Se­curely fas­tened

The observance of the highest safety and quality standards is of central importance for rail infrastructure. Depending on use and location, steel parts installed in rail systems need to be able to permanently withstand aspects such as salty air on the coast or conditions in damp tunnels. The high speeds of rail traffic mean that these elements are also subjected to enormous dynamic forces and are constantly required to compensate extreme oscillation. This is joined by mechanical stresses such as stone impact or high temperature differences and chemical stress – for example from lubricants and alkaline or acidic environmental conditions. In view of this, parts such as rail clamps, sleeper screws or nuts used to secure the rail to the track need to be equipped with a suitable corrosion protection. In the case of fasteners, the selected corrosion protection can also enable the setting of individual coefficients of friction. A tailored corrosion protection system should also be selected for other steel parts installed in the area of the rails, such as washers or rail support plates, to ensure the long-term, safe function of the overall construction, avoid expensive downtime and guarantee the safety of passengers and train personnel.

Whether it is rail clamps, screws, washers, nuts or concrete anchors: parts used in the securing of rails need to be given long-lasting protection against corrosion as well as chemical and mechanical stress.

Typical requirements for surface protection of steel components for rail fastening:

• Corrosion protection (1,440 h according to DIN EN ISO 9227)
• Coloring
• Chemikalienbeständigkeit
• Chemical resistance
• Chromium (VI) free
• No risk of hydrogen embrittlement and its consequences

High-performance zinc flake technology has proved its suitability for all of these requirements. Zinc flake systems deliver very high protective performance at low coat thicknesses of just 8–20 μm. A zinc flake system typically comprises a basecoat and a topcoat. The key task of the basecoat is to actively protect the coated steel surface against corrosion - it offers so-called cathodic corrosion protection. The scale-like zinc coats create a barrier effect. As the less noble metal, the zinc “sacrifices” itself and protects the steel below from deteriorating when faced with a corrosive attack. The additional topcoat strengthens corrosion protection as well as lending the coating multifunctional properties such as temperature resistance, chemical resistance to aggressive cleaning agents and defined glide and friction properties. The individual coats are dried at comparatively low curing temperatures of max. 240°C. Using this proven system structure – and dependant on layer structure, part geometry and application form – cathodic protection of 1,440 hours against base metal corrosion (red rust) and white rust in salt spray testing according to DIN EN ISO 9227 can be achieved. Further characteristics of this German-made green coating technology: it uses no chromium(VI) whatsoever.

Depending on the size and geometry of the parts (bulk products), application of the zinc flake coating occurs via either a dip-spin process or by spraying. The risk of hydrogen-induced stress corrosion cracking is avoided as no hydrogen is released in the application process. Also known as hydrogen embrittlement, it affects bolts, nuts and other fasteners made from tensile steel above a strength of > 1,000 N/mm², it is caused by hydrogen atoms diffusing into the steel. This can result in fasteners breaking at any time. This sudden material failure can have numerous negative consequences. From unplanned and expensive downtimes for maintenance to genuine safety problems and danger to life and limb.

The zinc flake systems from Dörken have already successfully proven themselves in numerous projects in the area of rail infrastructure. The application of a zinc flake coating that suits the requirements ensures the long-lasting corrosion protection of fasteners. This in turn guarantees ongoing function - reducing repair costs and extending track maintenance intervals.