Why the SKL Fastening System Needs Engineered Surface Protection
An SKL fastening assembly clamps the rail through a pre-loaded spring bar that is held down by spiral spikes, so every part in the load path works outdoors under moisture, chloride ingress, grit abrasion and cyclic stress. Corrosion on a guide plate or a spike shank is not cosmetic: section loss reduces clamping force, opens gauge tolerance and shortens the interval between tamping and re-tensioning work. Surface treatment is therefore chosen component by component, matched to the substrate, the exposure class and the mechanical duty of the part rather than applied as one blanket finish.
Surface Treatment Options for Spring Bars
Spring bars are produced from silicon-manganese and silicon-chromium spring steels such as 60Si2MnA, 60Si2CrA and 60Si2CrVA covered by GB/T 1222, quenched and tempered to a hardness range that keeps the bar elastic at the rated deflection. Four finishes are used on these bars. Light oiling leaves a thin film that separates steel from air and moisture and slightly reduces friction between bar and insulator. Black paint adds a uniform appearance with limited barrier value on its own. Electroplated zinc, described for threaded and non-threaded fasteners in ISO 4042, applies a thin sacrificial layer that protects the base metal at cut edges and scratches. Hot-dip galvanizing to ISO 1461 builds a much heavier coating, not less than 70 micrometres local thickness on steel 6 mm and above, and is the usual choice for coastal lines, tunnels and corridors treated with de-icing salt.
The metallurgical consequence matters as much as the coating weight. Quenched and tempered spring steel loses hardness if it is reheated above the tempering range, so hot-dip galvanizing is applied either to finished bars with a controlled thermal cycle or to bars whose grade tolerates the bath temperature. Where the loss of elastic limit cannot be accepted, electroplated zinc with a trivalent passivation is preferred even though its coating is thinner.
Treatment Selection by Component
Spiral spikes. Spikes in 38Si7 spring steel (EN 10089) anchor the assembly into the sleeper and are permanently in contact with a moist, sometimes chemically aggressive interface. Electroplated zinc or hot-dip galvanizing, or a zinc-rich black coating, keeps the shank from rusting so that the anchoring force and the clamping height stay stable over the service interval.
Guide plates and gauge baffles. These parts carry lateral rail force and are abraded by grit and ballast dust. Galvanizing, hot-dip galvanizing or a paint system that includes a primer is specified so that dimensional accuracy and the contact face with the rail foot are preserved.
Flat washers. Washers in Q235 carbon steel per GB/T 700 are galvanized or oiled to prevent fretting rust between washer, nut and sleeper face, and to distribute the nut load without crushing the sleeper surface.
Insulating sleeves, bushings and rail pads. Plastic and rubber parts rely on formulation rather than metal coatings. Anti-ageing additives, controlled cure and, where ultraviolet exposure is severe, a protective coating keep electrical insulation, buffering and vibration damping stable instead of hardening and cracking.
Matching the Coating to the Service Environment
| Service environment | Dominant corrosion driver | Typical finish | Coating target |
|---|---|---|---|
| Dry inland, low rainfall | Atmospheric humidity | Oiling or black paint | Barrier film only |
| Humid inland and hilly line | Condensation, ballast dust | Electroplated zinc, trivalent passivation | Thin sacrificial layer |
| Coastal and salt-fog belt | Chloride deposition | Hot-dip galvanizing or zinc plus top coat | Heavy zinc per ISO 1461 |
| Tunnel and wet cutting | Permanent damp, sulphur gases | Hot-dip galvanizing | Heavy zinc, sealed edges |
| De-icing salt zone | Chloride splash and spray | Hot-dip galvanizing plus duplex coating | Heavy zinc plus organic top coat |
Inspection and Acceptance of Coated Components
Coating quality is verified before the parts enter the track. Magnetic or eddy-current gauges measure local coating thickness on ten pieces per lot at defined positions: bar bearing area, spike shank, plate face and washer rim. Corrosion performance is confirmed by neutral salt spray testing to ISO 9227, and the assembly is then re-tested for clamping force, because a heavy coating on a bearing face can change the installed toe load. Threaded parts are checked for gauge fit after coating so that spikes and nuts still assemble by hand without galling. Parts that show blistering, bare patches at sheared edges or coating thickness below the drawing value are rejected and stripped before re-coating, since a partially protected surface corrodes faster than an uncoated one.
Bridge From Coating Choice to Track Performance
Surface treatment is the cheapest lever available for extending the replacement interval of a fastening system. A galvanized spring bar keeps its clamping force curve over repeated wet-dry cycles, a protected spike keeps its anchoring depth, and a protected guide plate keeps the gauge adjustment stable so that track geometry work is driven by ballast condition rather than by corroded hardware.
Frequently Asked Questions
Q: Which surface treatment gives the longest life for a spring bar in coastal track?
A: Hot-dip galvanizing to ISO 1461 is the strongest single-layer option because the coating is thick and sacrificial. Where the tempering temperature of 60Si2MnA or 60Si2CrVA bars cannot be risked, electroplated zinc with a trivalent passivation plus a top coat is used instead.
Q: Does galvanizing change the clamping force of an assembled fastener?
A: It can. A zinc layer of 70 micrometres and above adds measurable thickness on the bearing faces and changes friction between bar, insulator and plate, so clamping force is re-checked on coated assemblies and the drawing thickness is taken into account in the design gap.
Q: Is black paint a real corrosion protection?
A: It is a marginal barrier. Paint alone is used on low-exposure inland track and mainly for appearance and identification. In humid, coastal or de-icing-salt zones, painted parts are expected to be galvanized underneath.
Q: How is coating thickness verified on a finished component?
A: Non-destructive magnetic or eddy-current gauges are used on defined measuring positions, with several readings per part and ten parts per lot. Destructive sectioning is reserved for first-article approval and for disputed lots.
Q: Why are plastic sleeves and rail pads not galvanized?
A: They are non-metallic. Their durability depends on the compound, on anti-ageing additives and on ultraviolet protection, so the purchase specification controls formulation, cure and hardness rather than coating weight.
Q: What happens if a coating is damaged during installation?
A: Bare steel at scratches and cut edges becomes an anode and corrodes first. Field practice is to touch up damaged areas with a zinc-rich repair coating before the assembly is finally tightened.