1. How Cold Forming Affects the Corrosion Resistance
The superior corrosion resistance of Q355NH (and all weathering steels) comes from its specific chemical composition, which includes alloying elements like Copper (Cu), Chromium (Cr), Nickel (Ni), and Phosphorus (P).
During the cold forming process (stamping, bending, rolling), two main things happen that impact this protective ability:
Microstructural Changes: The metal undergoes plastic deformation at a microscopic level. Grains become elongated and work-hardened. This strained microstructure can create pathways for corrosion to initiate slightly faster than in the unaffected, more homogeneous base metal.
Disruption of the Protective Patina: The most significant factor is the disruption of the initial, pre-formed "mill scale" and the very early stages of the protective patina that may have started to form. The forming process can abrade or crack this thin protective layer on the surface.
2. The "Silver Lining": Self-Healing Properties
This is the crucial part. While cold forming creates areas of increased corrosion activity initially, the alloying elements in Q355NH are still present in the deformed areas.
Over time, when exposed to wet/dry cycles in the atmosphere, these same alloying elements will diffuse and form the stable, protective rust layer (patina) just as they do on the rest of the surface.
The patina will eventually "heal" over the cold-worked areas. The long-term corrosion resistance is restored because it is a bulk material property, not just a surface coating.
3. Practical Implications and Recommendations
The effect is most pronounced in the short term and in severe environments (e.g., coastal areas with high chloride salt). For most applications, it is not a catastrophic issue but should be considered.
Industry best practices to manage this effect include:
Post-Forming Cleaning: After cold working, it is highly recommended to clean the steel to remove oils, dirt, or metal fines that could trap moisture and accelerate corrosion. Methods include brushing, abrasive blasting, or high-pressure water washing.
Promoting Patina Formation: The best way to ensure uniform and rapid patina development over the entire structure, including cold-worked areas, is to allow it to weather naturally. The patina will form and stabilize within typically 3-6 years in a suitable atmosphere.
Accelerated Patination (For Critical Applications): In environments where early rust runoff is a concern (e.g., staining architectural surfaces) or for faster protection, an accelerated weathering process can be used. This involves cyclically wetting and drying the surface with a salt solution to quickly develop the protective patina over the entire structure, ensuring the cold-worked areas are protected uniformly.
Avoiding Coatings (Unless Intended): Do not apply impermeable barrier coatings (like standard paint) unless you have a full coating system designed for it. The goal is to let the patina form. If painting is desired, the surface must be properly prepared (e.g., abrasive blast cleaning to a SA 2.5 standard) to ensure adhesion.
