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Multi-Part Report

Corrosion at Cabo da Roca – 1

The first of a four part series

In conducting this work I have been greatly assisted by Luis Fernandes Silva and Rui Rosado who have provided multiple samples and photos for analysis. Luis has spent a great deal of time measuring spot-sulphate levels, and the pH profiles of bolts he has extracted during route refurbishment. We now know a lot more about why the Cabo da Roca crags eat stainless steel.

Introduction:

In an earlier post I discussed the granite/syenite sea cliffs of Cabo da Roca, and how the ground-water weeping from crevices in the rock is saturated with calcium sulphate, which then crystallizes out on the rock surface.

Given readily available sulphate, combined with a reduced oxygen environment, such as exists in the crevice where a bolt enters the rock, and we set the scene for the arrival of sulphate reducing bacteria (SRB). The SRB derive energy by reducing sulphate to sulphide, the latter of which is released into the narrow confines of the crevice. Multiple corrosive chemistries are enabled, and stainless steel types that are normally quite stable under marine conditions are rendered extremely vulnerable.

We will be looking at bolt failures at crags both north and south of Cabo da Roca. I don’t believe there is any distinction to be made between these locations. They are all on steep granite at the very edge of the sea. All are free of vegetation. All show sulphate seepage where ground water exits the rock.

Cabo da Roca, itself, is located on syenite, whilst the climbing crags to the north and south of the cape are on granite.

All climbing locations are to be found on clean, granite faces, and it is reasonable to assume the only influences on the geochemistry must come from seawater or ground-water. Even the connection to ground-water seems tenuous given the fact that the cliffs often are partially isolated from the land mass. The Google Earth pic below illustrates the situation for Aroeira.

The crag of Aroeira is somewhat separated from the main land mass. The granite is extensively fractured, and exhibits multiple volcanic intrusions seen here as dark bands.

This granite seems to be uncannily porous for such a rock type. Luis reports that on one occasion, drilling perfectly dry rock at Espinhaco revealed considerable moisture. Furthermore, at Aroeira, at the location labelled 5 in the picture above, the drill dust tested positive for sulphate. How does sulphate get up there?

Two samples of drill dust show positive responses for sulphate.

By all standards of geochemistry this finding is bizarre. For there to be a positive test against barium chloride like this, the sulphate species has to be soluble in the dilute hydrochloric acid used in the test. My guess is that we are looking at a granite that is sufficiently micro-fractured to be porous to ground-water, and, in our particular case, sulphate-saturated ground-water.

Types of Corrosion Observed:

We can makes some broad observations about the type of bolt corrosion we see at various sites along the sea cliffs of Cabo da Roca.

Bulk Corrosionplated steelSufficient bulk material is lost to weaken the partsignificant rust buildup
Stress Cracking304 (A2) stainless steelRemoval of metal is minimal, but environmental conditions overcome the innate fracture toughness of the steel, propagating stress cracks though the bulk material.some minor rust buildup
Embrittlement304 (A2) stainless steelBolt can be sheared with a single hammer blow. The metal loss through corrosion is insignificant, yet the bolt is seriously weakened by embrittlement in the anoxic zone just below the rock surface.corrosion products may be so few as to be not obvious obvious to the naked eye.

There are other broad observations we can add –

  • A totally compromised bolt may exist much less than a metre from one that is ok, yet both bolts can be of the same type and service age.
  • It seems that brittle fracture is restricted to expansion bolts.
  • Documented failures of 316 components are unknown. There is a marked difference in the performance of 316 compared with 304.

Bulk corrosion looks alarming, but is of no great concern as the condition of the component is never worse than it looks, and thus users are warned-off, long before it becomes hazardous.

Take the quick-link/FH combo below. This is what happens when you use a galvanised steel component on a sea cliff. It’s true that being in contact with a stainless steel hanger would have hastened its demise, but really there are no surprises here. There is no special Cabo da Roca flavour of this very typical maritime corrosion process.

The remains of a galvanized quick-link on a home-made hanger and commercial expansion bolt at Aroeira. The bolt and hanger are thought to be 316 SS

However, what about the hanger? It looks good to go surely? And herein lies the danger. The more I learn, the less I’m be inclined to trust any stainless steel bolt on a sea cliff where we have evidence of abundant sulphate. Surely the bolt is going to be stronger than that quick-link? In this particular case, local experience says these hanger/bolt combinations are all holding up well, but, without such local knowledge, there is no way of ascertaining that the bolt wouldn’t fail before the quick-link.

It is possible for a perfectly respectable-looking bolt to be hiding stress cracks of sufficient size that it would fail under body-weight, or the demand of a lead fall.

I believe the loss of structural integrity to be due to sulphide stress cracking (SSC) mediated by SRB that take up residence at the point where the bolt enters the rock.

This is a Goldilocks location for a sulphate reducing bacterium like Desulfovibrio vulgaris. It is close enough to the surface to have access to the energy source, sulphate, yet far enough from the surface to meet the requirement for a low oxygen environment. The location also retains the moisture that is essential for life processes. Finally, as if this ecological niche isn’t already specialised enough, we have the fact that this bacterium can survive on bare rock devoid of organic matter provided it has access to metallic iron.

Sherar et al published this nice pic of an SRB connected up to an iron surface. Those threads are an electrically conductive protein structure that conduct electrons from the metallic surface into the cytoplasm of the organism.

SRB electrically connected to a steel surface by means of conductive pili

SRB are widely-distributed organisms in swampy, low-oxygen environments where there is an abundance of organic matter to provide the reduction potential that fuels their metabolism. In analogous terms to a fire, the organic matter is the wood, and sulphate is the oxygen. It is therefore remarkable to find such an organism occupying a niche on a barren cliff that is free of organic matter. It seems that there is a very special adaption that lets this organism swap over to direct, electron-transport reduction when starved of organic substrate. Even given the abundant sulphate, and the availability of anoxic crevices in the rock, survival would be a struggle until such time as a climber comes along and places a bolt. The bacterium literally plugs into the bolt and thrives as it “burns” metallic iron in place of organic matter. And as SRB thrive, they create a sulphide rich chemical environment that radically alters the typical corrosion processes at play in the marine cliff environment.

It is sheer chance that rock climbers have created the perfect environment for an ancient organism that dates back to a time when our planet’s atmosphere was devoid of oxygen. This primordial chemistry, low in oxygen, rich in sulphide is tough on stainless steel.

Apart from the development of stress cracks, the other phenomenon degrading the bolt’s structural integrity is brittle fracture. Sometimes we find that a blow or two in the same direction with a hammer is sufficient to snap a bolt at the point it enters the rock.

It is easy to harvest bolts. All you need do is to rap down and hit each bolt a few times with a hammer to build up a good collection – all these, snapped off some 6mm below the surface of the rock. Note the lack of deformation, and the “stepped” nature of some fractures (red arrows). There may be some partly stress-cracked bolts here, but the majority have failed through hydrogen embrittlement (HE).

I believe that what we are seeing here is sulphide-mediated hydrogen embrittlement (HE). It is related to SSC, in that the migration of atomic hydrogen through the bulk metal is a common factor. Thus, it is not unusual to encounter a bolt where a stress crack has progressed part way through, before the act of tapping it with a hammer, finishes the process with brittle fracture.

Coming Next:

In Part 2 of this series I will look at the phenomenon of sulphide stress cracking (SSC) and compare it with stress corrosion cracking (SCC) which is a term better known to the climbing community.

In Part 3, I will take a look at the phenomenon of hydrogen embrittlement and see what we can learn from the published literature to guide future materials choices.

In Part 4, I will consider why it is we seem to be seeing no failures for 316 and consider what this means for future material choices.

1.
Sherar, B. W. A. et al. Characterizing the effect of carbon steel exposure in sulfide containing solutions to microbially induced corrosion. Corrosion Science 53, 955–960 (2011).

2 replies on “Corrosion at Cabo da Roca – 1”

Nice work, we had a bolt failure in a 304ss expansion bolt in a seacrag in Trondheim in 2007 ( unfortunately the bolt got lost before we could analyse it ). Litle corrosion on the outside but full of it on the inside, broke on the inside like your photos. The sulphate content in the north sea is around 2.8 g/liter. Is this amount enough

That is a good question. There is plenty of research that shows SRB are very active below the sea-bed where they attack the base of structures like oil platforms. For these, the normal concentration of sulphate in seawater must be sufficient.

However, as far as corrosion on sea-cliffs is concerned, with very few exceptions, I have found that corrosion is always associated with environments where there is an external sulphur source that pushes the sulphate to chloride ratio at least ten times higher than that for sea water.

I have a couple of cases where I’m “missing” the sulphate, so maybe yours is the third? In any event, exceptions to any rule are very interesting, and if you come across another failure, and it doesn’t land in the sea, 🙂 ,I’d be very happy to take a look at it for you.

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