1. The Functional Requirement: Stability, Not Thickness
The primary "requirement" for the patina is not a specific thickness but its stability and protective function. A good, protective patina should be:
Adherent: It should be firmly bonded to the steel substrate and not flake off easily.
Dense: It should form a continuous, compact layer that impedes the penetration of oxygen and water.
Self-Limiting: The corrosion rate should drastically slow down after the initial period (typically 1-3 years), reaching a steady, very low state.
A patina that meets these criteria will automatically find its own "equilibrium thickness" for the given environment. This is why standards focus on performance, not dimension.
2. What the Standards Say
The relevant European standard for S355J2W is EN 10025-5:2019. This standard specifies the chemical composition and mechanical properties of the steel itself to enable the formation of a protective patina. It does not specify a final patina thickness.
The standard ensures the steel has the right "ingredients" (like Cu, Cr, P) to promote the formation of the insoluble rust layer that makes up the patina.
3. Practical Thickness Expectations and Influencing Factors
While not specified, the typical, stable patina on S355J2W in a conducive environment will have a thickness in the range of 50 to 150 micrometers (0.05 to 0.15 mm).
This thickness is influenced by several critical factors:
Corrosion Mass Loss: The patina forms from the steel itself. A common rule of thumb is that during the initial stabilization period, the steel may lose 50 to 100 micrometers of thickness from its surface to form the patina. This initial loss is accounted for in the design of structural elements (by adding a "corrosion allowance" to the base thickness).
Environment (The Most Important Factor):
Ideal (Rural/Urban): With regular wet/dry cycles, a stable, dense patina of ~50-100 µm will form.
Aggressive (Coastal/Industrial): In environments with high chloride (sea salt) or sulfur dioxide (industrial pollution), the patina may be less stable, thicker, more porous, and less protective. It may never properly stabilize, leading to continued corrosion.
Time: The patina thickens and densifies over the first 1 to 3 years before stabilizing.
