What is Corrosion?
Corrosion is the physicochemical interaction between a metal and its environment, which results in changes to the metal’s properties and which may lead to significant functional impairment of the metal, the environment, or the technical system of which they form a part.
For many reasons corrosion can be a serious issue for sport climbing:
- Firstly many bolts placed prior to the millennium are now beginning to corrode or fail.
- Increasing numbers of new climbers are starting via sport climbing with commensurate low levels of experience and knowledge regarding fixed protection.
- Development of new areas is growing in regions with aggressive corrosion.
- Climbers with varying levels of knowledge are placing increasingly more bolts.
- The majority of climbers make assumptions regarding the reliability of fixed protection without considering the impact of failure.
- The majority of development is not funded therefore anchor specification may be compromised as cheaper and less suited anchors are used to meet an individual developer’s budget.
Most materials deteriorate over time and steel is no exception. Although stainless steels are often chosen because of their resistance to corrosion, they are not immune to it. Whether a stainless steel is corrosion resistant in a specific environment depends on a combination of its chemical composition and the aggressiveness of the environment. How a product is manufactured also plays an important role in resisting corrosion.
Passivation is a term often heard when the corrosion of steel is discussed on climbing forums and this is a thin film that forms spontaneously on its surface in oxidising environments provided the steel has a minimum chromium content of approximately 10.5%. As the film adheres strongly to the metal substrate and protects it from contact with the surrounding environment, the electrochemical reactions that cause corrosion are effectively stopped. If locally destroyed, for example by scratching (tightening the nut upon installing an expansion bolt), the film can ‘heal’ by spontaneously re-passivating in an oxidising environment.
All types of corrosion affecting stainless steel are related to the permanent damage of the passive film, through either complete or local breakdown. Factors such as the chemical environment, pH, temperature, surface finish, product design, fabrication method, contamination can all affect the corrosion behaviour of steel and the type of corrosion that may occur.
Corrosion by Condition
Effectively corrosion occurs in wet conditions or when subjected to very high temperatures associated with industrial processes. The former is of relevance to climbers whereas the latter clearly is not. Wet corrosion can be sub divided by the type of corrosion process with crevice, pitting and stress corrosion cracking of particular relevance to climbing bolts.
Wet corrosion involves steel exposed to moisture (e.g. humidity) or liquids (e.g. sea water) and involves the exchange of electrons from a cathode to an anode within the material via an electrolyte.
Electrolytes comprise water with salts dissolved in solution that enable the electro chemical reaction to occur. The metal oxidizes (corrodes) at the anode, forming rust or some other corrosion product whilst at the cathode, a reduction reaction takes place, this typically is the reduction of oxygen (which can include hydrogen production). Preventing corrosion involves stopping these reactions taking place.
Corrosion by Type
There are up to 11 types of wet corrosion that affect steel however the types of interest to climbers are:
- Stress corrosion cracking
1. Uniform corrosion
Occurs when the aggressive environment destroys the passive film however unlike other types, the whole surface corrodes uniformly and metal loss can be expressed as μm/year. This is a particular problem with carbon steel bolts (up to 2.1% carbon) and one reason why stainless steel is recommended as the minimum specification of material.2. Pitting corrosion
Pitting corrosion is a localised form of corrosion that leads to the creation of small holes or “pits” in the metal.
The corrosion initiating process starts with a local breakdown of the passive layer. Local corrosive attack can be initiated on stainless steels, for example, by chloride ions and stainless steels are particularly susceptible to pitting and crevice corrosion in environments where halide ions such as chlorides are present so coastal sport-climbing areas are high-risk environments.
Pitting attacks steel in very localised spots where surface irregularities exist in the steel or non-metallic inclusions are present, the break down of which, prevents the steel from re passivating as the ‘pit’ in the steel structure grows. Pitting corrosion can be the initial form of corrosion that then enables stress corrosion cracking to develop later as the grain boundaries within the metal become exposed and cannot self repair by passivation.3. Crevice Corrosion
Crevice corrosion occurs where water or moisture becomes trapped in gaps or pockets and can be a particular issue for expansion bolts considering there are always voids between the rock and hanger.As the oxygen content is limited inside a tight crevice, the passive layer is weakened and, just as with pitting, dissolved metal ions in the crevice lower the pH and allow chloride ions to migrate into the crevice. There are lower and upper limits to the size of a crevice in which corrosion may be induced. If the crevice is too tight, no electrolyte for corrosion will be introduced. If the crevice is too wide to reduce oxygen entrance, the aeration cell and consequently different concentrations of oxygen cannot develop. However, the critical crevice width depends on several factors such as the type of metals involved, the corroding environment and wet / dry cycles.Eventually the passive layer breaks down and the aggressive environment facilitates the corrosion attack. Compared to pitting, crevice corrosion results in larger but shallower attacks however both forms are considered initial types that lead to the more aggressive type of stress corrosion cracking.
4. Galvanic Corrosion
Here dissimilar metals are electrically connected by an electrolyte and exposed to a corrosive environment. This type of corrosion is practically restricted to mechanical expansion anchors for they involve separate metal components (bolt and hanger) that developers may sometimes inadvertently mix to reduce the overall cost of the fixing. An example would be pairing a plated carbon steel wedge bolt together with a stainless steel hanger. With the dissimilar metals in contact and water present, the internal carbon steel bolt has the potential to preferentially corrode whilst the external hanger gives no indication of the corrosion hidden from sight.
The following conditions are required for galvanic corrosion:
- Dissimilar metals sufficiently separated on the electrochemical series scale (e.g. stainless paired with aluminium).
- Metal-to-metal contact.
- Metals in the same conductive solution (an electrolyte – e.g. salt water).
5. Exfoliation Corrosion
This is a form of intergranular corrosion which involves selective attack of a metal at or adjacent to grain boundaries. In this processthe metal corrodes along sub surface paths parallel to the surfaceand corrosion products formed force metal to move away from the body of the material, giving rise to a layered appearance.
The only example of exfoliation corrosion that climbers could realistically encounter would involve aluminium caving bolt hangers that have been used to protect a sport route. The hanger and bolt are made from aluminium and intended to be removed from the plated steel displacement style anchor after use. They can be occasionally found dating back to the 90’s when climbers resorted to installing them in the absence of anything better.6. Stress Corrosion Cracking (SCC)
Without debate, SCC is the most serious form of corrosion affecting fixed protection and for two distinct reasons:
- The corrosion rate.
- The disparity between the severity of the corrosion and any visual indication that reflects how severe the corrosion has developed.
Apparently ‘ok’ looking bolts, while riddled with unseen microscopic cracks, can snap under body weight and the occurrences of SCC affecting climbing bolts worldwide is increasing.
The combination of the points above make it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and the specific environment is of crucial importance for only small concentrations of certain highly active chemicals are needed to produce catastrophic cracking.
Requirements for SCC
Simplistically SCC requires a combination of a susceptible material; steel in this case, tensile stress above a threshold and lastly a corrosive environment.
With chlorides of any salt except pure sodium chloride, a deposit left long enough with stress in an anchor from fabrication, will result in SCC. The process is more complex however and research by T. Prosek, A. Iversen, C. Taxén, D. Thierry established that there are two independent mechanisms of low temperature SCC, supplementing the well documented occurrences of SCC in 304L and 316L stainless steels involved with the collapses of swimming pool roofs.
The above does not account for why certain environments are prone to aggressive SCC however.
Work by David Reeve, on behalf of the UIAA Crag Chemistry Project, has identified that fixed hardware on coastal cliffs is subjected to a very different chemical environment than that of maritime exposure. While the role of elevated chloride levels in this process is not doubted, in order to achieve rapid (aggressive) SCC there must be another mechanism involved, especially when SCC is occurring on other non limestone marine sport crags.
“One of our earliest findings was that calcium levels for wall-wash samples taken at Tonsai (Thailand) were unexpectedly high given the constraints of the basic calcium / carbonate / bicarbonate / CO2 system. This led to the discovery of high sulphate levels in solution, and high levels of precipitated solid calcium sulphate on the surface. Analysis of the rock material itself showed it to be free from even traces of sulphate, and it became clear that sulphur was being introduced into the system via an external agency”
Key Point: A high rate of corrosivity is associated with an active sulphur process.
This is supported by the project data of all crags with SCC that exhibit an active sulphur process. Crucially the wall wash sampling data has identified the presence of both chlorides and sulphates at distinctly elevated levels of aggressive corrosion and in reverse, explaining benign conditions where chloride and sulphate salts are not elevated.
Key Point: elevated levels of chlorides and sulphates have been detected for all corrosive crags sampled during the UIAA Crag Chemistry Project.
The presence of elevated chlorides and sulphates also explains why aggressive chloride based SCC is occurring on other crags that are not tropical limestone. Long Dong is a very hard quartz conglomerate climbing area in Taiwan where acidified aluminium sulphate is found at saturated levels in ground water. Bolts have failed from aggressive chloride SCC in an identical manner to failures in Southern Thailand.
From formal materials analysis of failed hardware, cracks were found to be transgranular and typically originated from pits and/or crevices therefore pitting and crevice corrosion is recognised as initiating forms of corrosion that enable SCC to develop later.
Key Point: pitting and crevice corrosion act as chloride SCC initiators.
The crag chemistry project of the UIAA Anchor Working Group has made inroads into defining what compound salts are present and therefore responsible for corroding fixed protection. Data compiled to date is supporting why in the same region some crags are very corrosive and others benign and on the basis of data obtained to date it should be possible in the future for developers to base material choice based on capturing wall wash samples or some other form of chemical sampling. The challenge however will be ensuring the sampling density is sufficient to capture any salts present for a range of rock types in vastly different climatic conditions.