This note was posted to the Titanium Bolting FB Group 10th March 2020
Warning: This gets a bit geekey, but I’ll simplify as much as possible.
For the past few years I have concentrated on providing sufficient evidence for the theory that all marine crags showing SCC of 304SS are associated with high sulphate levels. I have now analysed almost 100 samples from crags across the world. All, but a few exceptions point in the same direction.
The possibility that such sulphate could be feeding sulphate reducing bacteria (SRB) within the anaerobic spaces between glue and steel, between nut and bolt, washer and hanger etc is a no-brainer. These little guys are the last representatives of a world when oxygen wasn’t present, and they now live on, wherever oxygen is low and sulphate is available. They have retained the archaeic ability to use sulphate as we use oxygen to metabolise a carbon source like sugar.
It has long been suspected that if starved of sugar or a similar carbon source, SRB can metabolise metallic iron as a substitute for a carbon source. Recent studies have verified this conjecture as fact, and the bacterium connects to the metallic surface via conductive fibres, and extracts a stream of electrons, ie an electric current, that is drawn within the bacterium by virtue of the reduction of sulphate, whilst simultaneously ferrous (iron) ions are released into solution. It is a biochemical form of electrolytic etching, and will remove metal very effectively.
If we had such a process operating we would expect to see three things.
a) the build-up of dark ferrous sulphide deposits.
b) locally acidic conditions at the point of attack.
c) alkaline conditions surrounding the general environment of the attack.
As far as a) goes, John Byrne argued long and hard to get anyone to listen to him when SCC first showed up in the Caymans all those years ago. He coined the term ‘black ring of death’ for the dark ring that circled the base of the nut or washer on a bolt that was suspect. It is only recently that I have developed a spot test sensitive enough to show that the ‘black ring of death’, or any dark material showing at the edge of any anaerobic space, is in fact metal sulphide. There have been some serious spats on social media over whether the ‘black ring of death’ was a bad omen or not. John was absolutely right. It is to be avoided.
We got an early indication of both acid and alkaline conditions when Martin Roberts did the first investigation at Railay some three years back. If you file or chip the dark flakes off a bolt, and immediately test the pH then it shows as acid, say pH 2 to 3. I was super happy to see this result because it was an indicator of locally corrosive conditions. However, when Martin showed that the limestone underneath an old, and SCC raddled FH, was strongly alkaline, say pH 12, then I have to admit I went into denial. This isn’t what I wanted to hear.
However, last January, I got a chance to take a look for myself at a corroding FH within the big cave of Klao Luk Choe, I found to my surprise the same story, acid inside the corrosion product structure, but alkaline outside of it. This has to mean something, but what?
With a bit of research, the last bit of the puzzle dropped into place. It is likely that SRB living in the spaces around a bolt are going to be short of an organic carbon source, so the bug commences to directly eat the metal, and a very high rate of pitting corrosion is possible if sulphate is freely available. Maybe this is enough to initiate SCC given the inevitable presence of chloride, or maybe there are other mechanisms also in play. Sulphide is certainly a significant modifier of both anodic and cathodic processes.
In any event, it seems very likely that, if you have an active SRB, electicrally-mediated, corrosion process going on then pH conditions will be exactly what we found. Acid inside, alkali outside.
With the exception of the “black ring” pic, the photos below are all from Railay, but we can show the same, or similar story for the sea cliffs around Lisbon. I hope to present some data from there just as soon as Luis Silva and I can pull all the loose ends together.