard of Stress Cracking Corrosion then. 316 if placed in locations where the bolts are not washed off by rainwater can and do corrode. So for example under roofs... nothing lasts forever...

Just some musings out loud.

Are some rock types more corrosive than others? Limestone for instance, thats dead sea creatures as far as my limited geology goes.  Does it contain high chloride concentrations from those prehistoric seas and munch its way through 316 bolts faster than a nice dead Cumbrian volcano might?

Do we need to ask OP what rock he's going into?

Of course I've heard of SCC, but it's almost certainly not an issue inland in the UK's relatively cool climate.

In reply to @jdh90 

Definitely in the damp category, as I'm mainly after the chain draws for the bolts in the greener parts of the crag (where sling or rope draws with get a bit mouldy/deathy) . In the main, only thinking of relatively short lengths of chain (ie ~15cm).

In reply to @CantClimbTom

Tell me about it! shopping around for a good deal seems to mainly getting my hopes dashed by Google shopping when I click a link only to find out it's  a US/Canadian site.

The crag (The dry-tooling cave at Masson Lees) is covered by the Peak bolt fund, but I'm lead too believe the focus is on re-bolting rather than new routing and equipping (which makes perfect sense to me). For the latest round of development, We've had ~10 crag regulars chip in so far, but cash donations have been focused on bolting/re-bolting. As soon as we've finished the next round of routes, we'll start asking for donations to cover equipping/re-equipping. However, we'd like people's donations to equip as many routes as possible (without them all needing replace in a couple of years time). They don't have to last as long as the bolts, but it'd be good if they lasted 5-10yrs or until worn out.

As @CantClimbTom guessed, it's Matlock Limestone.... there's several types: the normal grey limestone prevalent throughout the peak, some dolomitic limestone and a fare amount of 'sandy limestone' aka choss. 

In terms moisture exposure: Some of the bolts are permadry as they go through the cave roof, and aside from a bit of seepage after heavy rain/ projectile sweating from climbing on the cruxes won't see any moisture. For the most part these routes are already equipped with mainly sling/rope permadraws, so we've started replacing these as they wear out. The unequipped routes are either exposed to the rain, subject to heavy seepage/run-off or both. These are the ones we're looking at chain draws for. The permadry ones we'll probaly replace with heavy duty sport draws.

Reason for SS Chain is that was what was what was availble from bolt products: http://www.bolt-products.com/MaillonsRingsandChain.htm

Just realised the captive pin crabs I've been looking at for preventing theft/cross loading are actually plated steel (not Alu): https://rope-master.com/en/steel-carabiners/697-carabiner-be-smith-captive-...