A tale of four krabs

[Penguin]

The story of four DMM Shield wire-gate alloy karabiners...

As named below, left to right: the grey, the silver, the silver-pink, and the grey-pink (the grey colouring i think is anodising of some sort, the silver ones seem to be self-coloured and uncoated).

Grey and silver were a pair on a quickdraw, grey-pink and silver-pink were also a pair on a quickdraw.  The ones with pink marking were bought first, the unmarked ones second, with maybe 12 months between them.  Both pairs were bought from the same shop and i think came from the same stock order. 

The pink marked ones were fitted to the bungees on my sidemount harness.  When used in the sea the harness was always washed in fresh water.  When used in 'fresh' (i.e. peaty cave) water the harness wasn't always washed unless it was muddy.  Whether washed or not the harness was either dried outside or in a warm garage in an open box - it was never stored damp in a sealed container or bag.  In between uses the harness was stored in an open box in a warm garage.  Sometimes the karabiners were left clipped to the stainless steel D rings on the harness. 

The grey-pink one corroded rapidly after a sea dive in late 2010.  After three weeks of careful storage although clipped to a SS D ring, it looked like this:

Its brother, the silver-pink one, was treated in exactly the same way but showed very little corrosion (also clipped to a D ring during the same three week storage period). 

After the corrosion appeared on the grey-pink one i often made sure they were touching no other metal in storage nor each other.  I left them on the harness and continued to use them both equally.  (I bought the other pair to use on bottles.)  Tonight i decided to replace the grey-pink one.  This is how they look now (remember, both have been treated exactly the same):

Silver-pink (still on bungee), grey-pink removed.

Grey-pink detail of corrosion around wide end (the end that i clip to the D ring when diving).

Grey-pink detail of corrosion around the gate hinge (part of this area was wrapped in the bungee in use).

And the grey-pink corrosion after a wee poke. 

What's interesting about the corrosion on this pair, and makes me now wonder about ascribing it solely to galvanic corrosion between the alloy and SS hardware, is that it's largely confined to the grey coloured karabiner and is very limited on the silver karabiner (i'd almost use the silver one on a cowstail, if i didn't know its history and its brother's history).  Even more peculiarly, i looked closer tonight, and the corrosion is confined largely to one side of the grey karabiner, and appears to me to be more related to the forging of the karabiner (note the pronounced separation between good and bad metal, and the 'delamination' of the corroded metal). 

The reverse side (the side without the DMM marking) of the grey-pink karabiner:

Here's a close-up of the broad end of the silver-pink karabiner for comparision:

And the same area on the silver karabiner (one of the newer pair, fitted tonight to the harness bungee in place of the grey-pink):

The two non-pink-marked karabiners have been used once or twice for climbing (until the grey-pink started corroding), and once or twice on tackle bags, maybe once or twice on bottles in fresh water.  On the whole they've had a much more gentle life than the pink marked ones. 

Here's a close-up of the grey karabiner, penknife blade pointing out the slight beginnings of corrosion (no corrosion is visible on its silver brother):

If you have any of these karabiners used anywhere as critical components you might want to check them...  :-\

 

[ianball11]

Thought provoking stuff, I do go to lengths to separate out dissimilar metals but not as a rule.

I doubt I'll ever use them in salt water though.

 

[Penguin]

Admittedly my usage probably wasn't envisioned by the engineers, but climbing on sea cliffs would expose hardware to salt, and throwing them altogether in a bag would bring them into contact with other metals.  And we all leave kit damp and muddy in a bag or car boot for a few days while caving or climbing... 

Here's a better photo showing the separation between corroded and not (visibly) corroded metal:

The same divide is apparent at the gate hinge too.  Also the hinge area rarely contacted the stainless steel.

All the (aluminium) karabiners i've seen that have galvanicly corroded have not shown this separation and have corroded much more evenly. 

 

[Burt]

I'm using mine on my lifejacket harness, so they don't get as heavy loads as in a climbing situation. Also depending on the boat I'm on they could be running along a stainless wire, or a nylon tape. They are so far as good as new.

The thing with Mr Penguin's that springs to my mind is a brinelling effect caused either by repeated "hammering" on to a bolt or wired trad gear, which has damaged the outer coating and started the corrosion in this area.

Have you contacted DMM? they are usually quite helpful and like to see stuff like this so they can make improvements on future kit - you may even get something in return! Even if not, you can't lose anything by sending them some pics and a few notes.

 

[Burt]

I should point out that brinelling is a frequent light tapping in one area, not a good whack with a mallet!

 

[Penguin]

Are you using the plain silver ones or the coloured grey ones?

I've not contacted DMM, yet.  It would be good to hear what they think. 

The most corroded one, the grey-pink, has never been used for climbing, it has only been used on a bungee cord on a sidemount harness to attach cylinders.  So fairly light loads - up to 16kg with a 12L cylinder.  Sea and fresh water dives.  And generally cared for. 

The silver-pink has had exactly the same life until this evening, and shows nowhere the same amount and depth of corrosion. 

Here they are (new) on my (new) harness, c. February 2010:

 

[jarvist]

I've certainly seen de-lamination like this in most badly corroded krabs I've come across. I imagine it comes directly from their method of construction, which I always imagined was via a forge rather than casting? You could certainly then imagine how these patterns would be imprinted by the mould, with all the weird grain flow you'd get.

I had some climbing krabs with a harness as a bosuns chair on a small yacht, within a few months of sailing (& no real use, just exposed to the salty air, perhaps a little spray) and they were badly corroded to the point of me not wanting to use them.

This summer I was derigging a short pitch in an Alpine cave. Someone had supplemented the rusty maillon for the hang with a standard Alu scregate krab two years ago. It looked in a very bad state, the metal flaking off like above. The screw gate was stuck, whereas the maillon could be easily spannered open. I hit the gate of the krab a couple of times with my rigging spanner to try and free it, and was amazed to see it pop open -- by the hook of the carabiner simply ripping through the gate like it was paper! Sobering stuff.

I think it's certainly true that the Alloy the make carabiners out of is not suitable for an environment with ions wandering around, whether that be in the sea, long term in a cave or on damp concrete.

I believe the standard Marine Alu Alloy is one of the corrosion resistant 5052 series (i.e. diving back plates, hulls of lightweight metal ships), whereas the starting point for lightweight stuff is 7075, which is known for being less corrosion resistant than even the average Alu Alloy (not surprising considering it's full of Zinc, the standard 'sacrificial anode' for marine applications!).

 

[hrock]

it is my obseravtion that some krabs are fare more prone to this than otheres and it seems to be more dependant on anodising than the age manifactura tipe etc

in fact the one thing i have noticed is the old ali krabs if anodised are not very prone at all

so i think there is a chance that as time has gone on manufaturas have been trying to give there kit less and less aodising  to make it look good and sell but this has made it more corrosion prone.

the different colours are from different prossesis and i would guess have different properties.

could ask and anodising expert to recommend a colour  (any one know one)

 

[darwen dave]

Black or dark grey anodising is actually harder than case hardened steel. Remember, aluminium oxide is what many grinding wheels are made of.

So, forget the aesthetics and go for the stuff that works. I speak from experience after having to machine both hardened steel and anodised ally on a regular basis. Hope this helps.

 

[Penguin]

Coincidently, DMM have just produced this video on anodising - http://dmmclimbing.com/knowledge/anodising-vid/

 

[pete_the_caver]

Very interesting...  I saw the same thing on crabs left in a French cave last year.  I have also seen this happen with aluminium cold welds on ladders in New Zealand.  We found that ladders would corrode badly in some caves when left and could be unusable even after two weeks. 

I was once presented with a cold weld made from 1/2" ally on a ladder where 'toluit' weld holding a clip onto the end had failed where the ally had become crystalised and then fractured though the crystals.

The lesson... always check iron/ally mongery before you hang your life on it.  saying that... titanium scares me even more.  It can fail suddenly due to work-hardening.