1. Why the Corrosion Resistance Decreases Temporarily
The protective patina of Corten relies on a specific chemical composition at the surface. Welding heat disrupts this in two key ways:
Burn-off of Alloying Elements: The intense heat of the welding arc can cause the volatile alloying elements crucial for forming the protective patina-particularly Copper (Cu) and Phosphorus (P)-to oxidize and burn off from the surface of the HAZ. This leaves a surface layer depleted of these elements, making it behave more like a mild steel than a weathering steel.
Scale Formation: The welding heat creates a thick, bluish-black iron oxide scale on the surface in the HAZ. This scale is brittle, non-adherent, and non-protective. It acts as a barrier, preventing the uniform formation of the desired stable rust patina.
2. The Result: Differential Corrosion
After welding, you will observe a visible difference in corrosion behavior:
The unaffected parent metal will begin to form its stable, reddish-brown patina.
The HAZ, with its depleted alloy content and scaly surface, will initially corrode differently. It often appears as a darker brown or black line along the weld, and in the early stages, it may even show slightly higher corrosion rates or uneven rusting.
3. The Key Question: Is This Effect Permanent?
No, it is not permanent. This is the most important point.
The Corten steel beneath the scaled surface still retains its full chemical composition. Over time, through natural weathering cycles (wet/dry), the non-protective scale will eventually spall off (flake away). Once this scale is gone, the underlying Corten metal is exposed to the atmosphere and begins to form its own protective patina.
This process of the HAZ "catching up" can take anywhere from 6 to 18 months of exposure. Eventually, the corrosion performance of the HAZ will become uniform with the rest of the parent metal.
