Corroded bolt :O
- Thread starter Brains
- Start date
Ed W
Member
Firstly thanks to Brains for posting this, and secondly apologies for a rather long and technical reply. I strongly suspect that this bolt was placed on a sea cliff and in a position where sea water could get into the gap between the hole and the bolt. All corrosion of steel is electrical (galvanic) in origin, and requires two things - an electrolyte and oxygen. In the case of a mild steel bar submerged in sea water, electric currents are formed between phases of the microstructure of the material. These small corrosion cells are distributed all over the surface of the material, hence more or less uniform corrosion. When differing materials are placed together, as stated above, larger electric potentials can be created (the voltage being dependant upon the position of the two materials in the galvanic series). The rate of corrosion is dependant upon the voltage, the higher the voltage the more rapid the corrosion, and the corrosion occurs preferentially at the anode. Place a bi-metallic cell into salt water, with lots of free chloride ions, and the result is not far short of Alka-Seltzer.
This can all be made much worse if the electrolyte can be made stagnant, such as in a narrow gap, as this allows concentration of the electrolyte to occur, rapidly increasing corrosion rates. This process occurs in pitting and crevice corrosion. Failure is seldom due to a single factor, and it can be dangerous to speculate, especially from a single photo. However, from what I can see, it appears that we have a mild steel bolt attached to a stainless steel hanger. This immediately means that the hanger is the Cathode and the bolt the anode, so we would expect the bolt to corrode preferentially to the hanger. Now, if the hole were wet, we would have (salt?) water in the gap between the bolt and the hole, which creates ideal conditions for crevice corrosion. The bi-metallic corrosion cell between the stainless hanger and mild steel bolt would start the process going, and the stagnation of the electrolyte in the hole would accelerate the process, and make the material in the hole more anodic, leading to preferential corrosion of the bolt within the hole in this case. The lack of corrosion on the mild steel outside the hole is therefore explained, and in fact this material would be partially protected as the crevice mechanism will tend to make the material outside the crevice more cathodic in nature.
If the bolt had been stainless, there would have been a much lower corrosion cell voltage in the first place, which at the least would have slowed the initiation of the crevice effect, and may have prevented it. I suspect that in fresh water that the process would be very much slower (though it can still occur) and of course as Roy has pointed out it can be accelerated by electrolytes other than salt. Fairly recently I removed a 50 year old half inch mild steel rawl bolt from a cave. The hole had been "moist" for all of that time, but the bolt was not corroded beyond some discolouration on its surface. In fact it screwed out of the sleeve perfectly.
Finally, this is a pretty spectacular example of corrosion, but there are other mechanisms that can lead to just as dangerous situations with just as little warning - stress corrosion cracking being perhaps the scariest. As others have stated, all bolts will eventually fail...
The links below provide some light background reading on corrosion...
http://www.marinecorrosionforum.org/explain.htm
http://www.steelconstruction.info/Corrosion_of_structural_steel
This can all be made much worse if the electrolyte can be made stagnant, such as in a narrow gap, as this allows concentration of the electrolyte to occur, rapidly increasing corrosion rates. This process occurs in pitting and crevice corrosion. Failure is seldom due to a single factor, and it can be dangerous to speculate, especially from a single photo. However, from what I can see, it appears that we have a mild steel bolt attached to a stainless steel hanger. This immediately means that the hanger is the Cathode and the bolt the anode, so we would expect the bolt to corrode preferentially to the hanger. Now, if the hole were wet, we would have (salt?) water in the gap between the bolt and the hole, which creates ideal conditions for crevice corrosion. The bi-metallic corrosion cell between the stainless hanger and mild steel bolt would start the process going, and the stagnation of the electrolyte in the hole would accelerate the process, and make the material in the hole more anodic, leading to preferential corrosion of the bolt within the hole in this case. The lack of corrosion on the mild steel outside the hole is therefore explained, and in fact this material would be partially protected as the crevice mechanism will tend to make the material outside the crevice more cathodic in nature.
If the bolt had been stainless, there would have been a much lower corrosion cell voltage in the first place, which at the least would have slowed the initiation of the crevice effect, and may have prevented it. I suspect that in fresh water that the process would be very much slower (though it can still occur) and of course as Roy has pointed out it can be accelerated by electrolytes other than salt. Fairly recently I removed a 50 year old half inch mild steel rawl bolt from a cave. The hole had been "moist" for all of that time, but the bolt was not corroded beyond some discolouration on its surface. In fact it screwed out of the sleeve perfectly.
Finally, this is a pretty spectacular example of corrosion, but there are other mechanisms that can lead to just as dangerous situations with just as little warning - stress corrosion cracking being perhaps the scariest. As others have stated, all bolts will eventually fail...
The links below provide some light background reading on corrosion...
http://www.marinecorrosionforum.org/explain.htm
http://www.steelconstruction.info/Corrosion_of_structural_steel
Simon Wilson
New member
Thank you Ed W.
The photos on the Train4underground website again show stainless hangers on carbon steel bolts. This is a very bad thing to do.
The photos on the Train4underground website again show stainless hangers on carbon steel bolts. This is a very bad thing to do.
royfellows
Well-known member
I have done further thinking on this and the situation is worse that I thought yesterday.
Most steel through bolts will be plated with zinc or cadmium so the galvanic reaction between these and stainless hangers is far worse that I thought. However the surface coating would be very thin and first to dissolve.
Where this has been done obvious remedy is to remove the hangers if they be left in situ.
There are permanent hangers in Coniston Copper Mines that have been there many years but those have hangers made up from heavy duty angle iron.
With reference to some of my earlier comments, anodic dissolution is not uniform but occurs mostly on the area facing the cathode. It is practice in electroplating vats to rotate the anodes so as allow a more uniform dissolution.
We are dealing with some very complex issues here.
Most steel through bolts will be plated with zinc or cadmium so the galvanic reaction between these and stainless hangers is far worse that I thought. However the surface coating would be very thin and first to dissolve.
Where this has been done obvious remedy is to remove the hangers if they be left in situ.
There are permanent hangers in Coniston Copper Mines that have been there many years but those have hangers made up from heavy duty angle iron.
With reference to some of my earlier comments, anodic dissolution is not uniform but occurs mostly on the area facing the cathode. It is practice in electroplating vats to rotate the anodes so as allow a more uniform dissolution.
We are dealing with some very complex issues here.
royfellows
Well-known member
Hope that I am not becoming a bore on this!
When I left school I worked as a chemist at an electroplating factory, hence talk from experience re my comments in my last above.
Where there is galvanic corrosion between the stainless hanger and steel through bolt I would expect this to happen on the threaded area of the bolt immediately inside the area of the hanger. The ultimate result being the sheering of the bolt head.
I stand on my comments re pyrite bearing rock in relation to the bolt in the photograph.
When I left school I worked as a chemist at an electroplating factory, hence talk from experience re my comments in my last above.
Where there is galvanic corrosion between the stainless hanger and steel through bolt I would expect this to happen on the threaded area of the bolt immediately inside the area of the hanger. The ultimate result being the sheering of the bolt head.
I stand on my comments re pyrite bearing rock in relation to the bolt in the photograph.
bograt
Active member
royfellows said:
I am in agreement with Roy on this one, if the corrosion is a result of galvanic action caused by the stainless/ non stainless dissimilarity, I would expect the corrosion to be worse at the point of least electromotive resistance, i.e. the junction of the two metals.
The suggestions that a sort of galvanic cell is created by the two dissimilar metals in an electrolyte would imply that the fluid in the hole is in permanent contact with the stainless hanger on the surface, unlikely to be the case as there appears to be a corroded carbon steel plug at the top of the bolt in the photo.
Brains
Well-known member
All very intereseting, and a little scary! I remember climbinmg at Gogarth one Easter many years ago (80's/90@s) and found several in situ pegs, many of which fell apart to the touch or just lifted away from the rock. I dont know the vintage, but we were quite happy using nuts and cams. Bottom line is check for soundness as best you can before hanging off it!
droid
Active member
Hence my reply to Badlad above.
Simon: stainless/mild might be bad, but I would think alloy/mild, or alloy/stainless would be worse. 'Stainless' is a bit of a misnomer anyway. There are many grades of stainless, all of different corrosion resistance/tensile strength. At least according to my late father, who used to sell the stuff....
Simon: stainless/mild might be bad, but I would think alloy/mild, or alloy/stainless would be worse. 'Stainless' is a bit of a misnomer anyway. There are many grades of stainless, all of different corrosion resistance/tensile strength. At least according to my late father, who used to sell the stuff....
Ed W
Member
Bograt,
As stated in the previous post, these things are rarely simple. With the available evidence I reckon this is primarily crevice corrosion, which was kick started by the dissimilar metals. Once the corrosion cell gets started, the fluid in the hole effectively alters the current flow so that the material in contact with the outside becomes the anode, and is protected, with the cathode in the hole. The shape of the corroded part of the bolt in the hole may well be the result of the original geometry of the thru bolt. As stated before, ALL metallic corrosion is electrochemical in origin, and the presence of dissimilar metals would be a good bet to be a causal factor here. Note that the presence of electrolyte does not require the bolt to be submerged, this could be provided by sea spray or even humid salt laden air.
Droid, as others have mentioned, the rate of corrosion is influenced by the relative position of the various materials in the galvanic series (a measure of the voltage generated between two materials) as shown in the table in this link http://en.wikipedia.org/wiki/Galvanic_series, and you would be right that Aluminium alloys would have a higher potential difference if paired with mild or stainless steel than for a stainless/mild steel combination. However, in this case the crevice effect may be even more aggressive.
There is a wide range of stainless steels on the market, I know a little about them as I have used them quite extensively at work - to the extent that a few years ago a project that I was involved in was buying enough 316 series stainless to apparently be affecting the market price in the UK.
As stated in the previous post, these things are rarely simple. With the available evidence I reckon this is primarily crevice corrosion, which was kick started by the dissimilar metals. Once the corrosion cell gets started, the fluid in the hole effectively alters the current flow so that the material in contact with the outside becomes the anode, and is protected, with the cathode in the hole. The shape of the corroded part of the bolt in the hole may well be the result of the original geometry of the thru bolt. As stated before, ALL metallic corrosion is electrochemical in origin, and the presence of dissimilar metals would be a good bet to be a causal factor here. Note that the presence of electrolyte does not require the bolt to be submerged, this could be provided by sea spray or even humid salt laden air.
Droid, as others have mentioned, the rate of corrosion is influenced by the relative position of the various materials in the galvanic series (a measure of the voltage generated between two materials) as shown in the table in this link http://en.wikipedia.org/wiki/Galvanic_series, and you would be right that Aluminium alloys would have a higher potential difference if paired with mild or stainless steel than for a stainless/mild steel combination. However, in this case the crevice effect may be even more aggressive.
There is a wide range of stainless steels on the market, I know a little about them as I have used them quite extensively at work - to the extent that a few years ago a project that I was involved in was buying enough 316 series stainless to apparently be affecting the market price in the UK.
paul
Moderator
There's a link on the FaceBook page at the start of this thread to a PDF document in French. Here is the English version of the same document: http://www.euro-inox.org/pdf/map/Contact_with_Other_EN.pdf
Simon Wilson
New member
Please read the last post by Ed W. Then think about what you often see done.
Stainless* hangers on mild steel bolts = very bad if it's left in place because the corrosion is hidden.
'Ally'* hangers on mild steel bolts = not good but not quite as bad because it's the hanger that corrodes more and you can see it.
Remember, corrosion involving dissimilar metals and unknown electrolytes is highly complex.
Any bolt providing it's strong enough is fine for short term use. For permanent anchors we really don't need to even think about the mechanisms of corrosion discussed above because there should be no mild steel, no ally and no dissimilar metals no matter how far apart on the galvanic index and if taken very strictly that would exclude all welding and plating.
There are now several designs of resin anchor available which minimise the known corrosion risks to the best of present knowledge. The new design from Titan Climbing is an excellent design which is more economical and eliminates the weld of the earlier design. http://www.titanclimbing.com/Titan%20Climbing%20products%20-%20Eterna%20Titanium%20glue%20in%20bolt.html
[* layman's terms - there are thousands of alloys in these general categories.]
Stainless* hangers on mild steel bolts = very bad if it's left in place because the corrosion is hidden.
'Ally'* hangers on mild steel bolts = not good but not quite as bad because it's the hanger that corrodes more and you can see it.
Remember, corrosion involving dissimilar metals and unknown electrolytes is highly complex.
Any bolt providing it's strong enough is fine for short term use. For permanent anchors we really don't need to even think about the mechanisms of corrosion discussed above because there should be no mild steel, no ally and no dissimilar metals no matter how far apart on the galvanic index and if taken very strictly that would exclude all welding and plating.
There are now several designs of resin anchor available which minimise the known corrosion risks to the best of present knowledge. The new design from Titan Climbing is an excellent design which is more economical and eliminates the weld of the earlier design. http://www.titanclimbing.com/Titan%20Climbing%20products%20-%20Eterna%20Titanium%20glue%20in%20bolt.html
[* layman's terms - there are thousands of alloys in these general categories.]
bograt
Active member
Ed W said:
I do not dispute most of what you conjecture, I do question, however, your suggestion that the "stainless" hanger had a part in this reaction. I would rather consider that the dissimilar metal elements came from within the bolt itself, as I said earlier, it could be between the expanding sleeve and the bolt, or, as Roy later suggested, the plating (maybe zinc?) on the bolt itself. Electrochemical reactions require an electronic circuit between dissimilar metals, usually provided by an electrolyte in contact with both electrodes. Paul's link explains this from page 3 onwards.
P.S. Thanks for that Paul, Good Link
Ed W
Member
Cavermark - you are of course right, brain fart on my part. Its the anode that fizzes away down the hole and the outside that's on the outside.
Bograt, it is of course entirely possible that there was some sort of dissimilar metal on the expansion collar, and of course any zinc plating is intended to create a corrosion cell, with the plating being the anode. These could have been the source of the original corrosion cell, but it's a bit difficult to tell given the state of the bolt now. I don't however see the problem about the electrical connection between the bolt and hanger. If we agree that this is a crevice corrosion issue, then the hole must have been wet for at least some time. If the hole is wet then I am sure that capillary action would ensure that wetness would be pulled into the narrow gaps between bolt, hanger and washer. This would be more than enough to ensure electrical connection between the stainless hanger and the bolt. From experience I would say that this is a very likely source of a corrosion cell. In a marine environment it is quite usual to see very rapid bimetallic corrosion between bolts and the materials they hold together. Generally in ship design, I would only consider a joint to be electrically isolated if some form of insulating sleeve is provided betweenthe bolt and the material it is attached to, only a small film of salt water being enough to create that connection. On sea cliffs I have seen very rapid corrosion of bolts in areas where they are well above the splash zone, humid salt laden air is often enough to provide that electrical connection.
I think Simon has hit the nail on the head, if you place a bolt where the hanger is the cathode then there is the distinct possibility of catastrophic corrosion of the bolt within the hole whilst the hanger looks perfectly OK. This may be an argument for using Aluminium hangers in some case, as at least then the hanger will corrode first and you would see it!
Bograt, it is of course entirely possible that there was some sort of dissimilar metal on the expansion collar, and of course any zinc plating is intended to create a corrosion cell, with the plating being the anode. These could have been the source of the original corrosion cell, but it's a bit difficult to tell given the state of the bolt now. I don't however see the problem about the electrical connection between the bolt and hanger. If we agree that this is a crevice corrosion issue, then the hole must have been wet for at least some time. If the hole is wet then I am sure that capillary action would ensure that wetness would be pulled into the narrow gaps between bolt, hanger and washer. This would be more than enough to ensure electrical connection between the stainless hanger and the bolt. From experience I would say that this is a very likely source of a corrosion cell. In a marine environment it is quite usual to see very rapid bimetallic corrosion between bolts and the materials they hold together. Generally in ship design, I would only consider a joint to be electrically isolated if some form of insulating sleeve is provided betweenthe bolt and the material it is attached to, only a small film of salt water being enough to create that connection. On sea cliffs I have seen very rapid corrosion of bolts in areas where they are well above the splash zone, humid salt laden air is often enough to provide that electrical connection.
I think Simon has hit the nail on the head, if you place a bolt where the hanger is the cathode then there is the distinct possibility of catastrophic corrosion of the bolt within the hole whilst the hanger looks perfectly OK. This may be an argument for using Aluminium hangers in some case, as at least then the hanger will corrode first and you would see it!
Whether the hang is involved or not, I have seen a similar thro' bolt corrosion, but not on the same scale, of the sleeve and wedge, after about a year in a relatively dry cave, Reservoir. There was no hanger or even the washer or nut, the exposed thread showed little, if any rust.
In this case I was very pleased to be able to pull them out with little effort, as I was tidying up from a climb I had done.
In this case I was very pleased to be able to pull them out with little effort, as I was tidying up from a climb I had done.
