Why Rail Surface Treatment Matters
Rail steel corrodes where water, chlorides and industrial pollutants sit on the surface and where the protective oxide does not form a stable barrier. Corrosion matters most on the web and base of the rail, on rail joints and on the fastening components, because these areas are hard to inspect and expensive to replace. Corrosion also changes the electrical behaviour of the track and can seize threaded connections so that they cannot be dismantled at the next maintenance cycle.
Wear is a separate duty. On the running surface the contact between wheel and rail is severe and no coating survives it, so surface treatment is applied to the parts that do not carry rolling contact: the rail body outside the running band, the joint bar faces, base plates, clips, bolts and gauge rods. Accepting that division is the first step in specifying a coating that will actually stay in service.
Hot Dip Galvanizing
| Process parameter | Control value | Reason |
|---|---|---|
| Zinc bath temperature | 440 to 460 degrees Celsius | Controls zinc fluidity, coating adhesion and drain-off |
| Immersion time | 30 to 60 seconds | Metallurgical reaction time that sets coating thickness |
| Pre-treatment | Degrease, acid pickling, fluxing | Removes scale, rust and oil so the zinc wets the steel |
| Coating thickness on steel 6 mm and above | 85 micrometres minimum average to GB/T 13912 | Required coating mass for thick sections |
| Post-treatment | Passivation sealing | Forms a protective film that limits wet storage staining |
Pickling before galvanizing is essential. Any remaining mill scale or oxide stops the zinc from alloying with the steel and produces bare patches that fail first in service. After galvanizing the coating must be continuous, free from uncoated areas, pimples and flux inclusions, and the coating thickness is measured by the magnetic method or by the stripping method of GB/T 13912. A 2000 hour neutral salt spray exposure to the GB/T 10125 method is a common verification of the finished coating system, with no red rust accepted on the assessed area.
Where the rail joint is the critical corrosion location, local treatments at the joint faces and at the bolt holes are applied in addition, because the joint traps water and holds it against the steel. Holes and cut edges are always treated, since coating thickness at a sheared edge is the weakest point of any galvanized part.
Zinc Flake and Non-Electrolytic Coatings
Zinc flake coatings use lamellar zinc and aluminium particles in an inorganic binder. A coating of about 60 micrometres gives corrosion protection comparable with, and in some environments better than, hot dip galvanizing, because the overlapping flakes form a barrier while the zinc still provides sacrificial protection at damaged edges. The process is applied by dip-spin or spray and is cured at a temperature well below the tempering temperature of high strength fasteners, which is why zinc flake coating is the usual choice for rail clips, bolts and nuts where hydrogen embrittlement and thread fit are both concerns.
Non-electrolytic zinc flake coatings on fasteners are specified in GB/T 5267.3, which defines the coating grades, the coating mass and the corrosion test requirements. Two practical advantages matter in track work: the coating does not build up on threads the way a hot dip coating does, so nuts run on without re-tapping, and there is no risk of hydrogen embrittlement because no electrolytic pickling and plating step is involved. The trade-off is that the coating is thinner and softer than a hot dip coating, so it is more sensitive to mechanical abrasion during handling and needs care in packing and transport.
Other Preparation and Protection Routes
Abrasive blast cleaning followed by a zinc rich primer and a finish coat, used for base plates, tie plates and large fabricated parts.
Rust preventive oil or wax, used as an interim protection for rail stored before installation rather than as a permanent coating.
Thermal spray aluminium or zinc, used where a thick, weldable and abrasion resistant coating is needed on large sections.
Phosphate and zinc flake combination systems, widely used on threaded components to hold a controlled friction coefficient in the thread.
Coating choice is finally decided by four questions: what the service environment is, how long the coating must last before the first maintenance intervention, whether the part has a thread or a mating face that limits coating thickness, and whether coating damage during installation can be repaired on site.
Inspection, Coating Damage and Repair
Inspection records the coating thickness at defined positions, the appearance of the coating against a written acceptance sample, the adhesion, and the result of the salt spray test where it is specified. The most frequent cause of premature corrosion on galvanized rail components is not the coating itself but damage caused by slings, chains and rough stacking during transport and installation, so handling instructions are part of the specification.
Small damaged areas are repaired with a zinc rich repair coating applied to a cleaned surface, building the dry film thickness to the value specified for the repair product. Cut edges, drilled holes and welded areas are always touched up. Where the running surface of a rail has been galvanized by mistake, the coating must be removed before the rail goes into track, because the zinc layer would be displaced into the wheel-rail contact and is not compatible with rolling contact fatigue behaviour.
Frequently Asked Questions
Q: What zinc bath temperature is used for hot dip galvanizing rail components?
A: The bath is held at 440 to 460 degrees Celsius, with an immersion time of 30 to 60 seconds to give a uniform coating.
Q: How thick must a hot dip galvanized coating be?
A: For steel sections 6 mm and above, an average coating thickness of at least 85 micrometres is required under GB/T 13912, measured by magnetic or stripping methods.
Q: Why is pickling necessary before galvanizing?
A: It removes scale, rust and oil so that the zinc wets the steel and alloys properly. Without it the coating forms bare patches that corrode first.
Q: When is a zinc flake coating preferred to hot dip galvanizing?
A: On fasteners and threaded parts, and where coating thickness must be limited. Zinc flake coatings are specified in GB/T 5267.3 and avoid thread build-up and hydrogen embrittlement.
Q: How is corrosion performance verified?
A: By coating thickness measurement, visual examination against an acceptance sample, and neutral salt spray testing to GB/T 10125, where a 2000 hour exposure without red rust is a common requirement.
Q: Can the coating be repaired on site?
A: Yes. Small damaged areas and all cut edges and drilled holes are touched up with a zinc rich repair coating applied to a cleaned surface at the specified film thickness.