The Shield Against Rust: UIC Leaflet 842-5 Surface Preparation

Introduction to UIC Leaflet 842-5

In the lifespan of a steel railway bridge, the enemy is not the train load; it is rust. A coating system is only as good as the surface it adheres to. Studies show that over 80% of premature coating failures are caused not by bad paint, but by poor surface preparation. UIC Leaflet 842-5, titled “Guidelines for the corrosion protection of steel structures and components – Surface preparation and application of coating systems,” is the execution manual for ensuring that protective layers last for 25 years or more.

While other leaflets define what paint to buy, UIC 842-5 defines how to apply it. It governs the dirty, critical work of abrasive blasting (sandblasting) and the precise climatic conditions required before a single drop of paint touches the steel.

Snippet Definition: What is UIC 842-5?

UIC Leaflet 842-5 is a technical guideline specifying the procedures for the surface preparation and application of anti-corrosion coating systems on railway steel bridges and structures. It mandates the cleanliness standards (typically Sa 2.5), environmental limits (dew point, humidity), and application techniques (stripe coating for edges) required to ensure long-term durability in harsh railway environments.

The Foundation: Surface Preparation

UIC 842-5 aligns closely with international standard ISO 8501 but applies it strictly to the railway context.

• Cleanliness Grade Sa 2.5 (Very Thorough Blast Cleaning): The standard typically requires the steel to be blasted until it is 95% free of all mill scale, rust, and old coatings. The surface must look like a “grey white” metal.
• Roughness (Profile): It is not enough to be clean; the steel must be rough. The blasting must create an “anchor pattern” (peaks and valleys) for the paint to mechanically grip onto. If the surface is too smooth, the paint will peel off like a sticker.

The “3-Degree Rule” (Climatic Conditions)

Applying paint on wet steel is fatal for the coating. UIC 842-5 sets a rigid “Go/No-Go” rule for painters based on physics:

• The Rule: The temperature of the steel surface must be at least 3°C above the Dew Point.
• Why? The dew point is the temperature at which moisture in the air condenses into water. The 3°C buffer ensures that no invisible film of condensation forms on the steel, which would trap water under the paint and cause blistering.
• Humidity: Relative Humidity (RH) is generally limited to a maximum of 80% or 85% during application and curing.

The Critical Detail: Stripe Coating

Corrosion always starts at the edges. Paint acts like a liquid; surface tension causes it to pull away from sharp corners (the “edge effect”), leaving a dangerously thin layer at the precise point where rust attacks.

UIC 842-5 mandates Stripe Coating: Before the full spray coat is applied, a painter must manually use a brush to apply a strip of paint to all edges, welds, bolt holes, and rivets. This ensures these vulnerable spots get a “double layer” and adequate thickness.

Comparison: UIC 842-5 vs. ISO 12944

UIC 842-5 adapts general industry standards to the specific needs of railways.

Quality Control Measures

The job isn’t done until the measurements confirm it.

• DFT (Dry Film Thickness): Measured using magnetic gauges. A typical railway 3-coat system might require 240-320 microns total.
• Adhesion Test: A “pull-off” test or “cross-cut” test to verify the paint is actually stuck to the steel.
• Holiday Detection: For critical areas (like submerged steel), a high-voltage spark test is used to find “holidays” (pinholes/missed spots) in the coating.