CORROSION is one of the most common causes of persistent and costly degradation of railway infrastructure across the world. Unlike sudden failures driven by overload or impact, corrosion is insidious: it progresses slowly, often going unnoticed until asset performance, safety margins or maintainability are compromised. For rail networks operating with increasing traffic density and higher axleloads, corrosion represents a significant technical and financial risk.
Among the many components exposed to corrosive environments, rail fasteners, including clips, bolts, anchors and associated hardware, are particularly vulnerable. Typically manufactured from high-strength steel, they are directly exposed to the environment, as well as being critical to track integrity.
To illustrate the scale of the problem, the global cost of corrosion has been estimated at $US 2.5 trillion a year, equivalent to 3.4% of global GDP, with 15-35% of that cost potentially avoidable using corrosion control.
Railways account for a significant share of corrosion-related costs due to their extensive steel infrastructure, long asset lives, exposure to aggressive outdoor environments, and safety‑critical performance requirements. While rail fasteners are relatively low-cost items, they are safety‑critical components and can be labour‑intensive to replace. Consequently, costs associated with corrosion are not necessarily driven by the price of materials, but by access constraints, labour, possession time, and associated operational disruption.
Corrosion is not an unusual problem, and can easily be taken into account at the design and specification stage in order to minimise maintenance requirements and optimise whole-life value. Railway assets experience a wide range of corrosive conditions. ISO 9223 classifies atmospheric corrosivity using first-year corrosion rates and links corrosivity to time of wetness, sulphur dioxide (SO₂) pollution and airborne salinity (chloride deposition), giving descriptive examples of the type of environment assigned to each corrosivity category.
In practical terms, rail infrastructure can be located in benign or aggressive sites. Benign sites are typically inland rural areas with good drainage and low pollution, lower wetness levels and minimal chloride. Aggressive sites are coastal routes affected by salt spray, urban/industrial corridors exposed to pollutants, tunnels and cuttings where there can be high time-of-wetness, and lines where salt is used for de-icing in winter. However, even when a route is classified as general C3 or medium corrosivity, local micro-environments can sometimes behave like C4 (high) and C5 (very high) sites and go unnoticed.
If rail fasteners are not adequately protected against corrosion, several technical and operational risks arise. One of the most obvious is loss of mechanical function. Corrosion can reduce effective cross-section, leading to reduced clamping force, which in turn may damage adjacent assets and compromise track geometry.
Another issue is seizure and loss of maintainability. Seized fasteners increase extraction time and often require destructive removal, which raises the likelihood of damage to sleepers or rail.
Corrosion can also lead to fracture. Corrosion pits act as stress concentrators, increasing the risk of crack initiation and reducing fatigue endurance.
While rail fasteners are rarely single-point failure items, widespread corrosion indicates broader degradation across the track system, increasing the likelihood of emergency interventions and service disruption. From a systems perspective, corroded rail fasteners can be costly in terms of maintenance relative to their unit price.
At the track design stage, the need for corrosion protection on rail fasteners should be assessed based on environmental conditions and the presence of pollutants, as these factors directly influence degradation risk. Where required, a range of coating options is available to improve corrosion resistance and extend the service life of the rail fastener.
Coating choice
Organic coatings, including paint, powder coatings and polymeric topcoats, provide an effective barrier to environmental exposure and can be applied using a range of methods, including spray application, dip‑spin coating, powder coating, and electrophoretic deposition. On fasteners and small hardware, damage may occur during handling or installation, and any local breach can allow corrosion to initiate beneath the coating, particularly where adhesion is reduced. Increased coating thickness improves barrier performance, although excessive thickness may lead to dimensional or interference issues. In more demanding environments, organic coatings are typically used as part of a multi-component system to enhance durability.
In addition to organic coatings, metallic coatings are widely used in several industries to protect steel components, with zinc the most common. It provides both an effective barrier to environmental exposure and offers sacrificial (cathodic) protection in the event of coating damage. These coatings can be applied using a range of processes (see panel below).
In practice, coating choice is driven as much by access and possession costs as by corrosion resistance, particularly where managing process-induced hydrogen embrittlement of high-strength steels requires additional post-treatment, increasing both lead time and lifecycle cost.
The strongest argument for corrosion protection is whole-life value, with corrosion management seen as an investment that reduces costs, not an unavoidable overhead. In rail operations, effective corrosion protection leads to improved reliability through reduced degradation, fewer seized rail fasteners, longer replacement intervals in aggressive micro-environments and reduced unplanned work. It also results in reduced carbon emissions associated with replacement and logistics.
Corrosion protection increases unit cost but reduces ongoing maintenance costs. A rail fastener may require higher initial investment to protect, but the asset has a lower cost of ownership because it is easier to maintain and lasts longer under real exposure.
Across the rail industry, effective corrosion protection of small, safety-critical components like rail fasteners is increasingly recognised as a lifecycle optimisation challenge. Good practice starts with understanding route-specific corrosivity and mechanical demands.
Suppliers have a vital role to play in supporting asset owners with corrosion protection solutions that are technically robust and operationally realistic. Some treat corrosion protection as a core engineering discipline, reflected in continued investment in corrosion testing, alongside enhancement of coating capability, specification development and manufacturing control. Pandrol, for example, is investing in dedicated sheradizing capability, including a new line at its rail fastener manufacturing facility in India, enabling consistent, repeatable protection aligned with real-world service conditions.
As railways pursue longer asset life and shorter possession windows, corrosion protection of rail fastening systems remains an important lever for reducing the maintenance burden, improving availability and controlling whole-life cost.
Metallic coating options
Zinc and zinc-nickel electroplated coatings provide sacrificial protection, but zinc-nickel offers improved corrosion resistance due to its lower corrosion rate and more stable corrosion products. Despite this advantage, coating thickness is limited for both systems, and application on high-strength steels requires strict process control to mitigate the risk of hydrogen embrittlement.
Hot-dip galvanizing produces a thick, durable coating comprising zinc-iron alloy layers and a zinc-rich surface, providing sacrificial protection is widely used on structural steel. However, its use may be restricted on high-strength steels, as processing can increase hydrogen embrittlement risk or alter mechanical properties or component dimensions.
Zinc flake systems deliver controlled thickness corrosion protection with no process-induced hydrogen embrittlement risk, but performance is sensitive to specification, curing control and mechanical damage. In more aggressive environments, such as coastal and urban lines, durability has proven variable in service and zinc flake is often specified with more durable organic topcoats or alternatives with demonstrated field performance.
Sherardizing is a dry thermal diffusion process in which zinc diffuses into the steel surface to form zinc-iron alloy layers that are metallurgically bonded to the substrate, providing sacrificial corrosion protection, uniform coverage on complex geometries, and strong adhesion with good resistance to handling damage. The process is compatible with high-strength steels, with no risk of hydrogen embrittlement, and produces a fully diffusion-based coating with no free-zinc layer. The resulting surface can be readily integrated into duplex systems, where specified, to extend durability in severe rail environments.
*Connor Fox is senior materials and improvement engineer at Pandrol.
