Brains
Well-known member
Posted on a facebook site in a language I cant read (French...): https://www.facebook.com/GrottesDeHotton?fref=photo
Guess a bad dose of corrosion!
Simon Wilson said:but surely everybody knows that.
Simon Wilson said:It looks like galvanic corrosion caused by a stainless steel hanger being used on a carbon steel throughbolt.
Stay away from any rusty bolts and only ever use carbon steel bolts for short term use - but surely everybody knows that.
royfellows said:Simon Wilson said:It looks like galvanic corrosion caused by a stainless steel hanger being used on a carbon steel throughbolt.
Stay away from any rusty bolts and only ever use carbon steel bolts for short term use - but surely everybody knows that.
I am sorry but completely disagree with you on this.
The difference in the galvanic index between two types of steel would be very minimal, beside the corrosion would occur at the point of contact not somewhere else.
I would say that the bolt has been inserted into pyrite bearing rock and the corrosion has been caused by the breakdown of the pyrite into hydrous iron oxide and sulphuric acid.
This is not going to be an issue in the huge majority of natural caves but is areal issue in mines. There has recently been occurrences of this in a very short time scale in mines in the Gwydir Forest.
SamT said:amazing... looks reasonably ok from the outside too.
royfellows said:What I am looking at in the photograph is typical of the effect that I describe in that the main area of corrosion is along the length of the bolt which is where the most of the acid would gather. Discharges from some mines have PHs low enough to be used as battery acid!
If you look at the point of contact between the dissimilar metals the corrosion is minimal.
Differences in the anodic index (sorry in last post said "galvanic index) of 0.15V or less even in high humidity environments can be discounted.
A good example of my point regarding the contact area of the dissimilar metals is the way that fingers used to fall off clocks. The clock mechanism would be of brass including the main spindle, and the fingers of aluminium pushed on as an interference fit. Where the two metals contacted would be the area of corrosion, causing the fingers to fall off.
The big issue here is the question of how this is likely to affect the safety aspect of what we do.
My advice here is to be suspicious of any rock showing iron coloration. To make matters worse the use of stainless through bolts only buys time, in sulphuric acid they will corrode. There is no easy answer to this, even the epoxy seated bolts designed for purpose will in time fail.
Of course, there is also the question of the effect of low ph water on our nylon ropes.
Trust me to open a can worms!
Simon Wilson said:royfellows said:What I am looking at in the photograph is typical of the effect that I describe in that the main area of corrosion is along the length of the bolt which is where the most of the acid would gather. Discharges from some mines have PHs low enough to be used as battery acid!
If you look at the point of contact between the dissimilar metals the corrosion is minimal.
Differences in the anodic index (sorry in last post said "galvanic index) of 0.15V or less even in high humidity environments can be discounted.
A good example of my point regarding the contact area of the dissimilar metals is the way that fingers used to fall off clocks. The clock mechanism would be of brass including the main spindle, and the fingers of aluminium pushed on as an interference fit. Where the two metals contacted would be the area of corrosion, causing the fingers to fall off.
The big issue here is the question of how this is likely to affect the safety aspect of what we do.
My advice here is to be suspicious of any rock showing iron coloration. To make matters worse the use of stainless through bolts only buys time, in sulphuric acid they will corrode. There is no easy answer to this, even the epoxy seated bolts designed for purpose will in time fail.
Of course, there is also the question of the effect of low ph water on our nylon ropes.
Trust me to open a can worms!
Agree with most of that but it is highly complex and very difficult to predict. All the more reason why we should assume that eventually all anchors will need to be removed no matter how good they appear at present.
paul said:And always back up main anchors.
cavermark said:Has anyone been able to decipher the translations to find out what rock type or environment the bolt is from? I think all of the previous suggestions are possible.
The photo below shows galvanic corrosion between stainless and carbon steel in marine environment. I'm under the impression that in a galvanic "cell", the wastage occurs on the point with highest electropotential (eg. sacrificial zinc anodes in seawater ballast tanks on oil tankers), but I'm no chemist.
Bograts idea sounds feasible - I'm sure I've seen cheap thrubolts with alloy sleeves (designed for building work not personal protection).
royfellows said:... there appears to be some corrosion of the stainless hanger.