Steel has many useful properties, which mean it is found in virtually everything around us – from the chair you may be sitting on to the vehicle you took to get to work today. Steel shapes our lives in more ways than you might initially think.
However steel has one property which isn’t so useful – it rusts. Steel is always trying to get back to its native state (Iron Oxide) and so, unless we do something to stop it, steel corrodes. How fast this happens depends on its environment and I’ll come back to this point as it is vital.
Protecting rail assets
If you look at pretty much any steel object in our rail industry, the steel used has some sort of corrosion protection. Whether this is galvanised overhead masts, paint protected vehicle chassis, line-side cabinets or even concrete sleepers, virtually all steel objects have corrosion protection built-in to stop the steel trying to return to its natural state. The most notable general exception to this rule is a fundamental part of our industry – the rail itself.
So why don’t we always protect the rails? – The key here is the expected life. In the majority of UK cases the rail will usually be replaced due to wear or rolling contact fatigue before it requires replacement due to corrosion, so protection in this case is unnecessary. However, as operating environments vary significantly, this is not always the case!
The rate of rail corrosion is highly dependent on the operating environment. There are several factors that accelerate corrosion vastly; the most common are water and salt, certainly something we are not short of in the UK.
Where these are present in areas such as coastal tracks, wet tunnels or level crossings, rail life may become determined by this accelerated corrosion. When this becomes the case, corrosion- protection of the rails becomes vital to optimise the life of the rail.
In severe environments, rail lives of just three months have been observed.
There are essentially two forms of corrosion-related rail failure. Rail gall is a general loss of rail section (usually affecting the foot). This can be general loss of section, but is usually more severe under rail clips/fastenings due to the localised environment here (water trap with abrasion from the clip/insulator).
Foot fatigue results from the combined effect of corrosion and fatigue. A corrosion pit forms on the foot of the rail and under traffic a fatigue crack can grow from this defect, ultimately causing complete rail failure. This is, in my opinion, the most problematic form of corrosion induced failure, due to the fact that you cannot measure it. Corrosion pits usually form on the rail foot (an area you can’t see or inspect), and in high load areas the pit only needs to be a few millimetres deep to start generating a fatigue crack.
The resulting crack is virtually undetectable via current in-track monitoring techniques, meaning control of this issue is problematic.
As the industry moves towards increasing rail life further by the use of more wear and rolling contact fatigue resistant rail steels, the proportion of rail replacement due to corrosion will likely increase unless we address this by adding protection where appropriate.
So how can we protect the rails against corrosion? – There are essentially two ways to prevent corrosion of steel.
This is one we are probably all familiar with; you create a barrier to stop the atmosphere/environment reaching the steel surface or, in other words, you apply a barrier coating to the rail. This is often a paint coating, but can be other things such as tar or rubber which is still used in some countries.
There are a number of drawbacks to this approach, though. The first is that any damage to the coating means that this area is unprotected and will corrode as fast, or indeed even faster, than if no coating was present. The second is that such coatings are usually unsuited to use where stray currents exist. Without going into too much detail, this means you can’t use them in third or fourth rail areas and also some overhead line locations, as any damage to the coating concentrates corrosion in the damaged area, resulting in extreme damage very quickly.
Another way to protect steel is to apply a sacrificial coating to it. This corrodes in preference to the steel (so protecting it at the same time). This is why overhead line poles and many street lamps are galvanised. The zinc coating corrodes (slowly), in preference to the steel. Any small areas of damage remain protected by the coating either side of the damage (called the throw effect). The size of uncoated area protected depends on the operating environment. Under seawater, for example, the area protected is large and this is why ships use lumps of zinc on their hulls to stop corrosion. In the atmosphere the area protected is much smaller.
Historic rail coatings have typically relied on the provision of a simple barrier coating; however, our latest generation of corrosion protection coatings provide both barrier and sacrificial properties to ensure real-world robust rail protection.
Real rail protection
The environment provides some unique challenges to providing corrosion protection for rails:
» Impact resistance – Passing vehicles can hurl ballast at the rails, so an impact resistant coating is needed to prevent excessive damage;
» Damage tolerance – Almost inevitably something, somewhere will manage to breach the coating. Damage may be from ballast or damage from installation or maintenance operations such as damage from tamper tines;
» Stray current protection – Stray currents third/fourth rail operations as well as overhead, external or even some signalling current sources can cause rapid corrosion to most barrier coatings;
» Abrasion – Coatings are subject to abrasion and erosion particularly where clips/insulators or other track furniture contact the rail;
» Removal – For maintenance and installation purposes the coating needs to be removable (and indeed replaceable) in order to facilitate track welding.
To provide a corrosion- protection system that lasts requires optimisation of all these items to deliver longer rail life. Tata Steel has spent many years protecting rails from the elements and its coated rail solutions have provided some simply staggering extensions to rail life. The replacement rail installed into the level crossing pictured above has now exceeded 23 times the life of the original. This means improvements for everyone – passengers have a safer and less disrupted journey, maintainers and network owners have a lower risk network with less rail replacement to do, and the general public don’t have their road/rail crossing closed for rail replacement.
In the constant strive to improve product performance our latest generation coated rail product, ZinocoTM, provides maximum sacrificial protection with a step change improvement in durability for the highest level of corrosion- protection.
Written by Daniel Pyke, product marketing manager at Tata Steel.
Article first published in Rail Engineer October 2015 Issue 132