Quick Note

Never Say Never

The benign cliffs of Campo Escola da Barra da Lagoa

I reported wall-wash results from this low-corrosion crag some time ago. The sulphate levels were not elevated, a fact which supports my mantra “no sulphate – no corrosion”.

However, it always interesting to take a somewhat deeper look, which I was able to do thanks to Rodrigo Castelan Carlson, who not only took the wall-wash samples, but also provided me with fixed hangers and nuts from bolts adjacent to the sample points.

These bolts have been installed for 11 to 12 years so it was of interest to see if any sulphide could be found in the anoxic zones within threads of the nut, or where the bolt fits against the hanger. I was not expecting any, given the crag’s history of low corrosion, and the fact that there was very little sulphate to feed sulphate reducing bacteria (SRB).

It turns out I was both right and wrong. Measure more speculate less!

The test I use is to take tiny scrapings of the corrosion product, and examine it under a microscope in the presence of an iodine-azide reagent. Although the metal sulphide is quite insoluble, it’s presence at the solid surface will promote the distinctive formation of tiny bubbles of nitrogen gas.

Sobre as ondas – high salt levels (15 mmole/sqcm)

looks pretty good ….
but what’s this?…. dark deposit in anoxic region.
well, none of that material is reacting to form bubbles … not sulphide
but hang on …. here’s a fragment showing a positive reaction

So it looks like SRB might well have been at work. Not the real demolition crew, but definitely a trace.

Duelo de Titans – moderate salt levels (10 mmole/sqcm)

something going on here?
some pitting for sure, but no dark deposits – perhaps crevice corrosion between rock and metal surface? The black material on the left of the hanger is organic detritus from vegetation at the top of the cliff.
a few dark deposits in the anoxic region
but this material is not sulphide containing

No sign of SRB activity with this FH.

Mau olhado – low salt levels (< 1 mmole/sqcm)

looks great, but whatever are those black lines?
scraping the deposit off revealed a number of straight line cracks
the cracks don’t go right through – the black material is organic detritus
the material covering the crack is very definitely a sulphide
no pH differentiation across the region of the cracks, or across the anoxic region where the bolt was fixed.

Never say never ….. so what is going on here? Because the cracks are not in the usual anoxic locations, I’m inclined to think that the crack came first, followed by the bacteria. That is, a pre-existing crack has provided an anoxic home for SRB, rather than the crack being the result of microbially induced corrosion (MIC). I’m also inclined to think that microbial metabolic activity is low given the lack of pH differentiation across the region of the cracks. Maybe the sulphate levels in sea water are insufficient to support a flourishing bacterial colony?

I’ll take a section through that crack to see what is going on. I can also check the hardness of the material. All work for another day. For now, I’ll classify this FH in the “well how about that?” basket.

Thanks to members of the Associação Catarinense de Escalada e Montanhismo (ACEM) for the collection of samples.

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